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Building Envelope Design Guide - Roofing Systems

by Tom Smith, AIA
TLSmith Consulting Inc.

Last updated: 10-21-2011

Introduction

Prior to the mid-to-late 1970s, almost all low-slope roofs were asphalt or coal tar built-up roofs. However, during the last two decades of the 20th century, a variety of other types of low-slope roof systems began to compete with traditional built-up roofs (BUR). These newer systems included modified bitumens, single-plies, sprayed polyurethane foam, and metal panels. While the modified bitumen systems are related to BUR, the other low-slope alternatives are radically different. Along with new choices of membrane materials, plastic foam roof insulations also emerged in the 1970s. The abundance of materials from which to choose has greatly complicated roof system design.

To select, detail and specify the most appropriate roof system for a project, the architect should ideally have at least a general understanding of the available material options. The purpose of this section is to provide design guidance to architects designing low- and steep-slope roof assemblies on new Federal office buildings.

Note: Low-sloped roofs are defined as those roofs with a slope less than or equal to 3:12 (25 percent). However, with the exception of metal roofs, most low-slope roofs have a slope of about ¼:12 (2 percent). Steep-slope roofs are defined as those roofs with a slope greater than 3:12 (25 percent). As discussed in the Description section, some materials can be used on both low- and steep-slopes, while others are limited to either low- or steep-slope.

Scope

The Description section discusses roof assembly materials, including roof decks, air and vapor retarders, roof insulations and roof coverings. The Application section discusses system selection criteria, warranty considerations, key elements of drawings and specifications, and construction contract administration. The Details section discusses and presents various details. The remaining section are Emerging Issues, Relevant Codes and Standards, and Additional Resources.

For below-grade waterproofing and plaza decks, see Below Grade Systems. For seismic considerations, see Seismic Safety. For blast considerations, see Blast Safety.

Project Delivery

Delivering a successful roof involves two distinct phases. The first phase is the design process. In this phase, the architect needs to have a basic understanding of roof assembly materials and system options, and an understanding of roof design considerations. After identifying the project's requirements, a roof system should be selected that optimally responds to an integration of the project's requirements and the system selection criteria. After the roof system is selected, the specifics of the system (such as deck type, insulation type(s) and thickness, fastener patterns, and warranty requirements) are developed and details are designed. This phase is culminated with the preparation of specifications and drawings that communicate the architect's design concept and requirements to a professional roofing contractor for execution of the work.

The second phase is construction contract administration. In addition to the traditional activities, such as submittal review and field observation, the architect should also inform the building owner about the importance of semi-annual roof inspections and routine maintenance.

Guide Limitations

This Guide is intended to give a relatively brief introduction to roofing and provide special criteria applicable to Federal office buildings. It addresses the basics, but does not delve deeply into the subject. After gaining a general understanding of the roof assembly options and various issues associated with them, an architect has a choice to make: Either elect to further expand his or her skills and knowledge, or work with professional roofing contractors or roof consultants. Years ago, it was uncommon for architects to work with a roof consultant or call upon a trusted contractor for advice. But the complexities brought on by the BUR alternatives now demand the inclusion of a roof consultant as part of the design team, if this expertise is not developed within the architect's office.

At the very least, architects should have some key reference materials in their office. These materials are discussed within the following Sections. If an architect desires to develop his or her expertise rather than use consultants, a concerted and continuing effort will be necessary.

The details associated with this section of the BEDG on the WBDG were developed by committee and are intended solely as a means to illustrate general design and construction concepts only. Appropriate use and application of the concepts illustrated in these details will vary based on performance considerations and environmental conditions unique to each project and, therefore, do not represent the final opinion or recommendation of the author of each section or the committee members responsible for the development of the WBDG.

Description

When specifying roof assemblies, designers have many materials from which to choose. This section provides a brief overview of the primary roof deck, air retarder, vapor retarder, insulation and roof covering materials used in the U.S. For further information on these materials, refer to The NRCA Roofing and Waterproofing Manual (published by the National Roofing Contractors Association). Roof system selection criteria are discussed in the Application section. Combining the various materials into assemblies is also discussed in The NRCA Roofing and Waterproofing Manual and in the Unified Facilities Guide Specifications.

A design concern when designing roofs in cold climates is the possibility of falling ice and snow, as described in the WBDG resource page by Mike Carter, CET and Roman Stangl, CET.

The term roof assembly includes the roof deck, air or vapor retarder (if present), roof insulation (if present) and the roof covering. The term roof system refers to the air or vapor retarder (if present), roof insulation (if present) and the roof covering.

The following are included in this section:

Roof Decks
Air Retarders
Vapor Retarders
Insulation
Low-slope Roof Coverings (slope less than or equal to 3:12)

  • Built-up Roofs
  • Modified Bitumen
  • Single-ply
  • Sprayed Polyurethane Foam
  • Metal Panels

Steep-slope Roof Coverings (slope greater than 3:12)

  • Metal Panels and Shingles
  • Asphalt Shingles
  • Slate
  • Tile

Roof Decks

Office buildings typically have steel or concrete decks, although plywood or OSB decks are also used on smaller buildings. The deck can have significant influence on the roof system.

Of the deck types, steel is the most common. Although prime-painted steel decks with welded connections are typically specified, it is recommended that galvanized decks be specified in order to obtain greater corrosion protection in the event of roof leakage. It is also recommended that screw-attachment be specified in lieu of welding, because screws provide more reliable attachment.

Air Retarders

If the roof membrane is monolithic (i.e., a membrane roof) it serves as an air retarder. However, separate air retarders are sometimes incorporated into roof systems. When air retarders are incorporated into wall systems, they are normally included to address moisture and/or energy consumption issues. When an air retarder other than the roof membrane is incorporated into a roof system, it is normally included to address wind performance issues as discussed in Wind Safety.

The deck itself can be a retarder if it is monolithic, such as cast-in-place concrete. When the deck is used as an air retarder, deck penetrations such as plumbing vents should be sealed, and the deck should be sealed at parapets. However, a separate sheet material such as 6 mil polyethylene, housewrap, a two-ply built-up membrane or a one-ply modified bitumen sheet is typically used to create an air retarder. Air retarders are further discussed in A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs (2005).

Vapor Retarders

Vapor retarders are typically constructed with sheet materials such as 6 mil polyethylene, a two-ply built-up membrane or a one-ply modified bitumen sheet. Housewrap should not be used for a vapor retarder because it has inadequate vapor flow resistance.

Insulation

There are two categories of roof insulation, rigid and non-rigid. Rigid boards are typically used in low-slope assemblies. Non-rigid insulations are typically used in attic spaces and in pre-engineered buildings. See the section on Sprayed Polyurethane Foam for more information on this type.

Rigid Insulation Boards

Board-stock insulation has sufficient compressive resistance to support the roof membrane. The following common types of rigid insulation boards are available:

Perlite: This is a low R-value insulation (R-2.78 per inch). It is commonly used as a cover board (see Note below). It has good fire resistance, but when exposed to water, it looses compressive resistance, turns to mush and can be easily compressed. ½" thick boards have a greater percentage of organic material content than do ¾" or thicker boards. Hence, when hot asphalt is applied over ½" boards, the potential for development of blisters in built-up and hot-applied modified bitumen membranes is increased.

Note: A cover board is a thin layer of insulation (such as perlite or wood fiberboard) or glass mat gypsum roof board. Cover boards are commonly placed over the primary thermal insulation (typically one of the plastic foam insulations) in order to provide an enhanced property, such as improved fire resistance, compressive resistance, or to avoid blistering or avoid a compatibility problem. Cover boards are also commonly used in re-covering to provide a separation layer between the existing and new roof membranes. Some types of cover boards are sometimes specified for use directly over steel roof decks in order to provide a thermal barrier to provide fire protection between steel decks and certain types of plastic foam insulation (the IBC specifies thermal barrier requirements).

Polyisocyanurate: This is a high R-value insulation (R-5.6 per inch using NRCA's "in-service" recommendation, or approximately 6.0 for one inch using the Long-Term Thermal Resistance (LTTR) method for determining resistance. This is one of the plastic foam insulations. It is widely used in low-slope roof systems.

Polystyrene: There are two types of polystyrene insulation, molded expanded and extruded expanded. The two types have distinctly different properties. Polystyrene is one of the plastic foam insulations.

Polystyrene boards should not be in direct contact with PVC membranes, otherwise the polystyrene will leach plasticizers out of the PVC. A suitable separator needs to occur between polystyrene and PVC.

Molded Expanded Polystyrene (MEPS or EPS): This is moderate R-value insulation (from slightly less to slightly more than R-4 per inch, depending upon density). The low-density product is relatively inexpensive. Solvent-based adhesive and hot asphalt disintegrate MEPS. Hence, if either of these are used, a suitable cover board needs to be installed over the MEPS. MEPS can also be decomposed at high temperature. Therefore, MEPS should not be used underneath a black membrane unless a suitable cover board is installed between the MEPS and the membrane.

MEPS cells are filled with air. Therefore, unlike the other plastic foam insulations, MEPS does not thermally age (i.e., loose R-value over time). MEPS is not very resistant to water vapor—when exposed to water vapor drive, MEPS can absorb a considerable amount of moisture.

Extruded Expanded Polystyrene (XEPS): This is a high R-value insulation (R-5 for products with a minimum compressive resistance of 25 psi, R-4.6 for products with a minimum compressive resistance of 15 psi).

XEPS is very resistant to water vapor drive. However, as with MEPS, XEPS should not be exposed to solvent, hot asphalt or very high temperature. But unlike MEPS, in order to avoid membrane splitting, XEPS should not be used below a built-up or modified bitumen membrane (even if a cover board is installed over the XEPS).

XEPS is the only insulation suitable for use above the roof membrane in protected membrane roof (PMR) systems (see section on this topic). However, boards intended for PMRs need to be specifically manufactured for this application. Some minor water absorption may occur in boards located above the membrane during the roof's service life. To account for the R-value reduction due to the water absorption, it is recommended that the roof designer reduce the board's initial R-value by 10%.

XEPS boards with extremely high compressive resistance are available for use in plaza decks where high compressive loads occur.

Wood fiberboard: This is a low R-value insulation (R-2.78 per inch). It is commonly used as a cover board. This board has good compressive resistance. However, when exposed to water, it looses compressive resistance and can be easily compressed.

Composite boards: Composite boards typically consist of two layers of different types of insulation that are laminated together in a factory. The primary insulation is typically polyisocyanurate or MEPS. The secondary layer is typically perlite, wood fiberboard, oriented strand board (OSB), plywood or gypsum board. Composite boards made with OSB or plywood are commonly referred to as "nail base." Some nail base products have a small ventilation cavity between the primary insulation and the OSB or plywood.

With some composite boards, the secondary layer (which is typically the top surface) is superficially adhered to the primary layer. With these boards, it is important to mechanically attach the composite board rather than adhere it. Otherwise the secondary layer could easily detach.

Batt, Blanket and Blow-in Insulation

Batt, blanket or blow-in insulation is commonly used to insulate attics spaces. Blanket insulation is commonly used to insulate roofs of pre-engineered metal buildings. Fiberglass insulation is the most common batt/blanket insulation, and it is also available as a blow-in product. Cellulose (recycled newsprint) is also a common blow-in insulation. If cellulose is specified, specify a product that has been treated for mold and fire resistance.

Note: Batt insulation is insulation that is factory pre-cut into lengths of approximately 4', 8' or 9' and bundled without rolling. Blanket insulation is insulation that is supplied in a roll.

Low-Slope Roof Coverings

The following membranes are typically used on low-slope roofs, but may also be used on steep-slope roofs. When used on steep-slopes, the system's fire resistance may be reduced and/or special precautions may be needed when used on steep-slopes.

Note: Liquid-applied membranes are available, but are not commonly used. They should only be considered when unique circumstances occur.

Built-up Roofs (BUR)

Bitumen is poured and aggregate surfacing is shoveled onto a built-up roof

Figure 2-1. Bitumen is poured and aggregate surfacing is shoveled onto a built-up roof.

Built-up membranes are composed of alternating layers of bitumen (either asphalt or coal tar) and reinforcement sheets (felts). Fiberglass felts are typically used for asphalt BURs, however, polyester felts are available. The asphalt is typically hot-applied, however, cold-applied asphalt is available (cold-applied asphalt incorporates solvent). The membrane is either adhered to the substrate in bitumen, or a base sheet (i.e., a heavy felt) is mechanically attached. When a BUR is installed over polyisocyanurate, NRCA recommends a suitable cover board be installed over the polyisocyanurate. Four plies of felt are recommended (if a nailed base sheet is installed, four plies are recommended in addition to the base sheet). "Heavy duty" fiberglass felts are available (ASTM E 2178 Type VI), but because of their stiffness, it is easier to construct unwanted voids in the membrane. Therefore, Type IV felts are recommended.

Exposed asphalt is susceptible to relatively rapid weathering. Therefore, BURs are surfaced with aggregate, a field-applied coating or a mineral surface cap sheet. If aggregate is specified, wind blow-off should be considered, see Wind SafetyRoof Systems. Coatings include aluminum-pigmented asphalt, asphalt emulsion (reflective or non-reflective), and acrylic. Coatings can enhance fire resistance. However, if coatings are specified, periodic recoating will be required. Because of future maintenance demands, coatings are not recommended. If a cap sheet is specified, it should be in addition to the 4 plies of felt.

ASTM standard D 312 is the product specification for asphalt. There are four Type of asphalt. Type I is much more susceptible to flow than Type IV. ASTM D 6510 provides guidance for selection of asphalt Type in BURs.

Base flashings are typically constructed with modified bitumen sheets.

Although coal tar is still available, the vast majority of BURs are constructed with asphalt.

Modified Bitumen (MB)

MB membranes exhibit general toughness and resistance to abuse. They are typically composed of pre-fabricated polymer-modified asphalt sheets. Polymers are added to bitumen to enhance various properties of the bitumen. The quality of MB products is highly dependent on the quality and compatibility of the bitumen and polymers, and the recipe used during the blending process. There are three primary types of MB sheets, as well as field-applied modified mopping asphalt:

This APP modified bitumen cap sheet is being applied with a multi-head torch.  The seam is being rolled with a roller.

Figure 2-2. This APP modified bitumen cap sheet is being applied with a multi-head torch. The seam is being rolled with a roller.

Atactic polypropylene (APP): APP polymer is blended with asphalt and fillers. The mixture is then factory-fabricated into rolls that are typically one meter wide. The prefabricated sheet, commonly referred to as a cap sheet, is typically reinforced with fiberglass, polyester or a combination of both. The sheets are available smooth (i.e., unsurfaced); embedded with mineral granules of a variety of colors; or factory-surfaced with metal foil such as aluminum, copper or stainless steel. The aluminum foil is available in colored finishes. APP MB membranes are generally resistant to high-temperature flow.

To avoid surface cracking, a field-applied coating (such as aluminum-pigmented asphalt, asphalt emulsion or acrylic), factory-applied surfacing (granules or metal foil) or a sheet with protective reinforcement near the top should be specified.

APP MB membranes are typically composed of a base sheet and an APP cap sheet. The cap sheet is either heat-welded (i.e., torched) to the base sheet, or it is adhered in cold adhesive. Mechanically attached systems are also available.

Note: APP MB sheets are also available with a factory-applied tackifier on the underside of the sheet, which permit them to be self-adhering. Several manufacturers introduced these products in the early 2000s.

Sometimes one or more fiberglass ply sheets (as used in BUR) are mopped to the base sheet and then the cap sheet is installed. This configuration is often referred to as a hybrid system, as it essentially is a BUR with a MB cap sheet. The ply sheet(s) provide additional redundancy.

This PMR is composed of two layers of XEPS.  The top board has a factory-applied mortar surface.

Figure 2-3. This PMR is composed of two layers of XEPS. The top board has a factory-applied mortar surface.

APP MB membranes can also be used in a protected membrane roof (PMR) configuration. In a PMR, XEPS insulation is placed over the membrane. The insulation is protected from ultraviolet (UV) radiation and wind blow-off by concrete pavers or large aggregate. When aggregate is selected, a filter fabric should be specified between the aggregate and insulation in order to keep the aggregate from getting into the board joints and underneath the boards. Alternatively, insulation boards with a factory-applied mortar surface may be specified.

Styrene-butadiene-styrene (SBS): SBS polymer is blended with asphalt and fillers. The mixture is then factory-fabricated into rolls with reinforcement and surfacing similar to APP MB sheets. SBS sheets generally have good low-temperature flexibility.

SBS MB is susceptible to premature deterioration when exposed to UV radiation. Although the sheets could be coated in the field, factory-surfacing is recommended.

SBS MB membranes are typically composed of a base sheet and an SBS cap sheet. The cap sheet is either heat-welded (i.e., torched) to the base sheet, or it is adhered in cold adhesive or hot asphalt. Mechanically attached systems are also available. Hot asphalt is not recommended for attachment of cap sheets because of the great potential for development of blisters.

As with APP systems, sometimes one or more fiberglass ply sheets are placed between the base sheet and the cap sheet.

Note: SBS MB sheets are also available with a factory-applied tackifier on the underside of the sheet, which permit them to be self-adhering. Several manufacturers introduced these products in the early 2000s.

SBS MB membranes can also be used in a PMR configuration. If a PMR system is specified, a minimum 4 mil [0.1 mm] polyethylene slip sheet should be placed between the membrane and the XEPS to prevent the insulation boards from bonding to the membrane. Otherwise, membrane tearing could occur when the insulation floats during a rainstorm.

Styrene-isoprene-styrene (SIS): These self-adhering sheets are blended with SIS polymer, asphalt and fillers. The mixture is then factory fabricated into either 3 feet or 1 meter wide rolls. The top of the prefabricated sheet is available with embedded mineral granules or a factory-laminated UV-protective surfacing, such as aluminum foil. The bottom surface has a release paper to keep the sheet from bonding to itself while rolled.

A similar product is commonly used under steep-slope roof coverings to provide ice-dam protection. However, the steep-slope underlayments do not have a UV-protective surfacing. SIS MB roof membranes currently capture a very small share of the low-slope market.

Styrene-ethylene-butylene-styrene (SEBS): SEBS polymer is blended with asphalt in a factory. The SEBS modified asphalt is then reheated at the job site in specially-designed tankers or kettles. The hot modified asphalt is applied in a manner that is virtually identical to BUR. The membrane is typically surfaced with aggregate. SEBS modified mopping asphalt is extremely expensive, and therefore not commonly used.

Modified mopping coal tar was introduced in the mid-1990s, but it has very limited market share.

Single-Ply

The single-ply family of roof membranes is composed of thermoplastic and thermoset products. Single-ply sheets are factory-fabricated and installed in a single thickness. Single-ply membranes are relatively easy to install on steep or complex roof slopes. In comparison to BUR or MB membranes, they are also very light weight (except for ballasted systems). However, unless used in a PMR configuration, they do not offer nearly as much toughness and resistance to abuse as do BUR and MB membranes.

There are five primary methods for securing the membrane the roof deck or other substrate:

Fully adhered: The membrane is adhered in a continuous layer of adhesive. Some single-plies are also available with a factory-applied tackifier on the underside of the sheet, which permits them to be self-adhering (several manufacturers introduced these products in the early 2000s).

Application of a fully adhered single-ply membrane.Application of aggregate ballast over a single-ply membrane.

Left, Figure 2-4. Application of a fully adhered single-ply membrane.
Right, Figure 2-5. Application of aggregate ballast over a single-ply membrane.

Ballasted: The membrane is loose-laid over the substrate and then covered with ballast to resist wind uplift. Ballast can either be large aggregate (for example, 1-½ or 2-½ inches nominal diameter, depending upon design wind speed), concrete pavers weighing 18 to 25 pounds per square foot (psf), or specially designed lightweight interlocking concrete pavers weighing approximately 10 psf [49 kg/m²]. Ballasted systems are limited to a maximum slope of 2:12.

If crushed aggregate is specified, a stone-protection mat between the membrane and aggregate should be specified to avoid puncturing the membrane. A stone-protection mat is also recommended when smooth aggregate is used because some sharp fragments are often among the smooth aggregates. Also, aggregates sometimes fracture into very sharp pieces after they have been installed. It is also a conservative practice to specify a mat to protect against abrasion and puncture from fragments during paver installation. A somewhat thinner mat is normally sufficient for paver-ballasted jobs.

This EPDM membrane was cut by a piece of aggregate.  Installation of a stone-protection over the membrane would avoid aggregate punctures.  These types of punctures are very difficult to find.  The end of an ink pen shows the scale.

Left, Figure 2-6. This EPDM membrane was cut by a piece of aggregate. Installation of a stone-protection over the membrane would avoid aggregate punctures. These types of punctures are very difficult to find. The end of an ink pen shows the scale.
Right, Figure 2-7. The stone in the center of the photograph shattered into several sharp pieces after installation on a ballasted single-ply. Installation of a stone-protection over the membrane would avoid punctures sharp fragments such as these.

Mechanically attached: The membrane is loose-laid except for discrete rows of fasteners. There are a variety of fastening and seam fabrication with this method, as described in A Guide for the Wind Design of Mechanically Attached Flexible Membrane Roofs (2005).

This mechanically attached single-ply membrane uses the 'bar-over' attachment method.  After installation of the membrane, a batten bar is placed over the membrane and screwed to the deck.  A stripping ply is then installed over the bar.

Figure 2-8. This mechanically attached single-ply membrane uses the "bar-over" attachment method. After installation of the membrane, a batten bar is placed over the membrane and screwed to the deck. A stripping ply is then installed over the bar.

To avoid tear propagation in the event that the membrane is torn, it is recommended that only reinforced membranes be specified for this attachment method.

Protected membrane roof. See the Modified Bitumen (MB) section above.

Loose-laid air-pressure equalization system: The membrane is fully adhered around the roof perimeter, but elsewhere the membrane is only loose-laid. This system should only be used over an air-impermeable roof deck or over an air retarder. To compensate for minor air leakage between the membrane and the deck/air retarder, air-pressure equalization valves are installed at prescribed intervals. The valves are one-way: they allow air underneath the membrane to vent out, but outside air is prevented from flowing through the valve and underneath the membrane. As with mechanically attached systems, it is prudent to only specify reinforced membranes for this attachment method. This type of system is susceptible to wind blow off if future roof penetrations are cut through the deck/air retarder and left unsealed.

Thermoplastic Single-plies

Thermoplastic materials do not cross-link, or cure, during manufacturing or during their service life. Field-fabricated seams are typically welded with robotic hot-air welders. Hand-held hot-air welders are used to weld seams at flashings and penetrations. Thermoplastic membrane seams are typically extremely reliable, resulting in a very low incidence of seam failures. These sheets are normally around 5 to 12 feet wide [1.5 to 3.6 m]. Some manufacturers weld the sheets together in the factory to form large sheets that are then welded together on the roof. Primary membrane types in this category are:

Polyvinyl chloride (PVC): PVC membranes are among the oldest single-plies still available. If in contact with polystyrene insulation, the polystyrene will cause the plasticizers in the membrane to leach out. To avoid such membrane embrittlement, a separator sheet needs to be installed between the membrane and the polystyrene. To avoid membrane damage, a separator is also needed to isolate PVC from asphalt and coal tar products. The ballasted attachment method is not recommended because fine dust particles from the ballast or particulate fall-out from the atmosphere may leach plasticizers from the membrane. PVC membranes are available in a wide variety of colors. This membrane is often selected for steep-slope roofs where a strong or unique color is desired.

PVC Alloys or Compounded Thermoplastics (also referred to as PVC blends): These membranes are related to PVC membranes. They are primarily compounded from PVC, but they have additional polymers that provide somewhat different physical properties. Only a very small number of manufacturers make these products. The primary types of membranes in this category are: copolymer alloy (CPA), ethylene interpolymer (EIP) and nitrile alloy (NBP).

Thermoplastic polyolefin (TPO)

TPO is the latest thermoplastic membrane introduced into the marketplace. It was commercialized in North America in the early 1990s. It is formulated from polypropylene, polyethylene or other olefinic materials. Unlike PVC and PVC blends, TPO membrane do not rely upon plasticizers for flexibility, so embrittlement due to plasticizer loss is of no concern. TPO membranes are typically white, and are available in sheet widths up to 12' [3.6 m].

Ketone ethylene ester (KEE)

This membrane is also referred to as a tripolymer alloy (TPA), and the polymer is known by the trade name of Elvaloy. KEE sheets are similar to PVC.

Thermoset Single-plies

Thermoset materials normally cross-link during manufacturing. Once cured, these materials can only be bonded together with a bonding adhesive or specially formulated tape. Primary membrane types in this category are:

Ethylene propylene diene monomer or terpolymer (EPDM): EPDM is a synthetic rubber sheet. As of 2005, EPDM enjoys the largest market share of the single-plies in service in North America. EPDM membranes are extremely resistant to weathering, and they have very good low-temperature flexibility. However, EPDM is susceptible to swelling when exposed to aromatic, halogenated and aliphatic solvents, and animal and vegetable oils such as those exhausted from kitchens. On portions of roofs where the membrane may be exposed to these materials, an epichlorohydrin membrane can be specified over the EPDM as discussed below. EPDM membranes are suitable at airport buildings, provided liquid fuel is not spilled on the membrane.

The sheets are typically available in widths of 10, 20 and 45 or 50 feet [3, 6 and 14 or 15 m], and lengths up to 200 feet [61 m]. Hence, on large roofs with very few penetrations, this type of membrane can be very economical to install. Most EPDM sheets are black, although white sheets are available. White sheets, however, are not nearly as resistant to weathering as black sheets. EPDM is typically non-reinforced. However, only reinforced sheets are recommended for mechanically attached and loose-laid air-pressure equalized applications. Reinforced sheets also offer some increased resistance to puncture and tearing when used in fully adhered and ballasted applications.

In fully adhered applications, typically a contact adhesive is applied to the substrate and the sheet. After the adhesive dries, the sheet is mated with the substrate. Another method of application uses fleece-backed EPDM, which is set in a low-rise sprayed polyurethane foam adhesive.

Field seams are fabricated with either a liquid-applied adhesive or specially formulated tape. Although tapes offer performance advantages over liquid-applied adhesives, the contractor still needs to exercise care in cleaning the EPDM prior to tape application, priming the EPDM and diligently executing the seam work as recommended by the manufacturer.

Epichlorohydrin (ECH): This sheet is similar in appearance to EPDM. ECH, however, is resistant to hydrocarbons, solvents and many greases and oils, so it can be used in areas of the roof that are exposed to chemical discharges that are harmful to EPDM. Because of its permeability, the ECH manufacturer recommends placing ECH over an EPDM membrane. Because it is so specialized, ECH is seldom used. Only one manufacturer produces it in North America.

Sprayed Polyurethane Foam (SPF)

This SPF roof is being installed with a robotic sprayer.

Figure 2-9. This SPF roof is being installed with a robotic sprayer.

SPF is a very unique type of roof system. The membrane is constructed by spraying a two-part liquid onto a substrate. The mixture expands and solidifies to form closed-cell polyurethane foam. The substrate can be either the roof deck, an existing roof membrane (provided the existing roof is suitable for re-covering), gypsum board or rigid insulation. The foam is applied with hand-held sprayers or by robotic sprayers. Each pass (or lift) of foam is typically between ½ to 1-½ inches [13 to 38 mm] thick. If a greater total thickness is desired, two or more passes are normally required. The total thickness of the foam can be easily varied to provide slope for drainage. The foam needs to be protected from UV radiation. This is typically accomplished by using one of the following surfacings:

Acrylic coating: This is the least expensive of the coatings, but generally offers the shortest service life (although the best acrylics can last longer than some of the polyurethane coatings). With acrylics, re-coating is required about every 10 to 15 years, depending upon the quality of the coating material, application and climate. They are typically white.

Polyurethane coating: When properly formulated, this coating offers long service life. This can be the toughest coating available in terms of impact and tear resistance, although a wide range of physical properties is available in this product category. Both one- and two-part coatings are available. One-part coatings are typically gray, although white is available. Two-part coatings are typically white.

Silicone coating: Silicone coatings offer exceptionally good weather resistance and long service life. These coatings are typically offered in a gray color, as silicone coatings pick up dirt (if a white silicone is installed, it will soon become gray). More than other coatings, silicone coatings are prone to being pecked by birds. To avoid the pecking, granules are commonly broadcast into the coating while it is wet.

Mineral granules: Mineral granules (similar to those used to surface asphalt shingles) can increase the durability of a coating and provide greater slip-resistance to persons on the roof. Course sand can also be used for these purposes. Granules or sand are broadcast into a coating while it is wet.

Aggregate surfacing: Properly formulated and installed SPF is quite resistant to liquid water. Therefore, aggregate of the size used on BUR systems can be applied directly over the foam. At parapets and equipment curbs, one of the previously described coatings is applied on the vertical surfaces and out several inches onto the field of the roof. Because water vapor can migrate through the foam, the aggregate surfacing option should not be specified in situations where the annual net vapor flow is downwards. As with aggregate-surfaced BUR, consideration should be given to aggregate blow off.

Other considerations

The worker performing the spraying must be very skilled and knowledgeable. If the qualifications of the contractor and the spray mechanic cannot be reasonably assured, it is prudent to specify an alternative system.

SPF systems have several important attributes. Besides readily lending itself to complex roof shapes, SPF roofs are exceptionally thermally efficient, since they do not have mechanical fasteners or insulation board joints, which create thermal bridges. Also, field research has demonstrated that they have exceptionally good wind resistance. And notably, an SPF roof is not in imminent danger of leaking if the coating is weathered away or ruptured or the aggregate surface is displaced, provided that the penetration does not extend all of the way through the foam (which is generally unlikely). This attribute, is in stark contrast with the other low-slope system options, in which leakage typically occurs if the membrane is punctured.

Metal Panels

Metal panels are not typically thought of as options for low-slope roofs. Some metal panel systems, however, can be used on very low-slopes. Although some manufacturers tout their systems as being suitable for slopes as low as ¼:12 (2 percent), NRCA recommends a minimum slope of ½:12 (4 percent). The greater the slope, the more reliable the leakage protection.

This section only addresses metal panels suitable for use on slopes of 3:12 (25 percent) and less. These panels can also be used on slopes in excess of 3:12. See Section 2.6.1 for metal panels that are only suitable for slopes greater than 3:12.

When installed on low-slopes (particularly slopes approaching ½:12 (4 percent) or less, a metal panel system needs to provide water infiltration protection all across the roof surface. Thus, low-slope metal panel systems should be designed and installed with the intent of making them membrane-like. To achieve this, the panel joints must be soldered or sealed together with sealant tape or sealant, or both. Also, fasteners that penetrate the panel at end-joint splices or flashings must be sealed with gasketed washers. It is more difficult to achieve a reliable and long-lasting watertight system on a very low-slope roof with metal than it is with the other low-slope membrane materials.

Steel or aluminum panels are typically specified for low-slope standing seam panels. Copper is also available, but not commonly used since low-slope roofs are not normally visible. For corrosion protection on steel panels, it is prudent to specify aluminum-zinc alloy (commonly known by the trade name Galvalume). Until the late 1990s, unpainted aluminum-zinc alloy panels had a factory-applied lubricant to facilitate roll-forming. The lubricant eventually weathers away, but installation smudges and fingerprints result in uneven appearance for awhile. A thin clear acrylic coat can be specified to provide a more even appearance and show the effects of weathering more gradually, as the acrylic weathers away. Acrylic-coated Galvalume is sold under trade names such as Galvalume Plus and Acrylume.

Normally low-slope panels are not painted. However, if the roof can be viewed from above, and it is desired to have a painted finish applied to either steel or aluminum panels, there are several finish options. The most common factory-applied coil coating is polyvinylidene fluoride (PVDF), commonly known by the trade names of Kynar and Hylar. PVDF is typically specified since if offers a large range of colors and is extremely resistant to color change over time. Painting can also be specified when it is desired to have a high emissivity.

Internal gutters and parapets at the eaves of low-slope metal roofs should be avoided, as it is less problematic to have the water flow over the end of the panels and fall directly to grade or drop into an external gutter that is below the plane of the panels.

There are two primary approaches to low-slope metal roofs:

Standing seam hydrostatic, or water barrier systems: These panel systems are designed to resist water infiltration under hydrostatic pressure. The panels have standing seams, which raise the joint between the panels above the water line. The seam is sealed with sealant tape or sealant in case it becomes inundated with water backed up by an ice dam or driven by wind.

Most hydrostatic systems are structural systems (e.g., the roof panel has sufficient strength to span between purlins or nailers). A hydrostatic architectural panel (which cannot span between supports) may be specified if a solid deck is provided. Most architectural panels are hydrokinetic, or water-shedding, and therefore require a slope greater than 3:12 (25 percent).

Some panels have snap-together seams, while others are mechanically seamed with an electrically powered mechanical seaming tool. On slopes of ½:12 (4 percent) or less, it is recommended that mechanically seamed panels be specified.

There are two basic types of standing-seam panel profiles, the trapezoidal rib and the vertical rib. Because of its appearance, the trapezoidal rib panel is typically used on industrial buildings and warehouses. The trapezoidal panel is difficult to make watertight at hips and valleys.

Unless very well designed and installed, wind-driven water can infiltrate end-joint splices.  Full-length panels eliminate this problem area.

Figure 2-10. Unless very well designed and installed, wind-driven water can infiltrate end-joint splices. Full-length panels eliminate this problem area.

In addition to the standing seam panels, through-fastened panels (also referred to as R-panels) with exposed fasteners are available for low-slope systems with slopes in excess of 2:12 (17 percent). They should be considered hydrokinetic systems. This is a relatively inexpensive system. It has largely been replaced by standing seam systems, which eliminate leakage problems that are often associated with exposed fastener systems. Other exposed fastener systems include corrugated panels and 5-v crimp panels.

To avoid leakage problems at panel end-joint splices, it is preferable for the panels to be continuous from eave to ridge. If panels are quite long, job-site roll-forming may be necessary. However, full-length panels are sometimes impractical.

The Metal Roof Systems Design Manual by the Metal Building Manufacturers Association provides further guidance pertaining to metal roof systems.

Flat seamed architectural panels

This is also a hydrostatic, or water barrier, system. This traditional system requires a solid substrate. It also requires the use of metals that can be soldered, such as copper. This type of system is labor-intensive. Hence, it is relatively expensive. Because it demands diligent workmanship to provide long-term water protection, it is recommended that this system not be specified unless done so for architectural restoration or compatibility purposes.

Steep-slope Roof Coverings

The following roof coverings are commonly used on steep-slope roofs. These coverings are water-shedding, rather than waterproofing. Special underlayment provisions are required when slopes are relatively low. The NRCA Roofing and Waterproofing Manual provides underlayment guidance.

Metal Panels and Shingles

When used on slopes greater than 3:12 (25 percent), hydrokinetic, or water-shedding panel systems may be used. Hydrostatic (water barrier) systems may also be used. Architectural panels may be specified if a solid deck is provided. If a solid deck is not provided, structural panels need to be specified.

Metal shingles are also available in a variety of metals and designs. The performance varies greatly depending upon the product selected.

Asphalt Shingles

Shingles are available with either fiberglass or organic reinforcement. Fiberglass-reinforced shingles provide greater fire resistance and are therefore recommended. In hail-prone areas, SBS modified bitumen shingles are recommended because of their enhanced resistance to impact damage.

In addition to the traditional three-tab design, laminated (architectural) shingles are available where a different appearance is desired. Product standard ASTM D 3462 has limited criteria to distinguish various products in the marketplace. Therefore, warranty duration is normally used to attempt to distinguish commodity products from those that offer longer service life. However, the warranty duration is not necessarily an indication of performance. Shingles with a minimum warranty of 25 years is are recommended.

Slate

Natural slates have the potential to offer several decades of service life. However, slate is heavy and very expensive. If slate is specified, a very durable underlayment is recommended, so that it does not prematurely degrade.

Specifiers are cautioned that synthetic materials are often marketed as slate. Some of these products are made from slate particles, while others are made from polymers or other materials. Synthetics should not be expected to offer a service life equivalent to natural slate.

Tile

Tiles can either be made from clay or concrete. Tiles typically can be expected to offer a longer service life than asphalt shingles. However, tiles are heavy and more costly than shingles.

Applications

After identifying the project's requirements a roof system should be selected that optimally responds to an integration of the project's requirements and the system selection criteria discussed in System Selection Criteria below. After the roof system is selected, drawings and specifications are prepared to communicate the architect's design concept to a professional roofing contractor. This section also covers warranty considerations, key elements of drawings and specifications, and construction contract administration.

System Selection Criteria

For most roofs, several different types of systems could serve quite well. But some roofs have unique characteristics that lend themselves to perhaps only a few systems. In order to select the most appropriate system for a project, ideally the architect should have at least a general understanding of the material and system options described in the Description section.

If the architect lacks a general understanding of the roof system options (particularly if the project is complex, unusual or very expensive), the architect should seek system selection input from a professional roofing contractor or professional roof consultant who is knowledgeable about all of the system options.

In the context of this section, system selection refers to selection from the primary system types discussed in the Description section (such as BUR, modified bitumen, single-ply, sprayed polyurethane foam, metal panels, asphalt shingles, slate or tile), as well as selection of membrane materials within system types (such as type of modified bitumen, type of single-ply membrane, type of surfacing on an SPF, type of metal panel profile, or type of shingle or tile), and where applicable, the attachment configuration (fully adhered, ballasted, mechanically attached, PMR, or loose-laid air-pressure equalized).

With a general understanding of the available system options, consideration of the following technical and non-technical criteria can lead to the selection of the most appropriate system and details for a project:

system demise
contractor familiarity and availability
maintenance intensity
nearby Government roofs
technical considerations
cost
warranty
implications of sustainable roof design

It is critical that the selected system sufficiently satisfy all of the criteria. Specific system selection recommendations are given later below.

System Demise

The first step in the selection process is to determine what will likely cause the death of the roof system. For example: 1) is the project located in an area that experiences frequent and damaging hailstorms, 2) does the roof have numerous HVAC units, the service of which will generate perpetual abusive foot traffic, 3) will the roof be exposed to intense solar radiation throughout most of its life?

In some cases, one factor will likely cause system death. In other cases, perhaps two or three factors may be nearly equally as likely to end the roof's life. After identifying the likely cause(s) of death, it is incumbent upon the architect to select a system with characteristics that can combat the destructive force(s).

Contractor Familiarity and Availability

Good application is crucial to the long-term success of a roof. During the system selection process, the following should be considered:

  • Are contractors in the vicinity of the project site familiar with the system being considered? If not, either a system should be selected that the local contractors are familiar with, or a contractor should be brought in from outside of the project vicinity. It is important to avoid having a contractor install a system that he or she is not extremely familiar with.
  • It is preferable to select a system that can be installed by contractors who have an office relatively close to the project site. By doing so, the contractor will be familiar with local conditions such as historical weather conditions during the projected application period and logistics.

Maintenance Intensity

Example of a roof system having field-coated APP modified bitumen that has weathered away

Figure 4-1. This APP modified bitumen was field-coated. The coating is weathering away as expected. If a system with a field-applied coating were to be specified, periodic re-coating would be required throughout the roof's life. If re-coating is not performed, the roof's service life and or other properties such as fire resistance will be reduced.

Because of uncertainties pertaining to future budgets for periodic maintenance, a roof system should be selected that has limited maintenance demands. Therefore for example, a system that requires re-coating more frequently than every 15 years is not recommended. Hence, rather than specify a modified bitumen membrane with a field-applied coating, a granule or foil-surfaced membrane is preferable to avoid future re-coating costs (see Figure 4-1.)

Nearby Government Roofs

If there are other Government roofs in the vicinity where the new building will be constructed, it is recommended that the architect request information on the type of systems, number of roofs within each system type, and the experience that the Government has had with the various types. If a specific system has been a good performer, it is probably best to use that system on the upcoming project, unless the new project has unique characteristics that another system would be better able to accommodate. Also, if the Government has periodic inspection, maintenance, and minor repairs performed by in-house maintenance personnel, one advantage of keeping with the same type of system is that they will not have to become familiar with another system type.

Technical Considerations

Most of the topics discussed in this chapter are technical in nature. Many of those considerations strongly influence the system selection. The considerations that influence system selection vary from job to job, depending upon the project location and requirements. When selecting a system it is important for the architect to determine whether the proposed system should more than just meet the minimum requirements. For example, if external fire resistance is particularly important for a project due to a strong threat from wildfires, then rather than just specify a system that meets Class A fire resistance, a better choice would be a system that has enhanced fire resistance, such as a paver-surfaced system. In northern climates consideration should be given to the potential for falling ice and snow. See Considerations for Building Design in Cold Climates.

Cost

Many architects select a roof system primarily on initial cost. Although cost is an important element of a project, when cost is a governing factor in system selection, typically there are ramifications. If a less expensive system is selected, invariably something suffers in comparison with the system(s) that fell from consideration because of the greater cost. The cheaper system generally will not have the reliability or durability of other systems, it may be more maintenance intensive or it may not be as energy efficient. Over the life of the roof, the system with the lowest initial cost often is more expensive than other options that were discarded because of their higher initial cost.

In evaluating cost, it is important to look at the life-cycle cost (LCC). In addition to the initial construction cost, LCC includes energy consumption (for building heating and cooling), maintenance, repairs, length of service life, and disposal at the end of the roof's life. Of these factors, the most difficult to assess is the design service life.

The service life can have a dramatic impact on the LCC analysis. For example, if a 40-year service life is assumed, but the roof fails after 15 years, the true roofing costs will be much higher than calculated. Lack of good data on design service life is often a significant limitation to developing a reliable LCC. It is difficult to have confidence in a manufacturer's claim of, for example, a 30-year life for products that have been in the marketplace for only a few years. Accelerated aging testing is of limited help, as it has not progressed to the point where credible estimates of service life prediction can be made. The selection of a predicted service life should be conservative. For most low-slope systems, use of a service life in excess of 20 years should only be done with caution, evaluation and justification.

For most projects, the costs associated with eventual tear-off and disposal are seldom considered. Because some systems are inherently more difficult to tear-off than others, LCC analysis should consider this issue. Also, it may be possible to salvage or reuse some of the system components. For example, with a PMR, it would be reasonable to assume that much of the ballast and insulation could be reused on the replacement roof.

Although there are difficulties and limitations with the LCC approach, economic decisions based on LCC are preferable to those that only consider initial system cost. ASTM E 917, Standard Practice for Measuring Life-Cycle Costs of Buildings and Building Systems provides further information on LCC.

Warranty

Architects often give considerable weight to a manufacturer's warranty when considering a roof system and a specific manufacturer. As discussed in Section 4.2, many limitations are associated with most warranties. The warranty itself should not be the basis for selecting a system or a manufacturer.

Implications of Sustainable Roof Design

If an emphasis on sustainable roof design is desired, sustainable design criteria can become major factors in the selection process, depending upon the degree to which sustainability is pursued. At the very least, the selected system should be thermally efficient, with consideration given to both R-value, reflectivity and emissivity. And for those buildings that are intended to have a service life in excess of 20 years, a system with enhanced durability should be selected to reasonably maximize the life of the roof to the extent that the budget allows.

See Sustainability of the Building Envelope for further information on sustainable design considerations.

Specific Roof System Selection Recommendations

The following roof systems are recommended, assuming compliance with sections above. Note: Special site or building conditions may preempt the following recommendations.

Slopes less than 1:12

  • Metal panel systems are not recommended.
  • Protected Membrane Roofs (PMR) are recommended. For the membrane, modified bitumen is recommended. If there is compelling reason to specify a single-ply membrane, EPDM is recommended. For ballast, concrete pavers or XEPS boards with a factory-applied mortar surface are recommended in lieu of aggregate.
  • If there is compelling reason to not specify a PMR, a modified bitumen membrane is recommended. It is recommended that the membrane be set in cold adhesive or torched (where torching is appropriate).

    If there is compelling reason to not specify a modified bitumen membrane, a single-ply membrane or SPF is recommended (provided it is reasonably assured that a qualified SPF contractor and spray mechanic will execute the work). If a single-ply is specified, an EPDM membrane is recommended if high reflectivity is not required. If high reflectivity is required, a PVC membrane is recommended. Use of self-adhering modified bitumen or single-ply membranes is not recommended, except for very small roof areas as discussed below. Single-ply membranes should only be specified when very limited foot traffic is expected.
  • Very small roof areas: For very small roof areas, such as canopies and penthouses, self-adhering modified bitumen, EPDM or PVC membranes may be suitable.
  • If the building is located in an area prone to wildfires or severe hail storms, paver-ballasted PMRs with protected base flashings are recommended.

Slopes greater than 1:12 and less than or equal to 3:12)

Standing seam hydrostatic metal panel systems, as well as all of the systems recommended in the section above are recommended.

Slopes greater than 3:12

At slopes greater than 3:12, aesthetics often play a significant role is system selection. System selection primarily based on aesthetics is acceptable provided the provisions in the previous sections are not adversely compromised.

  • Standing seam hydrostatic or hydrokinetic panels.
  • Asphalt shingles are typically specified where initial cost is a primary factor in system selection.
  • Copper panel systems, slate and tile are typically specified where there is compelling aesthetic reason to specify these relatively expensive systems.
  • Modified bitumen, single-ply and SPF may be suitable on steep-slope roofs, provided snow slides are not problematic. If modified bitumen or single-ply membranes are specified, be aware of aesthetic issues (e.g., seam lines will be visible and highly reflective surfaces may become discolored and visually objectionable). Refer to the recommendations in section on slopes less than 1:12 if a modified bitumen or single-ply membrane is specified.

Warranty Considerations

A warranty is not a maintenance contract, nor is it an insurance policy. Furthermore, it does not ensure that leakage, damage caused by hail or wind, or another type of damage will not occur. Rather, a warranty defines specific legal rights and obligations of the building owner and warrantor. It includes remedies and exclusions. If the warrantor is out of business when a problem covered by the warranty is experienced, the warranty often becomes a useless piece of paper.

Since the 1970s, many warranties have become marketing tools rather than true reflections of demonstrated roof system performance. It is common to see 20-year warranties on roof systems that have only been in the marketplace for a few years. When architects rely on warranties for performance, rather than pay attention to the factors that actually affect performance, the potential for premature failure dramatically increases.

When warranties are specified, the specification typically requires the warranty to be issued by the roof membrane manufacturer. It is normally specified that the warranty include both the roof membrane materials (and perhaps other roof system components such as roof insulation) and the roofing contractor's workmanship. Material-only warranties are also available from manufacturers. A manufacturer's warranty establishes a direct contractual relationship between the building owner and manufacturer. For low-slope systems, the length of coverage for a manufacturer's warranty is typically 5 to 25 years, with 10 years being the most common.

Warranty Benefits

A warranty may have some merit if it means that the manufacturer will take steps to minimize the potential for future problems (such as reviewing the architect's specification and details and providing meaningful inspection during application). A warranty may also enhance the likelihood that a roof will be installed by a professional contractor. However, rather than relying on a warranty to obtain a qualified contractor, architects should specify contractor qualification requirements as discussed in the next section.

If a problem that is covered by the warranty occurs, and the warrantor is still in business, the presence of the warranty may lead to a quick resolution of the problem. Virtually every warranty issued by a manufacturer covers repair of leaks caused by defective materials and workmanship (if the warranty is not for materials-only) provided that the cause(s) of the leakage is covered under the terms of the warranty. Without a warranty, the Government might have to pursue legal action to obtain relief. Pursuing legal action may be too costly if the problem is small. Also, the presence of a warranty provides a direct avenue for the Government to purse a claim with the manufacturer if the manufacture does not respond to a problem covered under the warranty.

Warranty Limitations

Warranties are normally prepared to limit the manufacturer's liability to a narrow scope of provisions rather than to provide protection for the building owner. Warranties typically preclude claims based on other theories of liability, including negligence and breach of contract. In addition, warranties typically exclude the implied and express warranties established by the Uniform Commercial Code (UCC).

The UCC provides that goods shall be fit for their particular purpose. However, the length of coverage under the UCC is typically four years from the date of sale. While the provisions under the UCC may be more favorable to the Government than the provisions in a manufacturer's warranty, the Government can obtain coverage for a much longer time with a manufacturer's warranty.

Example of hail damage on modified bitumen membrane

Figure 4-2. Although modified bitumen membranes offer good resistance to moderate sized hail, this membrane was ruptured by large hail. Cap sheet granules were driven through the membrane and into the face of the insulation. Hail damage is typically excluded from manufacturers' warranties.

Most warranties contain several unfavorable provisions, the most significant being:

  • the exclusion of consequential damages (including damages to building interiors and contents, and business interruption)
  • the limitation of wind coverage to wind speeds that are typically well below the design wind speed prescribed in the building code
  • the exclusion of hail damage (see Figure 4-2)
  • the limitation of leak repairs to patching the membrane rather than removing and replacing wet insulation
  • only the manufacturer can determine the applicability of the warranty
  • the inclusion of several provisions that could result in the building owner's inadvertent nullification of the warranty.

Recommendations

Don't make a roof system or manufacturer selection on the basis of a warranty. Select a system for its suitability to the project, as discussed in System Selection Criteria above.

Do not automatically specify a warranty. Rather, discuss the benefits and limitations of a warranty with the Government representative and have them decide what is in the Government's best interest.

If specification of a warranty is being considered, review the warranty section of NRCA's Low-Slope Roofing Materials Guide (or the Steep-Slope Roofing Materials Guide). If a warranty is desired, recommend that a Government attorney identify provisions of the manufacturer's standard warranty that are unacceptable, so that warranty provisions acceptable to the Government can be specified. The following are examples of changes to standard warranties that should be considered:

  • delete warranty language that takes away Government rights
  • delete warranty language that excludes other rights and remedies
  • delete warranty language that sets a maximum dollar limit
  • delete or modify the unfavorable items listed above in "Warranty Limitations."

Key Elements of Drawings and Specifications

After a suitable roof system has been selected, appropriate specifications and drawings need to be prepared to help ensure that the architect's design concept is understood and executed by a professional roofing contractor. The fate of many prematurely failed roofs is often set by poorly prepared documents (Figure 4-3). The importance of the architect's diligence in preparing specifications and drawings cannot be overemphasized.

The wind load on this very wide gutter was transferred to the edge nailer, but the nailer attachment was not designed to account for the gutter uplift load

Figure 4-3. The wind load on this very wide gutter was transferred to the edge nailer, but the nailer attachment was not designed to account for the gutter uplift load. The nailer therefore lifted and caused a progressive peeling failure of the roof membrane.

The following elements are critical in communicating project requirements to the contractor:

Specifications
Roof Plan(s) and Details
Peer Review.

Specifications

To produce good specifications, the architect should first acquire a suitable guide specification. The following Unified Facilities Guide Specifications (UFGS) are available:

Roof decks: 03 51 00 (LWIC), 03 31 00.00 10 (cast-in-place concrete), 03 41 36.00 10 (pre-cast concrete), 03 52 00 (LWIC), 05 30 00 (steel), 06 10 00 (wood)

Insulation: 07 21 16 (blanket [attic]), 07 21 23 (loose fill [attic]), 07 22 00 (rigid)

Built-Up Roof: 07 51 13

Modified bitumen: 07 52 00

Protected Membrane Roof (using a variety of membrane types): 07 55 00

Single-ply: 07 53 23 (EPDM), 07 54 19 (PVC)

Sprayed Polyurethane Foam: 07 57 13

Metal Panels: 07 41 13 (architectural), 07 61 13.00 10 (structural standing seam), 07 61 14.00 20 (steel structural standing seam), 07 61 15.00 20 (aluminum structural standing seam), 07 61 01 (copper)

Flashing and sheet metal: 07 60 00 and 07 62 10 (copper)

Asphalt Shingles: 07 31 13

Slate: 07 31 26

Tile: 07 32 13 (clay and concrete)

After obtaining a guide specification, it is critical that the architect tailor it to the specific project. Information that is not applicable should be deleted. Applicable information should be adjusted if needed. Additional specification criteria should be added as necessary to suit the project. The following should be considered when developing Parts I, II and III of the roof specifications:

Part I—General

  • Fire and wind: Specify fire and wind resistance requirements. For wind uplift, specify the test method required to demonstrate required resistance (see Wind Safety Section 4.4 Roof Systems). (Note: For shingles, slate and tiles, normally the resistance for the corner areas is specified for use throughout the entire roof area because these products are typically used on small roof areas. For large slate and tile roofs, if there is sufficient cost savings, both corner, perimeter and field resistances can be specified.)
  • Submittals: Specify submittal of catalog data for all products, including installation instructions and, where applicable, maintenance and repair instructions. Specify submittal of samples only when necessary to evaluate the product. For example, it is normally not necessary to specify the submittal of an EPDM sample, because black sheets are typically specified and there is no factory-applied surfacing to evaluate. However, it may be appropriate to specify submittal of samples of a granule-surface SBS modified bitumen sheet in order to select the desired color and to qualitatively evaluate the granule coverage and embedment.

    Specify that the following be submitted: demonstration of specified contractor qualifications, manufacturer review letter, manufacturer inspection reports, certification reports demonstrating that materials comply with referenced standards, certificates of analysis (when specified), and documentation demonstrating enrollment in the MBMA certification program (when specified). These items are discussed below.
  • Contractor qualifications: Specify that the contractor has a minimum number of years of experience with the type of system specified (five years is usually a reasonable requirement). Also specify that the contractor is approved, authorized or licensed by the roof covering material manufacturer to install its product.
  • Manufacturer review: Require the roof covering materials manufacturer to review the specification and drawings and advise in writing of their acceptance thereof, or to submit in writing their concerns and recommendations to resolve these concerns. Specify submittal of the review letter within a short time after contract award, so that if changes are needed there is sufficient time to develop them before the roofing begins.
  • Manufacturer inspection: Require the roof covering materials manufacturer to inspect the roof application on the first or second day of application, and to perform an inspection upon completion of the application. Require submittal of the inspection reports.
  • Standards compliance certificates: To help ensure that products furnished to the project comply with referenced standards, specify that certificates demonstrating compliance be submitted.
  • Certificates of analysis: For those products where certificates of analysis are available, consider specifying certificates of analysis that provide quality control test results for specific manufacturing lots (lot numbers are printed on the product package label).
  • Certification program: If a metal roofing system is selected, consider specifying that the manufacturer be certified through the MBMA Metal Roofing Systems Quality Certification Program . Further information about the program is provided in the MBMA Metal Roofing Systems Design Manual.
  • Pre-roofing conference: Specify a two-stage pre-roofing conference. The first conference should be held a few or several weeks prior to the start of roofing, depending upon job size and complexity. The second conference should be held just prior to application. The purpose of the conference is discussed later in this section.

    Specify that the conference be attended by the general contractor, roofing contractor's project manager, superintendent (on large projects) and foreman, a representative of the roof covering materials manufacturer, and the mechanical, electrical and lightning protection system (LPS) contractors if mechanical, electrical or LPS work is associated with the roofing work. If a third-party inspection firm will be on the job, the inspector should also attend.
  • Example of procedures taken for cold weather application of BUR

    Figure 4-4. This BUR was applied during very cold weather. To obtain suitable ambient temperature during application, warm air was blown into an air-supported enclosure. The heater was placed outside of the enclosure so that the moist combustion vapor was not exhausted into the work area.

    Quality control documents: If a BUR, modified bitumen, EPDM or sprayed polyurethane foam system is specified, specify application compliance with the applicable quality-control document co-produced by NRCA:
    • Quality Control Guidelines for the Application of Built-Up Roofing
    • Quality Control Guidelines for the Application of Polymer Modified Bitumen Roofing
    • Quality Control Guidelines for the Application of Spray Polyurethane Foam-based Roofing
    • Quality Control Guidelines for the Application of Thermoset Single-Ply Roof Membranes
  • Weather limitations: If there are weather-related limitations, they should be specified. If work will be performed during cold weather, special cold weather procedures should be specified (Figure 4-4).
  • Work hour limitations: If there are limitations to the hours or days that can be worked, this should be specified.
  • Storage areas: If there are limitations to on-site storage areas, this should be specified or shown on the drawings.
  • Load limits: Specify the maximum allowable wheel load limits for roof application equipment, and specify the maximum allowable load (per square foot [per square meter]) for materials stored on the roof. Some rooftop equipment and some materials, such as pallets of pavers, are very heavy. Specifying load limits will allow the contractor to determine the appropriate type of equipment and material handling and storage techniques needed for the project.

Part II—Products

  • Materials: Avoid specifying unproven products. Specify that the manufacturer shall have produced the specified materials for a minimum of five years.

    If products are specified by referencing a product standard (such as ASTM), it may be appropriate in some instances to just list the standard. But many ASTM standards have grades, types or both within the standard. In these cases, the type and grade need to be specified. Also, additional information sometimes needs to be specified, such as product thickness. Products covered by some ASTM standards have a substantial range in physical properties. In some cases, it is appropriate to specify a physical property value(s) that is different from that specified in the standard so that a higher quality is obtained. However, this approach should not be used as a method to exclude essentially equal products just because one product has a slightly different value.

Part III—Execution

  • Protection of the completed work: If other construction work will occur over the new roof, or if other construction trades will be on the new roof, specify appropriate protection measures.

Roof Plan(s) and Details

See Details for discussion of the roof plan(s) and details.

Peer Review

On some projects, it is prudent for the architect to have their specifications and drawings peer reviewed by someone knowledgeable of the specified system. Peer review should be considered for office buildings with very valuable contents or operations, projects where the cost of the roofing work is very substantial, complex or unusual projects, and those projects where the architect believes his or her expertise is lacking.

Construction Contract Administration

During construction contract administration, the architect or the entity responsible for construction contract administration has several important tasks. These tasks need to be executed regardless of project size or location, although the amount of time devoted to the tasks will be dependent upon the roof size and other factors.

The following topics are discussed in this subsection:

Submittal Review
Pre-roofing Conference
Field Observations
Non-Destructive Evaluation (NDE)
Project Close-out

Submittal Review

It is incumbent upon the reviewer to be thorough, diligent and cautious during the submittal review process. (Note: In complex or unusual projects, or where the reviewer lacks sufficient expertise to adequately review the submittals, a professional roof consultant who is knowledgeable about the particular roof system should be retained to review the submittals.) The reviewer should:

  • Verify that all of the specified submittals are received and approved.
  • If a submittal item is to be resubmitted, make sure that it is. Sometimes items to be resubmitted are forgotten about. These items often become problems.
  • Be cautious in approving materials, systems and details that are not in accordance with the contract documents. Minor changes may affect code compliance or roof system performance.

Pre-roofing Conference

It is important for the architect to work with the contractor to arrange for the conferences at the appropriate times, and to remind the contractor that all parties listed in the specifications are required to attend. The purpose of the meeting is to review the drawings and specifications to ensure that there is understanding and agreement by all parties. If there are problems with the design or other aspects of the project, the intent is to identify and resolve them prior to commencement of the roofing work. The architect should prepare minutes of the conference and provide them to the contractor for distribution to the other parties.

The first conference should be held a few or several weeks prior to the start of roofing, depending upon job size and complexity. The second conference should be held just prior to application. The advantage of two conferences is that the first can be held sufficiently in advance of application so that, if problems are discovered during the conference, there will be time to resolve them without delaying construction. Having the second conference just before application also provides an opportunity to verify that unresolved issues from the first conference have been dealt with, and it allows for a second review of critical issues just before commencement of work.

The agenda for both conferences is generally the same. Some items may be briefly covered in the first conference and then discussed more thoroughly in the second, or vise versa. Items that were discussed in detail in the first conference can often be quickly covered in the second meeting with brief mention and referral to the meeting minutes. The following should be undertaken at the conference:

  • Review the salient features of the specifications, including schedule; product delivery, storage and handling; roof loading; weather conditions; unique or critical items specified in Part 3—Execution of the specifications; and protection of the completed roofing from work of other trades.
  • Review the drawings, particularly the unique or critical aspects.
  • Review submittal problems (for example, items that have not been submitted, or items that have been submitted but rejected).
  • Ensure that the contractor has a copy of the contract documents and approved submittals at the job site, and any changes thereto.
  • Discuss the contractor's responsibilities regarding notification prior to roofing (for the purpose of alerting the field observer).
  • Establish a line of communication between the field observer and contractor (and other parties that may be involved). If the field observer is not an employee of the architect, also establish a line of communication between the observer and the architect, and the extent of the authority that the observer has with respect to interpretation of the contract documents and the handling of unforeseen conditions.
  • Immediately after the second conference, all attendees should review all of the roof deck areas to verify that they are ready for roofing. They should also review the parapets, curbs and penetrations. If a roof area is not ready, a later review of that area should be conducted prior to roofing. If corrective work is required, the corrective work should be reviewed prior to roofing.

Field Observations

For most projects, periodic observation by the architect is sufficient. But for others, full-time observation by the architect or a roof consultant is prudent. The purpose of the observation is to help ensure that the work is being executed in accordance with the contract documents.

The amount of observation will depend upon:

  • Desired system reliability: If a highly reliable roof is desired, it should receive more field observation.
  • Characteristics of the roofing system: Some systems are more demanding, or less forgiving, than other systems with respect to workmanship and weather conditions at time of application. Even if the work is being performed by a knowledgeable and conscientious contractor, demanding systems present challenges. They should therefore receive greater observation in order to help avoid inadvertent mistakes. An example where increased observation is helpful is when a modified bitumen membrane is applied in cold adhesive when the ambient temperature is near the lower boundary recommended by the manufacturer. Increased observation could prevent the application from occurring if the temperature drops below the minimum recommended temperature.
  • Cost: As the cost of the roofing work increases, the amount of observation should also increase.
  • Complexity: Complex or unique roofs (such as unusually shaped roofs) should receive greater observation.
  • Qualification of the roofing contractor: If the contractor is marginally qualified, greater observation should occur (full time observation should be considered).

It is imperative that the observer understand thoroughly the system being installed. The observer should:

  • Be provided with portions of the contract documents related to the roof.
  • Be provided with a copy of the approved submittals.
  • Be provided with copies of all changes related to the roof.
  • Attend the pre-roofing conferences.
  • Verify that the materials on-site are those identified in the approved submittals, and that the materials have FMG or UL labels when so specified.
  • If a BUR, modified bitumen, EPDM or sprayed polyurethane foam system is specified, follow the quality control guidelines co-produced by NRCA.
  • If fastener pull-out tests were specified, verify that the results are acceptable. If the values are lower than anticipated, the architect should provide the contractor with a revised fastening pattern that is commensurate with the test results.
  • If it appears that wheel loads or stored material loads exceed the specified load limits, the contractor should be advised immediately.
  • Bring to the immediate attention of the contractor's job-site person (who was identified during the pre-roofing conference) any need for a change in the contractor's work practices or a need for corrective work.
Photo of a plumbing vent not centered on one of the standing seams as detailed.

Figure 4-5. This plumbing vent was not centered on one of the standing seams as detailed. To avoid mistakes such as this, this type of detail should be discussed a the pre-roofing conference, as it involves coordination between the plumbing and roofing contractors. Corrective action should be taken with this type of error.

  • As with submittal approval, be cautious in approving materials, systems and details that are not in accordance with the contract documents while performing field observations (Figure 4-5). For example, the roofing crew may desire to use different fasteners to attach the membrane because the approved fasteners were not sent to the job site. Before accepting the fasteners, determine if wind uplift test ratings will be affected and if the membrane manufacturer will approve the alternative fasteners. Another example is a penetration detail that the foreman desires to flash differently than detailed. Is the change proposed in order to provide a better detail, or is it being proposed because it is simpler and cheaper to install? If the proposed detail is not as conservative as the original detail, it should probably not be approved.
  • Write daily reports and give them expeditiously to the contractor (and to the architect if the observer is not an employee of the architectural firm that designed the roof). Report copies should be included in the file prepared for the Government, as discussed in section 4.4.5.

Non-Destructive Evaluation (NDE)

After completion of all building construction, but before occupancy, consideration should be given to NDE of the roof to check for excessive moisture within the roof system. The purpose of the NDE is to find areas of wet insulation caused by moisture entrapment during application or leakage caused by roof system defects. Infrared thermography is the recommended technique for those roof systems that lend themselves to this type of NDE. If rain has not fallen within a few weeks before the NDE is conducted, the NDE should be delayed until rain occurs. (Unless rain falls before the NDE is executed, the a recent membrane puncture could go unnoticed because water has not entered the roof system.)

At the pre-roofing conference the contractor should be notified if NDE is to be conducted. (Notification may result in greater diligence in application and care of the completed roof).

Project Close-out

At the end of the project, the Government should be provided a file with the following items:

  • contract drawings and specifications related to the roof
  • approved submittals
  • minutes from the pre-roofing conferences
  • field observation reports
  • pertinent construction correspondence related to the roof
  • warranty (if specified)
  • NDE of the roof.

Details

After the roof system is selected and the specifics of the system (such as deck type, insulation type(s) and thickness, fastener patterns, and warranty requirements) are developed, it is necessary for the architect to determine what details are needed and to design the details so that they are suitable for the project conditions.

Roof Plan: A roof plan should be drawn to scale and be sufficiently large to adequately convey information. It should show all penetrations and all expansion, seismic and area divider joints. The slope directions and approximate amount of slope should also shown. The different wind uplift areas (field, perimeter and corners) should also be shown and dimensioned. References to all penetrations, roof edges and roof-to-wall details should also be indicated on the plan. (Note: For some standard details such as plumbing vents and roof drains, rather than include the detail on the drawings, referencing the manufacturer's detail in the specification is typically sufficient unless project conditions require enhancement to the standard detail). An example roof plan is shown in the Typical Roof Plan Layout detail below.

General: Details should be drawn to scale and should be sufficiently large to adequately convey the information. Illustrating details in section typically suffices. However, with complicated details, an isometric drawing may be needed.

At parapet and roof-to-wall details, the low-point of the roof should be drawn and a dashed line placed at the high-point if the roof elevation varies. See Parapet Wall Schematic detail below.

After details have been drawn, it is recommended that they be reviewed by a manufacturer prior to bidding, as discussed in the Applications section.

Reference Details: There are a variety of sources to draw upon for design of details. Various details for low- and steep-slope systems are included in The NRCA Roofing and Waterproofing Manual. The NRCA details are available on a CD-ROM. These details can be modified with CADD or incorporated directly into contract drawings. The NRCA details are widely accepted in the roofing industry and are generally recognized as being suitable for standard conditions.

Two noteworthy NRCA details are BUR-10 and Table 4 (from the Fifth Edition, 2001). BUR-10 shows a column-supported equipment stand. The detail provides recommendations for column height as a function of the width of the equipment. By following this guidance, stand-mounted equipment will be mounted high enough to allow roof mechanics sufficient room to work underneath the stand to properly install the new roof and future reroofing. Table 4 provides minimum clearances between adjacent penetrations and between penetrations and roof edges. It is recommended that the guidance in Table 4 be followed (the figure in Table 4 should be included in the contract drawings or referenced on the drawings or in the specifications).

The Sheet Metal and Air Conditioning Contractors' National Association, Inc. (SMACNA) Architectural Sheet Metal Manual also has details that are widely accepted and generally recognized as suitable for standard conditions.

AIA's Architectural Graphic Standards also includes a variety of low- and steep-slope details. However, unlike the NRCA and SMACNA details, the details in Architectural Graphic Standards have not undergone extensive industry review. While several of the details are suitable for standard conditions, some may be inadequate. Hence, this reference source may be valuable for some details, but the architect should scrutinize them.

Manufacturers of roofing products also promulgate standard details. These may also be suitable for standard conditions. Many of these details are available in CADD. However, manufacturer's details typically include propriety names for various products used in the assembly. Hence, modification of the details to delete propriety names will typically be necessary.

Modifying Reference Details: Whenever reference details are considered for inclusion in the contract drawings (or via reference in the technical specifications), the architect should determine whether or not the standard detail needs to be modified to account for unusual weather or building conditions. Standard details typically provide suitable performance when properly executed, provided the weather at the site and the building itself is "standard." If unusual weather conditions are expected during the life of the roof (such as very high wind loads, frequent wind-driven rain, accumulation of slush under snow), standard details may need to be modified to accommodate the non-standard conditions in which they will be required to perform in. For example, in areas where deep accumulation of slush under snow is anticipated, the height of base flashings should be increased above the typical 8" (200 mm).

Knowing when standard details are appropriate and when they are not (unless they are modified) requires judgment. To make a proper assessment of the adequacy of standard details, the architect needs to be keenly aware of weather and other special conditions that the roof will likely be exposed to during its expected service life. The architect also needs to possess adequate roof design knowledge. As discussed in Section 1, if the architect's knowledge is limited, consultation with a qualified roof consultant or professional roofing contractor is recommended.

Guidance for modifying standard details and recommendations for custom details pertaining to wind resistance is provided in Wind Safety. Examples of custom details are given below.

DWG iconDWF iconPDF icon

The following details can be downloaded in DWG format or viewed online in DWF™ (Design Web Format™) or Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title. Download Autodesk® DWF Viewer. Download Adobe Reader.

Roofing System Cove Sealant Detail  DWG | DWF | PDF

Roofing System Curb Between Roof System and Deck Membrane  DWG | DWF | PDF

Roofing System Drain Detail  DWG | DWF | PDF

Roofing System Edge Detail  DWG | DWF | PDF

Roofing System Equipment Curb  DWG | DWF | PDF

Typical Roof Plan Layout  DWG | DWF | PDF

Roofing System Gooseneck Vent Detail on Concrete Deck  DWG | DWF | PDF

Roofing System Gooseneck Vent Detail with Metal Deck  DWG | DWF | PDF

Typical Ladder Layout  DWG | DWF | PDF

Parapet Wall Schematic  DWG | DWF | PDF

Roofing System Penthouse Edge Detail  DWG | DWF | PDF

Roofing System Pipe Penetration with Pitch Pan  DWG | DWF | PDF

Roofing System Collared Pipe Penetration Detail  DWG | DWF | PDF

Roofing System Plumbing Vent Smooth Bitumen Roof  DWG | DWF | PDF

Roofing System Plumbing Vent Granular Bitumen System  DWG | DWF | PDF

Roofing System Plumbing Vent Bitumen System / Metal Deck  DWG | DWF | PDF

Emerging Issues

The introduction and rapid acceptance of single-ply membranes into the U.S. roofing market in the 1970s was likely the most significant roofing industry change in twentieth century. Another notable development in the 1970s was the widespread acceptance of plastic foam roof insulations, although this pales in comparison with the development of single-ply membranes. It is doubtful that another issue will be as revolutionary as the introduction of the single-plies. Since the single-ply revolution, changes in the roofing industry have been primarily driven by environmental and worker health issues and the pursuit of methods to reduce the amount of labor needed to install roof systems.

Environmental and Worker Health Regulations

The most notable impact of worker health regulations on the roofing industry pertained to asbestos. Prior to 1990, asbestos fibers were used in a variety of products, including asbestos-reinforced base flashings for built-up roofs, asbestos-fibrated roof coatings and asphalt roof cements and cement-asbestos shingles. The asbestos-containing roofing materials generally offered very good performance, but due to health concerns of workers exposed to asbestos fibers during product manufacturing, product installation and roof system demolition, asbestos-containing fibers have for the most-part been phased out. In many instances, the reinforcing fibers and products that were initially introduced to replace asbestos offered very poor performance.

In the late 1990s, health concerns related to development of mold in buildings were raised. Though the water necessary to initiate mold growth can come from a variety of sources such as leaking pipes and windows, leakage from roofs is a common source of water. Although the mold issue is in infancy (as of the early 2000s), thus far it has taught building owners, designers, contractors and roofing materials manufacturers, the importance of quickly responding to leakage reports. With quick response, the source of the leakage can be identified and corrected and steps taken to dry the building before significant mold bloom occurs.

The most notable impact of environmental regulations on the roofing industry pertained to phase out of chlorofluorocarbon (CFC) blowing agents in the 1990s. CFC was used to manufacturer extruded expanded polystyrene, polyisocyanurate and sprayed foam insulation. CFC was phased out because of its role in global depletion of atmospheric ozone. As an interim measure, hydrochlorofluorocarbon (HCFC) blowing agents were used in the 1990s and early into the 2000s. HCFC had a much lower ozone depletion potential than CFC. It was not until introduction of the third generation blowing agent, hydrofluorocarbon (HFC) that a blowing agent with a zero ozone depletion rating was available. The development of the second and third generation blowing agents was technically challenging. Though the phase-ins of the new agents were generally successful, product performance problems were experienced.

Environmental concerns have also affected products containing volatile organic compounds (VOCs). With some products, the VOC (commonly referred to as "solvent") content has been reduced. In other instances, there has been a move to water-based rather than solvent-based products. It is uncertain how successful the reduced VOC products and the newer water-based products will be.

Environmental concerns resulted in the following roof design trends beginning in the mid- to late-1990s:

  • Highly reflective roofs: In response to the urban heat island phenomenon and concerns pertaining to smog, acid rain and climate change, there has been growing interest and regulations regarding highly reflective roof surfaces. Although originally this issue was aimed at large urban areas in the southern (hotter) areas of the U.S., highly reflective roofs are also being more frequently specified in those rural and northern areas where hot sunny summers occur.

    In cooling-dominated climates, there has also been interest and regulations regarding high emittance roof surfaces.
  • Green (garden) roofs: Green roofs offer several potential environmental benefits, including, reduction of the urban heat island effect (via evapotranspiration and reflectivity), oxygen generation and reduced storm water runoff.
  • Solar collectors: Following the 1973 Energy Crisis, many rooftops were retrofitted with solar collectors. These early collectors used solar energy to heat water. Many of these early collectors were inappropriately installed on the roof and resulted in significant roof membrane damage and water leakage. For a variety of reasons, the solar collector trend did not last very long. However, around the early 2000s, a new generation of rooftop solar collector was introduced. These new collectors use photovoltaic cells to produce electricity from the sun's energy. These collectors are prefabricated into mats that are integrated with the roof membrane. Photovoltaic cells can also be laminated onto metal roof panels.
  • Sustainable roof design: Highly reflective roofs, green roofs and use of solar collectors can all be considered as elements of sustainable design. However, sustainable design considers and incorporates many other issues.

Labor Reduction

Over the past several decades there have been a variety of application equipment, system designs and product developments aimed at reducing the amount of labor to install roof systems. Trends since the 1990s include the following:

  • Wider sheets: Wider single-ply sheets for mechanically attached application are now available. Originally, sheets were approximately 5' wide. 10' wide sheets were eventually available, and then 12' wide sheets. With the wider sheets, fewer rows of membrane fasteners are required and there are fewer time-consuming field seams to fabricate.
  • Use of non-bituminous adhesives in lieu of mechanical fasteners to attach insulation.
  • Availability of self-adhering single-ply membranes: Self-adhering modified bitumen sheets were available in the 1980s, but several performance problems limited their widespread acceptance. Around the early 2000s, a variety of self-adhering single-ply membranes emerged, along with renewed interest in self-adhering modified bitumen membranes. In addition to potentially being faster to install, the self-adhering sheets eliminate the need for adhesives and torches (and the environmental, health and fire concerns associated with some of these other attachment method).
  • Mechanized rooftop application equipment: Although a variety of mechanized application equipment (such as aggregate spreaders, roof cutters and tear-off machines) was in use prior to the 1990s, the weight of the equipment has increased. On larger jobs, it is not uncommon to see ATVs (four-wheelers) being used to transport materials on the roof. Larger, and thus heavier ballast spreaders are also available. While these heavier pieces of equipment should not be detrimental to buildings with strong roof decks and deck support structures, the heavier equipment can damage older buildings with weak (or deteriorated) decks and/or deck support structures.

The Future

It is likely that as the industry moves through the early part of the twenty-first century, there will be relatively minor, but important changes to products due to environmental, health or labor-saving issues. The introduction of significantly different types of roofing materials is unlikely. The trend towards more sustainable roof design and construction will likely continue.

The use of advanced roof design technologies, such as expert systems, may develop, but this is not likely to occur in the near future. Although, there may be somewhat greater use of computer programs to evaluate system options or performance issues, such as moisture gain within roof systems. The design of very robust roof systems may also become more common place on a limited number of buildings. For example, design of roof systems with much greater service lives on buildings that have very long service lives (such as a courthouse). Another example is design of a roof system with much greater resistance to the devastating effects of a strong hurricane (such designs have been implemented, but are not common - for further information, see Wind SafetySection 4.7.5 Roof Systems.

Work that should be useful to designers, contractors and manufacturers is an imitative that was launched by the National Roofing Contractors Association. Referred to as "Performance Criteria for Constructed Roof Systems" (PCCRS), this program seeks to develop meaningful performance criteria for a variety of roof system types. The initial work is aimed at built-up, metal panel and sprayed polyurethane foam systems. Performance criteria for these systems should be available before 2010.

The past has shown that introduction of new materials and system designs has not be easy. After commercializing a new material or system design, it has typically taken several years for unexpected problems to be identified and successfully solved. Minor changes to materials and system designs have also often resulted in problems, but these have generally been less problematic and more quickly resolved. This age-old trend is likely to be repeated in the future. It is therefore incumbent upon designers and contractors to be cautious when specifying and installing new products and system designs.

Relevant Codes and Standards

Prior to 1990, the three model building codes (BOCA National Building Code, Standard Building Code and the Uniform Building Code) contained few provisions pertaining to roof systems. Additional provisions were added to these codes during the 1990s. The International Building Code (IBC) has many provisions related to roof systems, including reroofing projects. Code requirements pertaining to roof systems originally primarily addressed life-safety issues, such as fire resistance. However, the IBC also includes provisions pertaining to general serviceability, such as minimum roof slope.

Building code requirements can be quite different from those of the membrane manufacturer or FM Global. It is therefore important for the roof system designer to carefully consider the code requirements. The designer should determine if a building code has been adopted for the locale where the roof will be installed and, if so, what edition of the code is to be used. If the building occurs in an area that has not adopted a building code, it is prudent for the designer to voluntarily comply with the roofing-related provisions of a current edition of a model code such as the IBC.

The building code may have specific requirements regarding documentation to be provided to the building department. For example, the IBC has construction document requirements in sections 106.1.1 and 1603.1.4 (2003 edition).

Most building departments possess little expertise related to roof systems. Designers should therefore not rely upon the building department to discover non-code compliance during their plan review. Also, most building department's inspectors do not inspect the roof system. Those departments that do inspect the roof likely possess insufficient knowledge of all of the systems that they could encounter.

Standards

There are a large number of standards pertaining to roof systems. The majority of them were developed by ASTM. The ASTM standards typically pertain to test methods (laboratory and field) and product standards. However, there are a few design and application guides:

There are a few ANSI standards that pertain to roof systems, including:

  • ANSI/SPRI RP-4 Wind Design Standard for Ballasted Single-ply Roofing Systems
  • ANSI/SPRI ES-1 Wind Design Standard for Edge Systems Used with Low Slope Roofing Systems
  • ANSI/SPRI FX-1 Standard Field Test Procedure for Determining The Withdrawal Resistance of Roofing Fasteners
  • ANSI/SPRI RD-1 Performance Standard for Retrofit Drains

Underwriters Laboratory (UL) and FM Global have also developed a number of standards pertaining to test methods. In addition, FM Global has several Property Loss Prevention Data Sheets (the majority pertain to wind performance). Although the Data Sheets are not "standards" because they did not go through a consensus development process, they have essentially become de facto standards.

Additional Resources

WBDG

Design Objectives

Functional / Operational—Ensure Appropriate Product/Systems Integration

Products and Systems

Section 07 41 13: Metal Roofing, See appropriate sections under applicable guide specifications: Unified Facility Guide Specifications (UFGS), VA Guide Specifications, DRAFT Federal Guide for Green Construction Specifications, MasterSpec®

Key books and manuals, periodicals and websites:

Books and Manuals

  • The NRCA Roofing and Waterproofing Manual: This very comprehensive document has information on roof decks, insulations, vapor retarders and a variety of low- and steep-slope roof coverings. The Manual is updated approximately every seven years. A copy of the current Manual should be in the office of every designer who designs roofs. Published by the National Roofing Contractors Association.
  • Low-Slope Roofing Materials Guide and Steep-Slope Roofing Materials Guide: These guides, which are updated every other year, provide information on low- and steep slope roof coverings, including warranties. The Low-Slope Roofing Materials Guide also includes information on insulation and fasteners. The Guides lists manufacturers of the various types of products and provides information about the products. It provides a quick and convenient way to compare products within a given product category. Published by the National Roofing Contractors Association.
  • MBMA Roofing Systems Design Manual: This manual addresses metal roof systems. Published by the Metal Building Manufacturers Association.
  • The Manual of Low-Slope Roof Systems by C.W. Griffin: This book provides information on low-slope systems, including discussion of many fundamental design issues. Published by McGraw-Hill.
  • The Science and Technology of Traditional and Modern Roofing Systems by H.O. Laaly: This very comprehensive book provides information on low- and steep-slope systems. Published by Laaly Scientific Publishing.
  • Modified Bitumen Design Guide for Building Owners by ARMA: This document addresses modified bitumen systems. Published by the Asphalt Roofing Manufacturers Association.
  • Flexible Membrane Roofing: A Professional's Guide to Specifications by SPRI. Published by SPRI.

Periodicals

Websites

Software

  • RoofNav—RoofNav is a free Web-based tool developed by FM Approvals™ that provides fast access to the most up-to-date FM Approved roofing products and assemblies.