Building Envelope Design Guide - Sloped Glazing

by Nik Vigener, PE and Mark A. Brown
Simpson Gumpertz & Heger Inc.

Last updated: 03-14-2006

Introduction

Skylights have been used for over a century to provide interior daylighting. Early skylight systems consisted of plate glass (later wire glass) in metal frames and frequently incorporated both an exterior skylight and a decorative interior "diffuser" or "laylight". Most contemporary skylights now consist of insulating glazing captured in aluminum frames that in many configuration (e.g. single slope, ridge, pyramid, barrel vault). Skylights are engineered systems that are assembled from standard or custom extrusions provided by skylight manufacturers, and i.g. units made by glazing fabricators, but they share common design elements required to make them perform. This page uses the term "skylight" to describe field-assembled systems of sloped glazing. In the construction industry, the term "skylight" is often applied to relatively small shop-fabricated unit-skylights, frequently with plastic glazing. These unit skylights are not specifically addressed in this page.

Description

The following covers brief descriptions of typical skylight components:

Supporting members: Rafters spanning from sill to ridge, cross bars between the rafters, and pressure bars that that clamp the edges of the glass to the rafters. The pressure bars are frequently covered by rafter caps to conceal the fastener heads and shield them from rainwater.

Infill panels: Generally glass (see Designing for Finish Durability—I.G. Unit Failure Avoidance and Fracture and Glazing Retention (below) for typical configuration and design advice), but also proprietary translucent products, such as fiberglass sheets or fiberglass sandwich panels (not addressed in this page).

Fundamentals (Functions)

Thermal Performance (Conduction, Solar Radiation, Thermal Break, Comfort)

Skylights experience significant summertime solar heat gain and wintertime heat loss that must be calculated and accounted for in mechanical design. This aspect of the design is discussed in the Building Envelope Design Guide page Atria Systems. The thermal performance of a skylight is largely a function of the thermal performance of the glazing; see the discussion in Glazing on thermal performance.

Moisture Protection (Water Penetration, Condensation Resistance)

Functionally, skylights are roofs and are therefore exposed to a larger volume of rainwater and are much more susceptible to water leakage than vertical fenestration systems. Even for new, well-sealed skylight assemblies, some leakage beyond the exterior glazing is inevitable. Similar to windows and curtain walls, skylights are subject to wintertime condensation when the surface temperature of the interior glass surfaces falls below the dew point for the interior conditions. The leakage and condensation must be collected and drained to the exterior with a continuous drainage system consisting of interconnected condensate gutters and sill flashing.

See the discussion on moisture protection in Glazing that applies here as well.

Visual (Daylighting, Aesthetics)

Skylights have historically been used to daylight interior spaces, taking advantage of both the energy savings and positive psychological effect of natural over artificial light. The exterior appearance of skylight surfaces is largely determined by the glazing used; see the discussion in Glazing.

Sound (Acoustics)

Like other glazing systems, sound transmission through skylights is largely a function of the sound transmission through the glazing. See Glazing for a discussion. Similar to window and curtain wall systems, reducing sound transmission through skylights requires an integrated strategy that includes not only decreasing the sound transmission through the glazing (e.g. by adding laminate thickness, laminate layers, glass thickness, or airspace), but also through the perimeter construction (i.e. reducing air leaks).

Safety

Skylights increase the hazard associated with glass breakage substantially. Building codes have long stipulated post-breakage glass retention for skylights in non-residential buildings to help protect occupants. Effective glass retention requires either laminated glass, glass with an anchored film on the inboard surface, or separate interior screens to capture falling glass.

Fire safety (e.g. automatic sprinklers and smoke evacuation requirements) are addressed in Atria Systems.

Health and Indoor Air Quality

Commonly used skylight framing and glazing materials (e.g. aluminum framing, rubber gaskets, cured silicone sealants) typically do not contribute to indoor air quality problems. But water leakage from poorly designed or constructed skylights, or from defective perimeter flashings, can and often do contribute to mold and mildew growth and resultant indoor air quality problems. Design and construction details that exclude water penetration are critical to avoiding indoor air quality problems. See Applications and Design Advice (below) for design detail guidance.

Durability and service life expectancy

Aluminum is inherently corrosion resistant in most environments; see the discussion in Windows, which applies here as well.

Causes of i.g. unit failure in skylights are discussed in Glazing. The additional exposure to the effects of ultraviolet light, which accelerates aging of glazing sealants, and to larger amounts of precipitation, tend to stress sloped glazing considerably more than vertical glazing and translate into reduced life expectancy of skylight glazing compared to windows. Typical industry warranties are five years for sloped insulating glazing, compared to ten years for vertical glazing.

Maintainability and Repairability

Service life and required maintenance for typical aluminum frame coatings are discussed in Windows. Finishes on sloped frame surfaces, and perimeter sealants on sloped glazing, deteriorate at a faster rate than for vertical windows for the reasons noted in the paragraph above.

Skylights require cleaning of both the interior and exterior glazing surfaces. For larger skylights, cleaning requires special access provisions, such as dedicated scaffolding or anchors for suspended scaffolding or industrial rope access equipment. These access provisions must meet Occupational Safety and Health Administration (OSHA) standards.

Sustainability

I.g. units in skylights have a shorter life expectancy than i.g. units installed in vertical windows and curtain walls. Nonetheless, where energy consumption is a concern, there are no reasonable alternatives to insulating glazing for skylight construction.

The natural light provided by skylights can reduce electricity demand for lighting. The USGBC's LEED rating system allows credits for daylighting, and skylights are a common design feature in Green buildings. Reduced lighting costs are often outweighed by increased energy demands for heating and cooling of the interior space as a result of increased heat loss or heat gain through skylights compared to conventional roofs. Case-by-case engineering analysis is required to determine the energy payback, and life cycle cost associated with skylights; see the discussion in Atria Systems.

The "recyclability" of glazing and aluminum framing is discussed in Windows.

Applications and Design Advice

Establish System Track Record

Similarly to curtain walls and discontinuous windows, the first step to a successful design is the selection of a skylight system which has a demonstrated successful track record in similar applications and exposures. ASTM E1825—Standard Guide for Evaluation of Exterior Building Wall Materials, Products, and Systems provides guidance for evaluating a system's track record. In addition to the system's track record, the designer should evaluate laboratory test results for the system (structural wind resistance, water penetration and air leakage resistance, condensation resistance, etc.) to establish that the stock system is capable of meeting the site-specific project performance requirements.

Design for Waterproofing Performance

A successful skylight design acknowledges that it is unlikely to prevent water penetration through the skylight under all conditions, and provides a collection system to guard against leakage and condensation drips. The following are good design practices for skylights:

Provide a continuous system of gutters, integral with the skylight rafters and cross members, to collect leakage and condensation. The cross member gutters must be notched at their ends to assure drainage into the rafter gutters. Water must be drained from gutter to gutter and never onto i.g. units below.
Provide an exterior wet seal. A wet seal consisting of non-curing butyl glazing tape and an exterior silicone cap bead, and installed with proper workmanship, provides better waterproofing performance than a dry gasket.
Select a system with continuous rafters. Skylights frequently leak where individual rafter sections are spliced together to make up a single longer section because the gutters are not continuous. Providing membrane patches or similar repairs to splice rafter gutters together is not reliable. Most skylight manufacturers can fabricate, finish, and ship rafter sections up to 30 ft. long. However, transporting, hoisting, and installing very long rafters is difficult and the logistics of this work (e.g. truck access, crane required for hoisting, etc.) must be worked out during design.
Provide a continuous metal sill flashing to collect leakage and condensation. The flashing should be sloped and drain to the exterior. See Exterior Wall for integration of the sill flashing with the exterior envelope of the building.
Select a system with snap-on rafter caps, rather than exposed pressure bars. Exposed fastener heads in pressure bars tend to cause leakage.
Provide flush-glazed horizontal mullions without exterior applied pressure bars to avoid bucking water run-off.
Provide a minimum skylight slope of 3/12.
Coordinate the waterproofing with the attachment details. Providing a sloped sill flashing typically requires a sloped curb.

Design for Condensation Resistance

Skylights are prone to condensation in cold regions. The skylight design should include establishing the required condensation resistance factor (CRF) based on anticipated interior humidity and local climate data, and selecting a system that meets this CRF. Similar to windows, the U-value for the glazing is not sufficient to define the energy use or condensation potential for the whole system, including the framing. The designer must evaluate the entire system including perimeter conditions, for condensation potential. For high interior humidity buildings, such as swimming pools or museums, computer modeling of the skylight and its thermal and moisture exposure, is required to prepare a design that limits or avoids condensation.

Coordinate with Mechanical, Structural, and Lighting Designer

Coordinate the skylight configuration and proportions with the MEP designer. Excessive heat loss or gain and excessive light levels or glare, are frequent complaint about skylights. The mechanical design must include provisions to accommodate the thermal loads imposed by the skylight. Sometimes shading is necessary to reduce light levels. See Atria Systems for additional discussion.

Access provisions for skylight cleaning may require significant structural capacity. Current OSHA-required design loads for safety tie-offs are 5,000 lbs. Scaffold anchors or safety tie-offs cannot be supported by typical skylight framing and require dedicated structural framing.

Design for Differential Movement

Most skylights are framed with aluminum rafters, which undergo significant movement in response to daily and seasonal temperature changes. This movement must be accommodated in the attachment detail to prevent the introduction of stresses caused by restrained movement. Rafters in typical single-slope skylights have a single pinned gravity load anchor and one or more wind load anchors in sliding connections. The structural attachment details must be determined on a case-by-case by the structural engineer and the skylight manufacturer. In some systems, structural steel framing supports the aluminum skylight framing. Since steel and aluminum have significantly different coefficients of thermal expansion, the aluminum-to-steel connections must be designed to accommodate the expected differential thermal movement.

Designing for Finish Durability

See the discussion in Windows. Note that the service life of finishes on sloped frame surfaces is shorter than for vertical frame surfaces.

Designing for Glazing Durability—I.G. Unit Failure Avoidance

As for other fenestration system, the key to avoiding i.g. unit fogging due to in-service conditions is to keep water away from the glazing perimeter. Skylights that employ discrete metal stops to support the bottom edge of the glazing are less susceptible to ponding water than systems with a continuous sill support that may allow water to accumulate. Design strategies that limit water penetration (see above) are critical to glazing durability. See Glazing and Windows for additional discussion of i.g. unit failure.

Designing for Glazing Durability—Fracture and Glazing Retention

As in windows and curtain walls, good glazing practice that prevents glass-to-metal contact is necessary for fracture avoidance. I.g. units must be installed with appropriately spaced setting blocks and anti-walk pads. See the discussion in Glazing.

Skylight glazing is sometimes broken by flying stones from adjacent ballasted roofs or gravel-surfaced roofs, or by fragments from glass breakage above the skylight. The skylight design should include a review of the potential for such damage, including the possibility of gravel ballast being blown off adjacent roofs. Providing roof parapets or avoiding gravel roof ballast altogether limit the risk of breakage. In locations where debris impacts are likely, skylights can be protected with a protective metal grate installed on the top surface above the glass. Where there is an elevated risk of glass breakage, or the consequences of glass breakage are significant, providing thicker-than-minimum PVB interlayers (e.g. 0.06 or 0.09 in. instead of typical code-required 0.03 in.) in the laminated interior lite is appropriate. All lites in the skylight glazing should be heat-strengthened to limit the risk of fracture. Monolithic fully-tempered glass should not be as the inboard lite of a skylight to avoid fall-out associated with spontaneous fracture; see the discussion in Glazing.

Logistical and Construction Administration Issues

Similar to other exterior wall and roof systems, the most important construction administration tasks for a successful skylight project are mock-up testing and quality assurance during construction. The following are critical:

Require continuity between the shop drawing process and construction. Some curtain wall and skylight manufacturers work diligently with the designer during the shop drawing process, but do not work equally hard to convey the important installation details to the field crews during the installation phase. Requiring a manufacturer's representative familiar with the system and the shop drawings on site during construction helps ensure implementation of the design as shown on the shop drawings. This is especially important at the beginning of the work.
Design the skylight and perimeter construction to allow component replacement. Match the life expectancy of components that are mated together into an assembly. For example, detail skylight transitions to adjacent roofs to allow replacement of the roof membrane without disassembly of the skylight. Use durable flashing materials, such as stainless steel sill flashing, instead of membrane flashing.
Skylights that avoid horizontal pressure bars (see above), rely on structural silicone glazing to transfer wind suction loads from the glazing to the cross bars. Adhesion testing (typically performed by the silicone manufacturer) is required to verify design values for structural silicone joints. Also, for structural glazing, the sealant manufacturer must review the skylight shop drawings to check for appropriate sealant joint design, sealant joint constructability, and field application methods.
Verify skylight performance using mock-ups. The mock-ups should include all representative perimeter construction details (sill, hip, head, rake), and should be tested for air and water penetration resistance.

Details

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.

Sloped Glazing—Eave Flashing (Detail 3.4-1) DWG | DWF | PDF

The detail shows a skylight eave detail on a framed curb.

The sill has a continuous metal sill flashing that collects condensation and leakage. Skylight rafter gutters are notched to allow drainage onto the flashing.
The curb has a sloped sill (shown here constructed with a bent steel plate) to provide a sloped substrate for the sill flashing.
The skylight framing incorporates weep holes in the glazing pocket (to limit the risk of i.g. unit failure), the condensate gutter in the sill closure piece, and the sealant joint between the sill closure and the metal sill flashing. Weep holes are covered with weep hole baffles or open-cell foam to close off entry ways for insects.
The skylight framing attachment to the building structure below incorporates provisions for thermal movement.

Sloped Glazing—Sill Detail at Transition to Steep Roof (Detail 3.4-2) DWG | DWF | PDF

The detail shows a skylight eave detail within a steep roof assembly. It is similar to Detail 3.4-1 except that the metal sill flashing also provides counterflashing for the steep roof assembly. See Roofing Systems for more discussion on steep-slope roofing systems.

Sloped Glazing—Vertical Rafter Detail at Transition to Steep Roof (Detail 3.4-3) DWG | DWF | PDF

The detail shows a skylight rake detail within a steep roof assembly. A metal counterflashing for the step flashing that forms the transition from the roof to the rafter is integrated into the pressure bar detail. See Roofing Systems for more discussion on steep-slope roofing systems.

Sloped Glazing—Ridge Detail at Transition to Steep Roof (Detail 3.4-4) DWG | DWF | PDF

The detail shows a skylight ridge with a steep roof assembly.

The top of the skylight is protected with a continuous cricket. The cricket is sloped to shed water toward the skylight rakes. The metal flashing of the cricket must be selected to limit the risk of galvanic corrosion of the aluminum rafter caps.
The membrane underlayment for the cricket flashing must be heat resistant to prevent premature aging or bleed-out of adhesives. See additional discussion in Roofing Systems on underlayment membranes.

Sloped Glazing—Horizontal Cross Bar (Detail 3.4-5) DWG | DWF | PDF

The detail illustrates the proper integration of the skylight cross bar with the vertical rafters.

Like the rafters, the cross bar incorporates integral gutters to collect and conduct condensation and small amounts of leakage. The cross bar gutters are notched to allow drainage into the larger rafter gutters beyond.
The weight of the i.g. unit is supported on aluminum stops that are keyed or screw-fastened into the cross bar.
A screw-applied shear block within the cross section of the cross bar transfers structural loads from the cross bar to the skylight rafters. The concealed shear block complicates skylight assembly significantly, but provides more reliable attachment than screwed connections.
The flush glazing sealant detail between the top and bottom light is more reliable than horizontal pressure bars, but typically requires structural silicone glazing to withstand wind loads.

Sloped Glazing—Vertical Rafter (Detail 3.4-6) DWG | DWF | PDF

The detail shows a vertical rafter cross section.

Integral rafter gutters collect condensation and small amounts of leakage and conduct them to the sill flashing; see above.
A continuous snap-on cover conceals the pressure bar and fasteners that retain the glazing unit. A separate cover provides more reliable protection against leakage than exposed pressure bars and fasteners, which are prone to leakage at fastener penetrations.

Sloped Glazing—Isometric View of Water Management System (Detail 3.4-7) DWG | DWF | PDF

The detail shows an isometric cut-away view of the main skylight components and illustrates the continuity of the water management system formed by the cross-bar gutters, rafter gutters, and sill flashing.

Condensation that forms on the inboard side of the i.g. units and on the cross bars drains to the integral cross bar gutters. Notched ends in the cross bar gutters allow drainage into the rafter gutters.
The rafter gutters are collected on the sloped sill flashing (see Details 3.4-1 and 3.4-2) and drain to the exterior at weeps in the sealant joint between flashing and skylight frame sill.

Emerging Issues

Easy-to-clean glass with titanium dioxide coatings is sometimes used in sloped glazing to reduce the maintenance effort associated with glass cleaning; see the discussion in Glazing.

Building-integrated photovoltaic (BIPV) systems are integrated into the building envelope to convert sunlight into electrical energy. The solar cells that are part of the system are sometimes built into roofs and skylights rather than exterior walls to take advantage of the additional sunlight captured by sloped surfaces. Because the solar cells are generally not specifically designed as waterproofing elements, they typically require installation outboard of the building envelope. See the BIPV Resource Page in the Whole Building Design Guide for additional information.