07 41 13: Metal Roofing

by Sheet Metal and Air-Conditioning Contractors' National Association (SMACNA)

Last updated: 01-22-2008

Introduction

Keeping the elements at bay is fundamentally the most important aspect of buildings, with the roof shouldering the lion's share of the burden. Unfortunately, this responsibility is almost entirely taken for granted in modern American society. Few prospective home or building owners ask about life expectancy, life-cycle cost, leakage rates, or moisture protection of materials for their new roof. In recent years, asphalt has been the material of choice for most homes due to its low first cost. However, metal roofs are durable and lightweight for both new and reroof applications and are growing in popularity. In fact, standing seam metal roofing can be constructed to meet the highest UL and ASTM performance ratings available for fire protection, resistance to wind uplift, and hail damage. Metal roofing is environmentally friendly—due to its recycleability and recycled content and solar reflectiveness which results in lower cooling requirements and it will not mildew, split, crack, or curl. Metal is virtually maintenance free and can last 50 years or more. There are also a wide range of material choices—copper, galvanized or coated steel, stainless steel, aluminum and other coated metals—and several different panel construction options—standing seam, decorative stampings, 5V, and others.

Description

The following metal descriptions commentary briefly examines the nature of several metals used for roofing applications. Neither the list of metals nor the material precautions are all encompassing; whenever in doubt contact the manufacturer of the products for special applications. Generally, avoid designs with large flat surfaces or waviness can result and always check for compatibility between the metals and chemically treated materials such as preserved wood.

Historically, copper was the metal roof of choice. Copper:

• is resistant to corrosion,
• is easy to work and join in the shop or field,
• is not corroded by masonry, concrete, or stucco when it is flashed or embedded in them,
• does not have to be painted, as copper ages oxidation gives "raw" copper an attractive green patina—which is widely imitated as evidenced by the green colors applied to many other types of roofing materials, and
• can be coated with lead to avoid staining of concrete and stone or where the gray color is desired.

Precautions:

• Use copper or copper-alloy fasteners
• Avoid direct contact with other metals

Tensile strength—36,000 psi

While copper is still widely used, coated and stainless steels are becoming very popular metal roof choices. Coated steels can be provided in virtually any color desired and the coating is generally applied using techniques that make the coating extremely durable.

Stainless steel is a durable, maintenance-free, corrosion-resistant material with a silvery appearance. Stainless steel:

• provides a lustrous finish that can be produced in variety of surface finishes but the 300 series are typically used for roofs,
• can be coated with terne (lead-tin alloy) coated for a flat grey appearance that weathers to a soft gray finish favored by many architectural firms,
• requires little or no maintenance,
• is unaffected by mortar or concrete,
• does not stain adjacent surfaces, and
• offers superior resistance to metal fatigue.

Precautions:

• Clean surface after installation to remove contaminations that lead to surface corrosion.
• Requires special flux—must be neutralized after soldering—and 50/50 solder, cooler soldering irons, and longer contact for soldering joints.

Tensile strength—80,000+ psi

Galvanized steel is often used in commercial and industrial applications and can be painted as the galvanic coating wears off to provide a virtually indefinite life. Galvanized steel:

• is economical,
• may be used in direct contact with concrete and masonry,
• is easy to work and join in the field and shop,
• is light weight and has low expansion, and
• is stain resistant and provides durable long-term performance.

Precautions:

• Do not use in severely corrosive environments.
• Insulate with bituminous coating when in direct contact with copper, redwood, or red cedar.

Tensile strength—50,000 psi

Aluminum is an easily fabricated silvery-appearing metal that is often used for flashing in conjunction with other roofing materials. Aluminum:

• provides a lightweight, corrosion-resistant option that will not stain adjacent surfaces,
• is ductile, malleable, and easily worked,
• weathers to a uniform gray in its natural state and is highly resistant to corroding atmospheres—except caustics or strong alkalis, and
• is available anodized, coated, or with textured surfaces.

Precautions:

• Cannot be soldered, use rivets or sealer or weld joints.
• Avoid direct contact with dissimilar metals, concrete, or mortar.
• Avoid moisture entrapment during storage to avoid staining.

Tensile strength—22,000 psi

Terne metal consists of copper bearing steel coated on both sides with a lead-tin alloy. Terne metal:

• is light weight,
• has low expansion, and
• provides durable, long-term permanence when painted and maintained

Precautions:

• Detail so that all surfaces drain.
• Do not nail through metal; use cleats.
• Contact with aluminum, copper, or acidic materials.
• Must be primed both sides before installation.
• Must be painted soon after installation.

Metal roofs have more of the desirable attributes than any other roofing choice. They are:

Environmentally Friendly

Millions of barrels of crude oil are used in the production asphalt shingles, which inevitably fail. Annually, over 30 billion pounds of asphalt shingles are disposed of in landfills. The steel used to make metal roofing products and accessories is composed of as much as 50% recycled material, and is itself 100% recyclable. In fact, metal is the most highly recycled material; 64% of all scrap metal in North America is recycled annually. In many cases, metal roofing can be installed over existing roofing, saving costly tear off and disposal charges.

Fire Resistant

Metal will not burn and many insurance companies offer reduced rates due to the high fire resistance provided by metal covered roofs.

Energy Efficient

Metal roofing can reflect as much as 70% of the sun's energy which minimizes the heat transferred into the building. Florida Solar Energy Center research has shown that metal roofs reduce home cooling costs more than other roofing alternatives.

Wind Uplift Resistant

Metal roofing is the product of choice in hurricane-prone areas because experience has shown that metal roofs outlast other roofing products in severe wind conditions. Metal roofing panels can be constructed to the highest wind uplift ratings from UL.

Custom Colors

The color of painted-metal roofing can be selected to coordinate with exterior color schemes or repainted to match new color schemes at a later date—repainting even further lengthens the life of the roof.

Weather Resistant

Metal roofing will decrease the likelihood of damage from the effects of bad weather—wind, rain, hail, snow, and ice. Metal roofing gives the best protection against hail damage; hail cannot penetrate a metal roof. Property owners in Texas and other states may qualify for a reduced homeowner's insurance rates based on the durability of metal roofs.

Other Attributes

Often, initial structural savings can be realized when compared to heavy-weight roofing options. For example, asphalt shingles average 2 to 2.5 lbs. per square foot compared to steel roofing panels which weigh .7 to 1.3 pounds per square foot. In most re-roofings, metal can generally be installed over existing roofing materials, saving costly tear-off and disposal.

Relevant Codes and Standards

Guide Specifications

• Department of Defense -
• UFGS 07 41 13 Metal Roof Panels
• Department of Veterans Affairs -
• VA 07410 Preformed Wall and Roof Panels

Standards and Guidelines

• Sheet Metal and Air-Conditioning Contractor's National Association (SMACNA)—Located in headquarters outside Washington, D.C., the Sheet Metal and Air-Conditioning Contractors' National Association (SMACNA), an international association of union contractors, has 1,965 members in 99 chapters throughout the United States, Canada, Australia, and Brazil.
  
  The voluntary technical standards and manuals developed by SMACNA Contractors have found worldwide acceptance by the construction community, as well as foreign government agencies. ANSI, the American National Standards Institute, has accredited SMACNA as a standards-setting organization. SMACNA does not seek to enforce its standards or provide accreditation for compliance. SMACNA standards and manuals address all facets of the sheet metal industry, from duct construction and installation to air pollution control, from energy recovery to roofing.
• The Architectural Sheet Metal Manual contains the most comprehensive set of recommended practices available for proper design and installation of custom-fabricated architectural sheet metal including roof drainage system design for scuppers, gutters and downspouts. The latest edition features new historical restoration section on skylights, cornices and spires; new moisture and maintenance guide; new mechanical fasteners and soldering chapter, and enhanced metals selection data. It offers new construction techniques and alternative methods of design and installation reflecting climatic conditions and rainfall rates. An upfront, fast "look-up" key speeds information retrieval. Also covers flashing, metal roofs, copings, gravel-stop fascia. Includes wind uplift pressure test reports confirming superior performance of custom-fabricated metal roofing. The manual applies to commercial, industrial and residential buildings. CD-ROM version contains a collection of SpinFire, 3-D drawings allowing the designer to magnify, rotate, section and view the most minute but often critical construction detail from any angle.
• Standard Practice in Sheet Metal Work is a 768 page, hardbound publication that contains a collection of design data that was originally published in 1929. With today's emphasis on restoration, refurbishing, rebuilding and retrofitting, architects will find this publication more than of just historical interest. Architectural sections include elaborate cornices, skylights, domes, spires, spun balusters, gargoyles, marquise, storefronts, metal buildings, and metal roofs.
• Underwriters Laboratories Inc. (UL)—Underwriters Laboratories Inc. (UL) is an independent, not-for-profit product safety testing and certification organization. Each year, more than 17 billion UL Marks are applied to products worldwide. There are several UL classifications for roofing materials and systems.

Additional Resources

Trade Associations and Other Organizations

• ASTM International—ASTM International is a not-for-profit organization that provides a global forum for the development and publication of voluntary consensus standards for materials, products, systems, and services. In over 130 varied industry areas, ASTM standards serve as the basis for manufacturing, procurement, and regulatory activities. Formerly known as the American Society for Testing and Materials, ASTM International provides standards that are accepted and used in research and development, product testing, quality systems, and commercial transactions around the globe.
• Sheet Metal Workers' International Association (SMWIA)—SMWIA, the Sheet Metal Workers' International Association, represents 150,000 skilled crafts persons in the unionized sheet metal industry throughout the United States and Canada.

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.