Metal Offers Durable, Energy-efficient Roofing Solutions

The Architectural Roof

A second major grouping of panel profiles are referred to as architectural panels. Like shingled or slate roofs, architectural roofs carry rainwater off the roof. They are said to be hydrokinetic. Such roofs generally have a pitch of at least 3:12, although some architectural panels have been designed for pitches as low as 2:12. Architectural profiles are suitable for projects in which the roof requires weather resistance and performs as a visible, decorative element in the overall aesthetics of the building. Dramatic visual effects can be created with architectural roofs that are not possible with structural roofs.

Architectural roofs are generally designed to be installed over a solid substrate, the most common of which include plywood, metal decking or OSB, an engineered wood product, laminated to rigid insulation. Z-purlins, which are stronger than standard purlins, can also be used in combination with rigid insulation. The appropriate span for Z-purlin installations is dependent upon the type of panel and material, and the thickness of the material. Appropriate purlin spacing information is available from manufacturers.

Roof panels are fastened to the deck. Virtually all architectural panel profiles feature the use of concealed fasteners and clips.

Design Variants. The most common variants of architectural panels are shown in Figure 2. The “snap-seam profile” is one of the strongest and most popular. It affords superior strength due to its male/female panel interlock, which has a one-piece design that requires no tools to install, enabling labor cost savings during construction. The integral panel profile is suitable for mansard and canopy applications. The snap-on profile is used where applications involve a slope transition. The pan and cap can be easily altered in the field to accommodate the change in slope. These panels should be installed over a waterproof solid substrate.

The Basics of Underlayments

A key consideration in any roof design is keeping the building’s contents dry. Since the roof is the primary barrier to water penetration, specifying the proper underlayment is critical in protecting the building from precipitation.

A properly installed underlayment Since the roof is the primary barrier can control water entering a roofing

system. In structural roofs, if metal

panels are installed over intermittent supports, an underlayment typically ^{protecting the building from} is not used. If there is a solid precipitation. substrate, the National Roofing Contractors Association recommends using an underlayment. To minimize leaks, an underlayment of 30# felt or an ice/water guard membrane underlayment can be installed on top of insulation and along roof edges, valleys, ridges and hips, which are particularly prone to leaking. While such a waterproof membrane won’t assure a totally leak-free roof, it will help prevent water from leaking directly into the building envelope.

As an alternative and an upgrade to traditional roofing felt, a peel-and-stick membrane can be applied as an underlayment. A fast-growing roofing trend, peel-and-stick products reduce the amount of time, labor and materials needed to install roof membranes and accessories because the adhesive is factory applied.

In architectural roofs, panels are usually installed over a moisture barrier such as roofing felt that acts as a secondary water barrier. The moisture barrier serves to direct off the roof any water that gets past the panels, flashing or trim.

Peel-and-stick membranes are the best solutions in architectural roofs, and are often recommended for use at all valley, ridge, hip and eave areas. It is also considered good practice to install barrel vault-type applications over a peel-and-stick membrane. One reason that this is prudent is the fact that a barrel vault application has zero slope at the top of the radius. All underlayment should be laid horizontally from eave to ridge. Peel & stick membrane is recommended for use at all valley, ridge, hip and eave areas.

Potential Installation Problem Areas

Architects should be aware of common installation issues and problems that may result from a faulty design or installation process for metal roofing. Note that in the design stage of the structure, the goal is to achieve positive drainage flow of the water off the roof into properly designed rain-carrying equipment, be this gutters, external or internal drains, or similar means or methods to get water off the roof.

Oil canning: Oil canning refers to deformation, buckling or waviness of the metal. While some oil canning is to be expected in a metal roof, excessive waviness in the panel can be unsightly and is often a source of dispute with the building owner. Oil canning results from either: an uneven substrate; bad source material; rolled-in oil-canning caused by poorly-adjusted rolling equipment; overdriven fasteners; expansion/contraction; or any combination of the above. Some of these conditions—particularly uneven substrates, overdriven fasteners and problems caused by expansion/contraction—can and should be remedied by the installer at the jobsite.

Underlayment: Underlayment should be installed horizontally and staggered (shingle fashion) from eave to ridge.

Fasteners: Typically, a minimum of two fasteners per clip are required per manufacturer’s instructions to provide better hold-down strength and prevent the clip from torquing around a single fastener.

Flashing Design: Care must be taken to insure that flashing is designed to accommodate expansion/contraction.

Eave Trim: It is essential that eave trim be stripped in properly.

Jobsite Damage: Care should be taken to make sure that panels are stored properly on the jobsite in a clean, dry place and not scratched due to roof traffic.

Valley Flashing: Valley flashing should be stripped in using a method similar to that for eave flashing. The proper size of valley flashing is determined by roof pitch, panel length, and expected snow loading and rainfall.

Clip Spacing: Proper clip spacing per the manufacturer’s details is necessary to insure proper performance of the complete system.

Metal Substrates

Metal roofs have been used for centuries, and have proven to be durable, with both structural and architectural systems remaining operational for a minimum of 30 years, and often for the life of the structure itself. Typically, metal roofs have long useful lives and favorable sustainability ratings as compared with other roofing materials. Metal roofing substrates include metallic-coated steel, aluminum, copper, zinc, and stainless steel. Each material has distinct characteristics and, in particular, the corrosion resistance of a metal roof can vary depending on which substrate is specified.

Steel. Strength, corrosion resistance, longevity and economic value make steel a popular roofing material. Metal roofs are predominantly steel. To increase longevity and anti-corrosiveness, steel undergoes special surface treatments.

Galvanized steel, which has been used for over 75 years in architectural applications, is rendered corrosion resistant by a zinc coating applied to its surface that bonds metallurgically to the steel's surface. Without this zinc coating, steel rusts with oxidization from moisture and oxygen in the air. Zinc provides "sacrificial protection," which means that zinc “repairs” itself if the steel surface is harmed by scratches, nail holes or other damage; zinc molecules move to protect exposed steel. Galvanized steel is available in several grades, specified by coating weight—for example, G-30, G-60, G-90—with the "G" rating referring to the amount of zinc per square foot, per side. G-90 is the weight generally accepted for commercial architectural application. Galvanized steel is sold bare (mill finish) or coated with a paint finish. It accepts many different finishes, and is notable for its low relative expansion and contraction rates. Most common is 24- gauge steel, though panels are available in heavier gauges. ASTM Grade 653 (formerly known as ASTM 446) steel is required for UL-rated assemblies.

More than 30,000 square feet of 24-gauge steel G-90 galvanized panels were used in the renovation of the Amicalola Falls Lodge in the Amicalola State Park near Dawsonville, Georgia. The lodge’s cedar shake roof had begun to leak within a year of its installation in the late 1990s. Given the short life span of the original roof, the Georgia Department of Natural Resources (GA DNR) sought a roof system that would provide longevity as well as energy efficiency. The galvanized steel panels feature a 1-inch leg height and a continuous interlock for improved structural performance and wind resistance—a key consideration in view of the severity of the mountaintop storms. Decking was repaired before installation of new standing-seam panels, flashings, vented ridge, gutters, downspouts and vented soffit on the entire structure. “Cool color” gray was an important factor for GA DNR in the selection process, particularly since the agency could obtain this “cool color” with no additional cost over other “non-cool” colors.

Steel coated with an alloy of aluminum and zinc, which has gained market share in architectural application, combines the corrosion resistance of aluminum with the sacrificial properties of zinc. In bare (unpainted) form, it can prove superior to galvanized steel in many applications in terms of longevity and maintenance. This coating is typically available in AZ-50 or AZ-55 coating thickness. It has less of the sacrificial properties inherent in galvanized steel, which may be a factor if the material is abraded. But the zinc component of the alloy offers good corrosion resistance at cut or slit edges. A twenty-year non-perforation warranty is available from most distributors. Aluminum- and zinc-coated steel accepts a wide variety of finishes. Care should be taken to make sure that bare product does not come into direct contact with lead, copper, graphite, or green, wet, or treated wood as it may be subject to galvanic corrosion.

Courtesy of Petersen Aluminium Corporation
Amicalola Lodge features a galvanized steel roof system that provides longevity as well as energy efficiency.

Stainless steel is an increasingly popular corrosion-resistant substrate for architectural application. In bare, unfinished form, stainless is used in many flashing applications including through-wall flashing. A typical alloy for architectural application is Type 304/304L. Type 316/316L alloys are generally reserved for more aggressive environments such as coastal areas and chemical plants. Manufacturers now offer stainless steel with a polyvinlidene fluoride (PVDF) finish—a combination that provides one of the most corrosion-resistant, weather-resistant materials available.

Copper. A popular material that has been in architectural use for centuries, copper is noted for its superior corrosion resistance. Its finish ages to a rich green patina. Though the patina occurs over many years, chemicals are available that can hasten the process. A provision for oxide washdown should be factored into trim design in order to prevent copper oxide from staining adjacent surfaces; a drip edge is usually adequate for these purposes.

Copper prices tend to be volatile and costs per foot are higher than those of steel or aluminum. As it is a soft metal, it is almost always installed over a solid substrate, roofing felt and red rosin paper. The most common gauge specified is 16 oz. copper; 20 oz. copper is a heavier-gauge alternative.

Aluminum. Aluminum is known for its superior corrosion resistance, flatness and surface characteristics resistance, lighter weight panels, ease of installation and life cycle advantages. While higher in cost per square foot than steel and subject to volatile pricing, aluminum’s light weight and workability can offset the difference in labor cost savings on many jobs. Because aluminum expands and contracts at twice the rate of steel, it may require modifications in flashing design on longer panels. Generally, aluminum is not available for structural panel applications, but developments in alloying technology may lead to wider panel availability in the future.

Nonetheless, aluminum is gaining acceptance among architects. Hill Road Middle School/Benjamin Davis Elementary School, Landover, Maryland was topped with 40,000

4 Metal Offers Durable, Energy-efficient Roofing Solutions

square feet of .032 aluminum seam panels. Says Eugene Delmar, FAIA, President, Delmar Architects, Olney, Maryland, whose firm designed the project, “We used to use galvanized steel for metal roofs but now we specify only aluminum. There are just fewer problems with aluminum.” Delmar utilized jumbo 4-inch by 12-inch brick for the building, complemented with colonial red roof panels and color coordinated window frames throughout. Produced in factory-formed lengths up to 55 feet, aluminum panels were correctively leveled in the field to provide superior flatness. Longer lengths are available from many distributors.

roofing systems across the United States in

order to compare three different types of low-slope roofing systems—metal, asphalt, and metal roofs offer very single-ply systems—on service life, life-cycle favorable life cycle costs. cost, and maintenance. The Ducker study found that metal roofs are expected to last 17 years longer than asphalt and 20 years longer than single-ply. The study further found the life-cycle cost of a metal roof to be significantly less than an asphalt or single-ply roof, with life-cycle costs of metal roofs reported at 30 cents per square foot per year; asphalt at 37 cents; and single-ply roofs at 57 cents per square foot per year.

Finishes

Roofing-system manufacturers offer many organic-base finishes in an increasing range of standard and custom colors to harmonize the roof with the building exterior and the surrounding environment.

Anodizing. Typically not used for roofing panels, anodizing has applications in coping, storefront, building siding cladding and curtainwalls. Essentially the controlled oxidation of aluminum, anodizing is a process that occurs naturally when aluminum is exposed to the atmosphere. The resultant aluminum oxide coating is one of the hardest architectural surfaces. In the anodizing process, the material is first cleaned, then chemically treated and etched. A light, medium or hard etch can be specified. At this point, color can be introduced to the process using one of two methods.

In the hardcoat process, an electrical current is applied to the aluminum in the anodizing bath. The material’s ultimate color depends on a number of variables including alloy consistency, composition of the chemical bath, level of electrical current, and the amount of time spend in the tank. As a result, color can be inconsistent, and a sample range should be pre-approved by the architect.

The two-step anodizing process is a newer, more cost-effective process that produces better color consistency and a wider color range. The material is actually anodized twice. The first step is undertaken to produce aluminum oxide and the second to allow for the deposit of metal salts in the porous surface. Tin, cobalt or copper salts are used in the second step to produce the color. The material is then immersed in a hot water bath to seal the coating. One drawback of this process is that for exterior use, the color palette is still limited to bronze and gold tones or clear.

PVDF. Polyvinlidene fluoride (PVDF) resin, a thermoplastic fluoropolymer that relies on the superior strength of the carbon-fluoride bond, is one of the strongest chemical bonds used in building applications. It is used in fluoropolymer resin coatings and paints that have become a popular family of finishes for architectural application and are reserved for metal building applications in abrasive, corrosive environments. PVDF is the premium paint finish for durability and fade resistance. These finishes offer excellent weathering properties, ultraviolet resistance, formability, abrasion resistance and resistance to airborne pollutants. They also have a self-cleaning finish. Because they are blendable, PVDF finishes have a broad range of colors. Newer PVDF formulations have broadened the available bright color palette. Up to four-coat formulations are available, with new product development focusing on two-coat metallics and two-coat bright colors, water-based primers and printed coats such as wood grain finishes.

At the new Creekside Medical Office Building in Douglasville, Georgia, designed by Hill Foley Rossi & Associates, Duluth, Georgia, the roof utilizes approximately 12,000 square feet of 24-gauge steel panels covered with an aged copper-colored PVDF finish. The building is highlighted by curved entranceways utilizing 1,000 square feet of 24-gauge panels, using a similar PVDF paint.

Polyester. Low-cost coatings available in a many hues including bright primary colors, polyester coatings feature good ultraviolet and color fade resistance. Polyester coatings do not perform as well as PVDF coatings in many respects, including fading and chalking. Typical applications for polyester coatings include metal buildings, agricultural panels, signs, gas station canopies and interior applications.

AIA/ARCHITECTURAL RECORD

CONTINUING EDUCATION Series

Warranties

Most architectural metal specifications call for the issuance of a finish warranty covering resistance to color fade, weathering and finish performance. PVDF finishes are typically warranted for 20 years when applied in coil form, and for five years when applied in spray-coat form, although some 10-year warranties are available. The most important performance characteristic is probably the finish resistance to color fade. Color fade is expressed in "Hunter" units, referring to the Hunter Colorimeter that is used to measure color variation. In terms of color fade, a lower number reading means higher performance. A rating of 5 or better is expected for PVDF over 20 years, while a rating of 8 or better is typical for polyester over 20 years. A rating of 5 or better is barely discernable to the naked eye. Another measure of finish performance is chalking, which is the result of a breakdown in the carbon bonds in the finish. This is evidenced by the white power that comes off the paint surface. Chalking is rated on a scale of 1 to 10. In this case, a higher rating connotes higher performance. Twenty year performance for PVDF is typically at least 8, with 20-year performance for polyester typically 5 or higher. Another parameter is coating thickness, which is critical to the overall finish performance. Standard specifications for PVDF call for

a .2 mil primer coat and a .8 mil ± .1 mil. PVDF coat. Total dry film thickness should be 1.0 mil ± .1 mil.

Not all warranties offer the same coverage, and should be reviewed carefully to see whether, for example, coverage is prorated. Finish warranties should also reference paint adhesion, abrasion resistance and chemical resistance.

one that reflects and emits

CONTINUING EDUCATION

Specifying the right combination of reflectance and emittance based on climate can increase the building’s energy efficiency. In cold climates with high heating bills, a metal roof with high solar reflectance and low thermal emittance will achieve maximum cooling and heating energy efficiency. The most appropriate option: an unpainted cool metal roof product. But for warmer climates with steep air-conditioning costs, a cool metal roof with high solar reflectance and high thermal emittance is better.

In conclusion, a properly designed and constructed metal roof will last for decades without tearing, puncturing, burning, or shrinking. It will withstand ultraviolet degradation, chemical contaminants, and exposure to the elements. By their nature, metal roofs are fire-and wind-resistant, have durability and low life-cycle costs, and many are reflective, easily vented, and help reduce heat gain into a building. Roofing systems can also be designed to reduce a building’s heating and cooling costs. ■

6 Metal Offers Durable, Energy-efficient Roofing Solutions

This article also appears online at: http://construction.com/CE/articles/0705petersen-1.asp

Program title: “Metal Offers Durable, Energy-efficient Roofing Solutions,” (05/07). AIA/CES Credit: This article will earn you one AIA/CES LU hour of health, safety, and welfare credit. (Valid for credit through May 2009). Directions: Refer to the Learning Objectives for this program. Select one answer for each question in the exam and fill in the box by the appropriate letter. A minimum score of 80% is required to earn credit. To take this test online, go to construction.com/CE/

LEARNING OBJECTIVES

6. Galvanized steel is rendered corrosion resistant by:

After reading this article, you should be able to:

❑ a. reflective pigments.

• Describe the two major metal roofing types

❑ b. a strong underlayment.

• Explain the importance of underlayments ❑ c. zinc coating.
• Compare the advantages of metal substrates ❑ d. anodizing.

•

Discuss recent trends in high-performance, energy-efficient finishes

7. Life-cycle costs of a metal roof: QUESTIONS ❑ a. are higher than single-ply roofs.

1. Architectural roofs are differentiated from structural roofs as measured by ❑ b. are higher than asphalt roofs. which of the following?: ❑ c. are 57 cents a square foot.

• a. aesthetics. ❑ d. are lower than both asphalt and single-ply roofs.

❑

b. finish.

• c. slope. 8. The premium paint finish for durability and fade resistance is:
• d. age. ❑ a. polyester.

❑ b. two-step anodizing.

2. Architectural roofs are used on: ❑ c. PVDF.

• a. steep slopes. ❑ d. hardcoat anodizing.

❑

b. low slopes.

• c. slopes of 1/4: 12. 9. The most important finish characteristic is:
• d. hydrostatic roofs. ❑ a. chalking resistance.

❑ b. chemical resistance.

3. Structural metal panel systems: ❑ c. paint adhesion.

• a. are designed for a 3:12 slope. ❑ d. fade resistance.

❑

b. fit over structural metal framing and purlins.

• c. are hydrokinetic. 10. A cool metal roof:
• d. never require underlayment. ❑ a. reduces a building’s cooling needs.

❑ b. improves aesthetics.

4. Peel and stick membranes: ❑ c. absorbs infrared waves.

• a. are not as effective as roofing felt. ❑ d. refers to the roofing surface area.

❑

b. are an upgrade to roofing felt.

❑

c. are not recommended at valleys and eaves.

❑

d. are more labor-intensive than roofing felt.

5. Oil canning:

• a. is a finish problem.
• b. refers to waviness in the panel.
• c. can not be remedied on the job site.
• d. results from improper clip spacing.

Last		First
Name		Name
Firm Name
Address
City	State		Zip
Tel.		Fax
E-mail
AIA ID Number:		Completion date (M/D/Y):

Check one: ❏ $10 Payment enclosed. (Make check payable to Architectural Record and mail to: Continuing Education Certificate, PO Box 5753, Harlan, IA 51593-1253.) For customer service, call 877/876-8093.

Charge: ❏ Visa ❏ Mastercard ❏ American Express

Card#

Signature Exp. Date

Check below:

• To register for AIA/CES credits: Answer the test questions and send the completed form with questions answered to address at left, or fax to 609/426-5592.
• For certificate of completion: As required by certain states, answer test questions, fill out form, and mail to address at left, or fax to 888/385-1428. Your test will be scored. Those who pass with a score of 80% or higher will receive a certificate of completion.

Material resources used: Article: This article addresses issues concerning health and safety.

I hereby certify that the above information is true and accurate to the best of my knowledge and that I have complied with the AIA Continuing Education Guidelines for the reported period.

Signature Date

057SPOND

Petersen Aluminum Corporation produces a complete line of metal roofing products. Their Cool Colors Palette meets cool roof certification requirements. PAC-CLAD^®, a full Kynar 500^®/Hylar 5000^® paint system, with a 20 year non-prorated warranty, is available in 34 colors on aluminum and 36 on steel. Visit us at http://www.pac-clad.com or call 1-800-PAC-CLAD.

www.pac-clad.com