Metal Offers Durable, Energy-efficient Roofing Solutions

Understanding the complexities of metal roof systems, substrates and finishes

May 2007
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Educational Advertising Section Provided by Petersen Aluminium Corporation

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.

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.

 

Owner: Prince Georges County Public Schools
Architect: Delmar Architects, P.A.
General Contractor: Hess-Guilford Joint Venture
Roofing Contractor: Kim-Side Contractors, Inc.
Hill Road Middle School/Benjamin Davis Elementary School in Landover, MD, features an aluminum roof system.
Architect: Hill Foley Rossi & Associates
General Contractor: RAMA Construction
Roofing Contractor: C & M Roofing Services
A PVDF finish was used on the roof at Creekside Medical Center in Douglasville, GA.

Images: Courtesy of Petersen Aluminium Corporation

 

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 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.

Life Cycle Cost Comparisons

A metal roof may have higher initial costs than other roofing materials (at least twice that of an asphalt shingle roof). Installation cost of a metal roof is considered medium to high, on a par with tile or premium shake, although below slate, the most expensive roofing material. But installation costs are only one factor in total cost of a roof. Due to their durability, longevity and minimal maintenance requirements, metal roofs offer very favorable life cycle costs.

In 2003 and 2004, the Ducker Research Company conducted an in-depth analysis of 36 roofing systems across the United States in order to compare three different types of low- slope roofing systems—metal, asphalt, and single-ply systems—on service life, life-cycle 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.

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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.

 

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