How Metal Roofing and Walls Stand Up
By BOM Editorial Staff May 2007 - Roofing
Metal is experiencing a renaissance of sorts. As knowledge of metal’s suitability for walls and roofs grows, it increasingly is being used on a variety of building projects, including schools, warehouses, health care facilities and office buildings, among others. Facilities executives are recognizing benefits such as durability and longevity, and this has led to wider use, say industry sources.
According to Toy Henson, director of education and market development with The Metal Initiative (TMI), metal’s share of the commercial building market has about doubled over the last 20 years. While no firm statistics exist, Henson estimates that metal currently accounts for 20 to 30 percent of the market. What’s more, about 70 percent of specifications for building materials issued by the federal government include metal, says Henson.
A life-cycle cost analysis conducted by Ducker Research Company in 2004 helps explain the growing popularity for metal as a building material. When analyzing roofing systems, researchers found that the expected life of a metal roof is 40 years. By comparison, the average built-up roof lasts 10 to 15 years and single-ply roofs last about 20, according to the study.
Because metal roofs need to be replaced less frequently than most other roofing systems, the total cost of the roof, when measured over its expected life, becomes much lower. The Ducker study estimated that the expected life-cycle cost per square foot for a metal roof is 30 cents. In contrast, the life-cycle cost per square foot was 57 cents for single-ply roofs.
Metal does have a higher first cost than other building materials, says Scott Kriner, technical director with the Metal Construction Association (MCA). However, Kriner cautions that it’s difficult to make broad generalizations given the wide range of metal substrates and finishes on the market.
Even so, the initial price disparity for metal roofing and wall systems can often be made up in savings in the costs of other construction materials, say industry insiders. That’s because metal is significantly lighter than other roofing materials.
According to Green Seal, a nonprofit organization that promotes the manufacture of environmentally responsible products, metal weighs between 50 and 270 pounds per 100 square feet. That compares with 250 to 400 pounds per 100 square feet for built-up roofs. Because of metal’s lighter weight, support columns and footers needed within the frame of a building can be spaced farther apart. This can reduce the cost of construction materials.
It’s important to note that metal’s lighter weight doesn’t compromise its strength, says Dick Bus, president of ATAS International and president of the Metal Construction Association. “Pound for pound, metal is stronger than other materials.”
In addition, a metal roof can be part of the building’s structure, rather than an addition to it, says Joel Voelkert, vice president of Rigid Building Systems and chair of TMI’s market development committee. Commercial metal roofs may not require a separate deck, although an architect may request one as part of the roof design. In contrast, a built-up roof system is placed atop a deck.
Another benefit of metal building systems: The installation work often can proceed more quickly than with other materials, such as brick, says Jeff Irwin, chief executive officer with MeTecno/Benchmark and incoming president of MCA. That’s because it is installed in sheets that can measure 36 inches wide.
Finally, metal building systems can be installed in most types of weather. This reduces the risk of a delay in construction due to snow, rain or other weather extremes.
Facilities executives may balk at using metal because they are concerned it will rust or corrode. This is a valid concern, because in certain climates corrosion can shorten the expected life of a metal building system. However, when the proper product with the right type of coating is installed correctly, metal is able to resist corrosion, say experts.
Key to metal’s durability is the type of substrate chosen and the type of coating that can be applied. Three types of paint systems are commonly used on exterior metal building systems, says Kriner. These are polyesters, silicone or modified polyester, and fluoropolymers — often considered the highest quality premium paint. While the most expensive, fluoropolymers generally resist color fading and chalking, and maintain gloss and solar reflective properties longer than other options. Because of their color stability, fluoropolymers often are used on wall systems. On the other hand, if a low slope roof won’t be seen from the ground, one of the other less expensive paint systems may do the job, Kriner says.
To achieve maximum corrosion resistance, facilities executives should specify a quality metal coating from a reputable manufacturer, says Kit Emert, president of Fabral and immediate past president of TMI.
Being aware of installation issues can help facilities executives prevent corrosion as well. For example, failing to consider fasteners and attachments can lead to corrosion. Any fasteners and attachments used within a roofing system need to work with the substrate material. If a roofing system consists of steel roof panels, for example, the fasteners can be manufactured from carbon or stainless steel. If aluminum roofing is used, the fasteners should be made of stainless steel. “You don’t want the possibility of corrosion caused by dissimilar materials,” says Emert.
After the roof is drilled to install the fasteners, it should be swept clean, says Kriner. This is more than just a housekeeping detail. “Those little pieces of steel can rust and stain the roof surface,” he says.
Additionally, the flashings used along the perimeter of a roof need to comply with the manufacturer’s recommendations. If they don’t, they may compromise the integrity of the metal roofing system.
Properly installing fasteners and flashings is important not just to avoiding corrosion, but also to limiting the chance for roof leaks. The Ducker study found that while all roofs, including metal, experience leaks, none of the leaks in the metal roofs was a result of a failure in the material. Instead, they developed because of improper installation, deteriorating grommets and other issues.
In addition, the choice of substrate and coating should take into account the use of the building and its environment, Emert notes. For example, a building that’s used for chemical processing may require a different type of metal than a facility used for light manufacturing.
When these steps are taken, metal is extremely durable, and holds up very well even against the elements that can degrade any roofing system, namely heat, moisture and UV-radiation, Kriner says. Most metallic-coated steel is warranted for 35 years, Kriner notes. It’s not unusual to come across metal roofs that have lasted a century. In contrast, most built-up or single-ply roofs tend to need either significant repairs or total replacement well before the building has reached the end of its useful life.
What’s more, maintaining a metal roof or wall that’s been properly installed is relatively simple. Most metal roofs are dirt-shedding, rather than dirt-retaining, says Kriner. Usually, a power-washing will remove dirt and debris.
Because the maintenance work required is nominal, the cost of keeping up a metal roofing or wall system is typically a very small percentage of the initial installed cost — 3.5 percent for metal compared with 19 percent for single-ply systems, according to the Ducker study. Even though the initial cost of metal is more, this still means maintenance over the life of the roof is less than for most other types of roofing.
One of the most persistent myths regarding metal as a building material is that it will mean settling for a boxy, colorless structure. In reality, metal is available in a wide variety of colors and designs.
As a starting point, metal can be ordered in any number of colors. Most manufacturers offer a palette of forty to fifty different colors, with hundreds more available as custom choices, Henson says.
What’s more, “you can bend, twist, curve and form metal in ways you can’t do with other building materials,” says Emert. “You can achieve many designs that you’re looking for, at an economical price.” It’s also possible to mimic the look of other materials, such as stucco or concrete, with the right metal finish.
In addition to looking good, metal building systems are an environmentally responsible choice, say industry experts. For example, recent tests indicate that a metal roof installed with offset mounting creates an air gap and natural air convection under the panel, which can reduce heat gain through the roof deck by 45 percent compared to a direct-to-deck attached roof system, Henson says.
According to Green Seal, a white-painted metal roof will reflect 70 to 80 percent of the sun’s energy. That compares with 40 percent for a built-up roof with white gravel. By reflecting, rather than absorbing, the sun’s rays metal roofs can reduce the amount of energy needed to cool a building. And because paint systems commonly used on metal roof systems tend to shed, rather than retain dirt, they maintain their solar reflective properties for years, Kriner says.
As a result, metal roofs can contribute to points for buildings participating in the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) program, says Kriner. “They’re energy efficient and sustainable.”
Metal’s low weight also enhances its energy efficiency, says Bus. Heavier roofing systems, such as those constructed with concrete tile, hold heat longer, especially in hotter climates. That means the air conditioning system has to work harder to cool the facility.
Insulated metal wall systems also can reduce energy consumption in a building, Henson says. That’s because, for some other building materials, there’s a gap in the building envelope occurring where the material attaches to the framing. With insulated wall panels, there is no break; insulation abuts to insulation and the steel abuts to steel. As a result, less heat or cooling escapes to the outside.
A 2-inch insulated metal panel will have an R-value of about 14.2, says Henson. In comparison, a wall with standard 6-inch bat insulation will have an R-value of about 7. A concrete block wall has an R-value of 2 or 3, says Irwin.
If facilities executives decide to remove metal building materials, the metal is recyclable. “In evaluating which materials have the least impact on the environment, metal stacks up well on every issue,” Henson says. “We think there’s a bright future for it.”
Facilities executives who have used metal as a building material show great loyalty to the material. A study conducted by Ducker Research Company found that 58 percent of owners of single-ply roofs, and 46 percent of owners of asphalt roofs would use the same material on a future project. The number for owners of metal roofs? Ninety-two percent.
About The Metal Initiative
The Metal Initiative is a coalition of manufacturers, individuals and associations that have come together to provide information on the features and benefits of metal in construction. Carrying its message of metal primarily to the building owner community, The Metal Initiative seeks to gather and disseminate useful information for decision-makers.