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Failure is not an option: Metal panels stand up to the elements

It seems that no matter where you live, the threat of extreme weather conditions or natural disaster is there. Recently we've seen record snowfalls on the East Coast, fires in California, a devastating earthquake in Haiti and hurricanes ravaging the Gulf Coast. These are extreme examples of the natural elements and occurrences that batter buildings throughout any given year. When designing a building, not only does it have to serve its function and meet whatever aesthetic criteria you've set, but it has to perform under these various conditions. You have to know that it can hold up against wind, rain, snow, fire and seismic shifts in the ground.

 

So the question is, does metal pass the test?
David Stermer, PE, is the director of engineering for Metal Sales Manufacturing Corp., Louisville,Ky. He says that metal is unique in that it can be an attractive and cost effective way to resist weather.At the same time, it comes in a variety of colors that are energy efficient.

"Metal is very strong, it's durable, it'll last along time," Stermer said. "It's relatively lightweight and that helps from a seismic standpoint."

 

Load Bearing
The primary weather phenomenon to deal with is wind, according to Stermer. He said the engineering department at Metal Sales does extensive testing to determine the capacity of a roof or wall application to withstand load.

"You can't stop wind from applying load, it's going to apply load," Stermer said. "But you want to ensure that you've got more capacity than is being applied. We also calculate the anticipated snow loads and determine a capacity for the panel."

Engineers typically use the ASCE 7, "Minimum Design Loads for Buildings and Other Structures," from the American Society of Civil Engineers to determine the necessary load capacity for a building,along with testing protocols for roof panels, including ASTM E1592, "Standard Test Method for Structural Performance of Sheet Metal Roof and Siding Systems by Uniform Static Air Pressure Difference," and UL 580, "Tests for Uplift Resistance of Roof Assemblies."

 

Stermer said metal roof and wall panels stand up quite well to winds of up to 90 mph, which is what they are designed for throughout the vast majority of the United States. However,when you move towards the eastern or Gulf Coast, where winds can kick up over 140 mph, you need to design for higher loads. This sometimes requires reducing the span, along with further testing and analysis, of a panel to be certain it will be effective.

Perhaps the most challenging wind event is a tornado. Stermer said while there are some panels designed to withstand up to 200 mph winds, building codes typically don't design for tornadoes because of their low probability of occurrence, and that when a Category 5 strikes, there is sometimes nothing that can be done.

"You hear of tornadoes passing through and picking up asphalt from the road," Stermer said. "It's hard to design a building that's going to stand up in those situations. Storm shelters are what you want in those cases."

 

Metal roofs, however, can prevent what we have all seen footage of countless times on the news during a hurricane-pieces of roofs flying off.

"Properly installed, the entire metal roof would behave as an envelope and you wouldn't have individual pieces coming off. Clay tiles can be that way," Stermer explained. "For metal, under a wind event, the typical rule is if you can keep the perimeter fastened and attached, you won't have a failure out in the middle of the roof. Out in the field of the roof, away from the edges, there's much lower pressures-pressures are much higher on the perimeter. The goal is to keep that peeling action from even getting started."

Metal can also handle snow with ease for the most part. The concern is in areas with extremes now conditions-Buffalo, N.Y., for instance- where you can have lake effect snow events or a snowdrift that can occur on any building with a roof step. These conditions typically require significant span reductions, according to Stermer.

 

Keeping Weather Out
"One of the goals is to keep weather out," Stermer continued, "to keep air from getting in the building or water from getting in the building. We test our products for air and water leakage. The tightness of the seam and the positioning of the sealant would be the two key criteria. It really takes both of these to get a nice tight panel system. Typically you get no water leakage and very little air leakage."

Along with wind and snow, metal is resistant to rain, hail and fire, as well as heat gain and sun damage.

"Sun will not damage a metal panel functionally.It can cause chalking and fading, however our products have very good warranty coverage,and have the capacity to reflect solar heat," Stermer said. "There are coatings for metal roofs that reflect more than 70 percent of the heat that arrives at the roof. That's an enormous amount of heat that's reflected right back into the environment. You want a very reflective roof in warm climates. In cooler climates, you want a darker roof that can absorb and hold on to the heat. The amount of heat that's absorbed is very closely related to the color and the pigment, as well. Organic pigments tend to absorb more than inorganic or metallic pigments, which have solar reflectance to keep the roof cooler.

 

Rain really has no effect on a metal roof (there is no mildew growth and termites, ants or others insects will not attack it), according to Stermer, and any damage from hail would be purely aesthetic.

"We never have a failure in performance," he said. "It's not pummeled and beaten like a shingle or other product might be."

While lightening is often thought to be attracted to metal, Stermer said that really has more to do with the height, size and the surroundings of a building than the building material. Metal is beneficial against fire, meanwhile, because it is noncombustible. If there are combustible materials nearby you may need some other kind of fire barrier, but with strictly steel construction no other fire barrier is needed.

Even in coastal environments where carbon steel may not be effective, metals such as stainless steel, aluminum, copper and zinc can provide longevity. In the case of earthquakes or seismic events, metal panels are an ideal building material because they have very little mass and are easy to hold in place. When the earth moves underneath a building, the metal panels impose only a small force on the building's framing system as they move along with the building.

Design Considerations
Jason Shumate, manager of technical services at Metal Sales, has years of experience inspecting construction sites for weather tightness. When architects specify a warranty for a produce the makes sure there is as little chance as possible for a failure. This includes the correct engineering, a valid paint material finish and proper installation.

"That's why we have an inspector go on every site and we train our contractors and use only certified contractors," Shumate said. "A properly installed metal roof is going to be leak free whether Metal Sales warrants it or not. Some systems are water shedding and some are watertight. The roof system is designed never to leak." "Metal roofs can prevent what we have all seen footage of countless times on the news during a hurricane-pieces of roofs flying off."

 

Shumate said architects sometimes run into trouble when they specify a panel that is not going to work with the type of system they're using.

"The overall application and the performance requirements that each type of panel system has should be considered," Shumate said. "There are panel systems only for solid substrates. We have products that are for open framing, those are structural standing seam. You can always take structural standing seam over solid deck, but other systems may not work with open framing].

"We have architects call in all the time and ask for guidance, asking what would be the best suitable product. Usually they have a system in mind and we work with them to find what will meet their needs. We're not always able to use the products that they want because it may be a misapplication. It's a good idea for them to check. The end goal is for the building owner to have a trouble free panel system that meets their expectations.

One other common mistake, Shumate pointed out, is to specify an underlayment as a second layer of defense against leakage to make up for a lesser roof. He said this could cause condensation problems and corrosion within the building.

"It's not worth compromising," Shumate concluded."A metal roof system is not the cheapest out on the market. For someone spending that kind of money, they want the performance out of it."

For more information from Metals Sales Manufacturing Corp., Louisville, Ky., visit www.metalsales.us.com.

www.metalsales.us.com