The Role Of Gauge And Width In Metal Roof System Performance
By Dale Nelson, Roof Hugger Inc.
I was asked recently by DesignandBuildwithMetal.com to respond to a question they’d received from a visitor to their site looking for information on metal panel widths and gauges. This gentleman—a general contractor from Roatan, off the north coast of Honduras—was trying to evaluate price quotes for roof panels of differing dimensions to determine which was the best value. All things being equal, this would seem a relatively easy thing to do. But when comparing products from different manufacturers, you have to not only be certain that all things are in fact equal—same quality steel, same quality paint, etc—but you have to know too that each product being considered will meet the project’s necessary performance requirements. To compare price before capability is akin to putting the cart before the horse.
On this particular project, the building owner was from Canada and wanted to use a 12”-wide, 24-gauge snap-on panel from a Canadian manufacturer he was already familiar with. The general contractor found two products manufactured in the United States that were less expensive, but the panels were 26 gauge and not available in widths less than 16”. So which panels are the better choice? It all depends.
Metal roof panel systems can be broken down into two basic types. The first is non-structural—also known as architectural—and the second is structural. Structural panels are applied over open framing—such as wood or steel purlins—while non-structural panels are applied over some type of solid decking. In the case of non-structural systems the deck provides the basic strength to the roofing system.
In looking at metal panels, there is a relationship between panel width, gauge, clip spacing and overall system capacity. The narrower the panel, the stiffer the panel. The heavier the gauge, the stiffer the panel. The closer the clip spacing, the higher the system capacity.
Open framing systems typically have structural wood or steel purlins spaced from 24" to 60" on center (o.c.) so the spacing of the clips is controlled by the spacing of the purlins. The greater the purlin spacing the more important the panel width and gauge. Greater purlin spacing (60" o.c.) requires heavier gauge panels and a narrower width to achieve the same capacity as closer-spaced purlins (24" o.c.). Regardless of purlin spacing, 24 gauge is usually the standard for structural panels.
Deck systems can place clips as close together are needed to achieve a desired uplift capacity. Since clips can be placed closer together, a lighter gauge and wider panel can be used to achieve the same capacity as a narrower panel of heavier gauge on an open purlin system. So now the cost of the clips becomes a factor, as does any additional time—though likely negligible—it might take to install them.
From a performance standpoint, the key to getting the correct panel and clip spacing is knowing the loads in your area. To do this you’ll need to determine the design wind speed for the project’s location. Once that information is known the next bits of information needed are as follows: the shape and pitch of your roof, the mean height of the roof and the surrounding geographic conditions. Collectively, this information determines the loads for the building’s roof.
It is important to note that the roof loads will vary from one part of the roof to the next. Typically, they are divided into three basic zones: the corner zone, the edge or perimeter zone, and the field zone. The design loads will be highest in the corners, elevated on the edges and lowest in the field (or center) of the roof areas. Each zone will have a specific pounds per square foot (psf) design and a safety factor applied, and the size of the zone will also be determined.
If, for example, the loads are determined to be 80 psf in the corners, 60 psf along the edges and 25 psf in the field, a panel and clip spacing would be selected that would achieve those loads. If a 16"-wide panel produced from 26 gauge material can achieve 80 psf with clip spacing of 10" o.c., and 60 psf with a clip spacing of 16", and 25 psf with a clip spacing of 30", then that would be the pattern used to install the panels in each area. Every panel will have its own capacity based upon gauge and clip spacing, but panel manufacturers can provide this information based on test data already on file.
I hope the information I provided to the contractor proved helpful, and I hope my explanation is of some assistance to others as well. Metal roofing is a great product but like anything, predictable performance is directly attributable to proper specification and installation.
Dale Nelson is president of Roof Hugger Inc., an Odessa, FL-based manufacturer of Roof Huggers. Roof Huggers are notched sub-purlins used to facilitate the quick and eay installation of new metal roof systems over old. Prior to launching Roof Hugger Inc. in the early 1990s, Dale and his business partner, Red McConnohie, were design-build general contractors with more than 65 years of combined experience. To contact Dale, call (800) 771-1711, or e-mail dale@roofhugger.com. To learn more about Roof Huggers, visit www.roofhugger.com.
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