Gauge measures weight, not thickness, although the thickness of different gauges are also usually given in inches. The higher the gauge number the thinner the steel. Most steel roofs are built with 24-, 26- or 29-gauge steel. The most appropriate gauge depends on the type of roof support and the conditions of your climate.
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Gauge is measured by the number of ounces per square foot. The American Iron and Steel Institute defines 24-gauge steel as weighing 16 ounces per square foot. Twenty-four gauge steel is .0239 inch thick; 25-gauge steel is .0209 inch thick; 29-gauge steel is .0135 inch thick. Heavier-gauge steel panels are marketed in narrower widths than those of lighter gauges.
Type of Support
There are two kinds of steel roofs. Structural panels are applied over wood or steel horizontal support beams called purlins. Nonstructural panels are installed on roof decking. If your purlins are spaced more than 60 inches o.c., meaning the center of one is 60 inches from the center of next, you should have narrower panels of heavier gauge steel. The standard for structural panels is 24-gauge steel. Use 26- or 29-gauge steel on a deck.
If you live in an area with heavy snow loads or areas prone to hurricanes then your roof should be at least 26-gauge steel. Steel metal panels are fastened into place with clips. Find the wind speed for your area. If you use 29-gauge steel, you have to put the clips closer together to withstand the same uplift of wind. That means you have to buy more clips and it takes more work to put them in place.
Steel Panel Covering
Steel roofing can be covered with galvanized zinc or with a coating of aluminum and zinc, called Galvalume, which has improved characteristics for reflecting sunlight and resisting cracking and peeling. Galvanizing steel or coating it with aluminum and zinc adds to its weight, but not its strength. Twenty-four-gauge steel that weighs 16 ounces per square foot weighs 16.6 ounces per square foot when it is galvanized.