Dimensions of a Metal Roof

Basics

• Metal roofs are initially more expensive than other roofing materials, according to the metal roofing alliance. But they can last 50 or more years, as opposed to the average 17-year lifespan of non-metal material. They can duplicate different materials such as slate, shingle, shake or tile, or employ a standing seam profile and contain factory-applied colors that do not need repainting. They are impervious to weather, rot and insects and can grant discounts on homeowner insurance due to their fire resistance. They naturally reflect heat, reducing cooling loads in the summer and adding insulation in the winter.

Shingles

• Because they are roughly the same size and shape as their wood counterparts, metal shingles also require a minimum deck slope of 25 percent. This is equal to a three-unit rise for every 12 units of horizontal movement, or a 3:12 ratio. In climates where ice formation along the eaves can back up water, the shingles require at least two layers of underlayment, or a self-adhering polymer-modified bitumen sheet. In areas with winds above 110 miles per hour, the underlayment must be secured with corrosion-resistant fasteners according to the manufacturer’s instructions. Surfaces that join to form a valley need flashing that extends at least 8 inches each way from a center line.

Panels

• For lapped, nonsoldered-seam metal roofs with no lap sealant, the minimum slope is 25 percent, or 3:12. For the same type with applied lap sealant, the minimum drops to 4 percent, or 0.5:12. For standing-seam roofs, the slope is 2 percent or 0.25:12. Steel roofs require galvanized fasteners. Copper roofs can use copper, brass, bronze or copper alloy fasteners. All metal roofs can also use stainless steel fasteners. Panels in areas with high winds require corrosion-resistant fasteners applied according to the manufacturer’s instructions.

Codes

• All local and state building codes use the same source for metal roofing requirements: the International Residential Code. The International Code Council, which was established in 1994, developed these regulations by combining three national standards, eliminating duplication and resolving conflict. They update the codes with their latest finding once a year. The council also maintains other constructions codes that may be necessary when erecting a metal roof. These include the International Building Code, primarily for commercial and public structures; the International Fire Code, which determines fire safety standards, and the International Energy Conservation Code, which discusses requirements for efficient energy use.