A metal roof is among the safest of all roofs because of its durability and non-combustibility. In fact, when wildfires rage through housing developments, often the metal-roofed homes are the only ones left standing. If you're considering installing a metal roof in a new construction or retrofitting an existing building with a metal roof, understanding the fire ratings can help you make an informed decision about the roof.
Function
The Underwriters' Laboratory (UL) has developed its fire ratings system to provide information about the fire safety of a variety of products---everything from electronics to construction materials. With regards to most construction materials, the UL provides ratings based on the hourly performance of the material when it's exposed to fire, including the transmission of heat and sufficient for igniting a fire and the load-bearing performance of the material. For roofing materials, the UL provides a class-based ratings system.
Types
There are three different fire ratings assigned to roofs: Class A, Class B and Class C. A roof with a Class A rating is non-combustible, and so will withstand severe fire exposure without igniting. The Class B rating is given to roof structures that can withstand a moderate amount of fire exposure. The Class C rating is the lowest rating applied to roofing materials, and can only withstand a small amount of fire exposure, but enough to allow those inside to escape. Because metal isn't combustible, metal roofing is fire-rated as Class A.
Significance
Though it's impossible to create a fireproof building, the fire ratings system allows builders to select materials that reduce the spread of fire, giving more time for occupants to vacate the building, as well as providing more time for firefighters to arrive and put out the blaze. Because metal roofs are fire-rated as Class A, they represent the most fire protection offered in a roofing material.
Misconceptions
Though metal roofs themselves receive a fire-rating of Class A, the entire roof structure may receive a different rating. When a building's roof is fire-rated, all the materials are taken into account--including whatever is underneath the metal roof. For example, if a metal roof is installed on top of wooden shingles, the shingles will lower the roof's fire rating because they are made of a combustible material. Therefore, though the metal roofing is rated as Class A, the entire roof structure may receive a Class C rating.
Benefits
Knowing the fire ratings for a metal roof can help you make the best building choices for the location of the structure. With a Class A roof, the structure will have much more protection from fire, and so is suitable for even the most fire-prone of areas, such as dry climates with frequent wildfires. Making the entire roof structure fire-rated Class A can be expensive, so in areas not prone to fire, you can choose a lower fire rating for the entire structure while using Class A metal roofing. There is a benefit of spending the extra money, though: having a Class A fire-rated roof can significantly reduce the cost of insuring the building.
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