Fire RatingsMore roofing definitions
The independent, for-profit company Underwriters Laboratories Inc (UL) has developed a series of laboratory tests to measure how well roofing materials (such as shingles) resist fires. These tests are widely accepted by the roofing industry, and the results are widely quoted in industry promotional materials.
UL 790, the most widely quoted standard, tests roofing materials' resistance to fires from external sources -- fires that originate outside of the house (such as from a lightning strike). It should be noted that the fire rating is not earned solely by the physical properties of the roofing material, but also by installing the material in the manner recomended by the manufacturer. The UL assigns roof coverings a rating that ranges from Class A (the highest level of protection) to Class C (the lowest level of protection).
A roofing material that receives a Class A, B or C rating should maintain its position on the roof following a fire, and is not expected to generate flying, burning, shards of shingle. Class A roof coverings afford a "high degree" of fire protection, Class B coverings afford a "moderate degree" of fire protection and Class C coverings afford a "light degree" of fire protection. The UL makes no guarantee that even Class A roof coverings will be usable after a fire.
So, if there's no guarantee that even a Class A shingle will survive a fire, why should you bother with the fire rating when choosing a roofing material? There is a practical reason and a philosophical reason: On the practical side, many building codes require Class A roofing materials. On the philosophical side, although you cannot guarantee the safety of someone inside a burning building, he or she has a much better chance of escaping unharmed if the building has a more fire-resistant roof.