Q. Aftermarket Fire Protection for Wood Roofing

Are there commercially available coatings that can be sprayed or brushed onto a cedar shake or shingle roof to give it a Class B fire rating?

A. Robert White, a wood scientist at the USDA Forest Products Laboratory in Madison, Wis., responds: While I can’t conclusively say that no such coatings exist, I’m not aware of any that can show the required documentation. Where a wood roof is to be used in a fire-prone area, the recommended solution is to choose shakes or shingles that have been pressure-impregnated with a flame-retardant treatment (FRT) rated by the ICC-ES. Depending on their performance in the ASTM E 108 (or UL 790) test — which includes a flame-spread test and a burning-brand test — FRT shakes or shingles may receive a B or C fire rating. Although a B rating is the highest available for pressure-impregnated FRT shingles alone, a Class A–rated wood roof system can be achieved by using Class B wood shingles with a specified roof deck and underlayment, such as approved gypsum/fiberglass panels.

Don’t be confused by ASTM E 84, a second test protocol that measures the flame-spread index of materials for nonroof applications, and that also provides an A, B, or C rating. While untreated wood products may be rated Class B or C under E 84, wood roofing materials need flame-retardant treatment to qualify for even a C rating under the E 108 protocol.

Durability is also a major concern, so the roofing test protocol includes weathering requirements to ensure that the flame-retardent treatment won’t lose its effectiveness when exposed to rain and sunlight.