Q: I’m concerned about shake roofs and fire risk. Is there a substance that can be applied to shake roofs that will make them more fire resistant?
A: To the best of our knowledge, and after a Web search to verify our belief, there are no products out there that we believe work in a retrofit application. In other words, if the roof is already installed, there’s nothing you can spray or brush on that will make it fire resistant. But if you’re planning to install a new shake roof, there are pressure-treated shakes that are fire-rated.
The generally accepted application of shake roofing is over skip sheeting — pieces of 1-by-4 wood nailed to the rafter with 3 inches of space between them. This allows exposure to air on both sides of the shakes and equalizes the expansion and contraction that naturally occurs with changes in humidity. Unfortunately, it also creates a fire hazard.
Roofing systems are fire-rated either A, B, C or not ratable, with A the highest rating, or most fire resistant.
Fire ratings are determined by literally trying to set the materials on fire in a laboratory. A mock-up of a roof structure is built and a burning piece of wood called a "brand," is ignited and placed on top of the roof to be tested. The larger the brand, the higher the rating.
For example, in a test conducted by the UC Berkeley Cooperative Extension, the A-rated brand is 12 by 12 inches. The B-rated brand is 6 by 6 inches. If the roof system being tested prevents the fire from penetrating to the underside of the roof and prevents heat transfer sufficient to ignite material in what would be the attic, it passes the test and achieves the appropriate rating.
At best, a new non-treated shake roof might attain a C rating. But there are no guarantees, and that rating won’t last. Shakes that have been exposed to the weather for a few years are not ratable.
There are many companies that will clean, repair and spray fire-resistant chemicals on an old shake roof. We would advise you to steer clear of them. At best, a cleaning with a power washer and oiling will prolong the life of the roof, but any fire resistance will be minimal.
If you are absolutely wed to a shake roof, the only way to mitigate the fire danger is to completely replace the shingles with a fire-resistant system.
Today, there are cedar roof shingles and shakes available that have been pressure-treated with fire-resistant chemicals at the factory and can achieve a class B or C fire resistance rating. This is accomplished by placing the shakes in pressurized treatment cylinders where fire retardants are injected into the inner cells of the wood.
The shakes then go through a polymerization process and the fire retardants are locked into the wood. Used in conjunction with a fire-resistant substrate, this material can attain a class A rating.
So, to sum up, you’re out of luck in trying to improve the fire resistance of your existing shakes, but if you decide to go with shakes in the future there are better alternatives in the wood world. For our money, and many jurisdictions in the country don’t allow wood roofs, a top-quality, architectural-grade asphalt shingle is a handsome alternative.