This is a photograph of a fire test on a fire-retardant treated shake roof (“stand alone” Class B, Class A “by assembly”). The burning “A” brand [12" x 12"] is ignited prior to placing on the roof. The pressure impregnation process, and “post impregnation” heat treatment is required to give the fire retardant the exterior rating. Products that are sprayed on “in-service” haven't been shown to weather well, and therefore aren't sufficiently durable.