When FRTW products are used in areas where the material is exposed to weather or high humidity, it must be treated with special non-leaching chemicals similar to those used for fire-retardant treated cedar shakes and shingles.
An accelerated weathering test (ASTM D2898) exposes FRTW to regular wetting and drying cycles to represent actual long-term outdoor conditions. FRTW must still achieve a flame-spread rating of 25 after undergoing this accelerated weathering in order to qualify for exterior use.
These requirements apply to exterior grade fire-retardant treated plywood siding over wood studs in exterior walls of noncombustible buildings and FRTW decorative cladding on exterior marquee fascias of noncombustible buildings.
Listed fire-retardant coatings applied to wood also reduce the flame-spread rating to less than 75 or 25. These coated products can be used for interior finish in noncombustible buildings except where the flame-spread rating limits apply not only to exposed surfaces but also to surfaces that may be exposed by cutting through the product in any direction. FRTW products are excluded from these requirements while products protected by fire-retardant coatings are not. This recognizes the permanency of the fire-retardant treatments.
The image shown here is a photo of the attic of a church. Trusses in the attic spaces were fire-stopped and coated with a fire-retardant paint. This avoided the cost of sprinklering the concealed spaces.