Code Requirements
Building codes regulate the use of combustible materials (including wood) in interior finishes based on occupancy type and construction classification. The International Building Code (IBC) classifies interior finish materials by flame spread index: Class A (0-25), Class B (26-75), and Class C (76-200). Most untreated wood falls in Class C.
For residential projects (R occupancy), untreated wood is generally permitted on walls and ceilings without restriction. For commercial projects (B, M, A occupancies), higher fire ratings may be required depending on the area of coverage and the presence of sprinkler systems.
Fire-Retardant Treatments
When codes require Class A or B flame spread ratings, reclaimed wood can be treated with fire-retardant chemicals. Intumescent coatings are the most common option for reclaimed wood — they are applied like paint and swell when exposed to heat, creating an insulating char layer that slows combustion.
Pressure-impregnation with fire-retardant chemicals is another option but is more commonly used with new wood since the process requires specialized equipment. For reclaimed wood, surface-applied intumescent coatings are more practical and do not significantly alter the wood's appearance.