Fire-Retardant Treated Wood (FRTW) is wood which has been impregnated with fire-retardant chemicals in solution under high pressure in accordance with CAN/CSA-O80 Series, Wood Preservation. The treatment reduces surface burning characteristics, such as flame spread, rate of fuel contribution and smoke contribution.
To dispel any myths that may still exist, the fire-retardant treatment does not make the wood noncombustible. This idea stems from certain earlier versions of building codes which equated a 25 surface flame-spread rating to noncombustibility.
FRTW contains different chemicals than products known as preservative treated wood. However, the same manufacturing process is used to apply the chemicals. The products are not interchangeable. FRTW products are harder to ignite than untreated wood products.
The use of fire-retardant treatment does not prevent ignition or charring. The rate of burn through treated wood is approximately the same as that for untreated wood.