How does fire retardant treatment (FRT) affect fire resistance?

Fire retardant treatments (FRT) on wood products retard the ability of flames to spread across the surface of wood products. However, fire retardants do not reduce the rate at which wood degrades when subjected to an external heat source. Accordingly, fire retardant treatments do not improve fire resistance ratings to any significant extent. The user is reminded that NDS 2.3.4 requires the effects of fire retardant chemical treatment on strength to be addressed in design.

Does fire retardant treatment (FRT) make wood non-combustible?

FRT wood does not meet the requirements for noncombustible materials. However, in recognition of its very low flamespread characteristic, the International Building Code (IBC) allows FRT wood to be used in many applications otherwise required to be of non-combustible construction. See the construction type definitions in Chapter 6 of the code, and also Section 603 which lists many applications for FRT wood in noncombustible construction types.

See also the Fire FAQ: “Where can I find publications on fire retardant treated wood?”