Building code requirements for using fire retardant-treated (FRT) lumber address exterior wall assemblies separately from the floor/ceiling or roof/ceiling assemblies that bear on those walls. The exterior walls of Type III buildings must be non-combustible. While FRT lumber is not classified as non-combustible, it is permitted in the walls of these Type III buildings in place of non-combustible construction. The ceiling assemblies of Type III buildings are not required to be non-combustible and therefore do not require the use of FRT lumber. Where walls and ceiling assemblies meet, joints need particular attention in terms of fire protection. This report steps through the details of these requirements. 

The same principles apply to floor/ceiling and roof/ceiling assemblies. Throughout this report, the term “ceiling assemblies” is used to include both possibilities.