Even in noncombustible construction types (Types I and II), many elements of the building, such as floor coverings, windows and doors, interior finishes, and roof structures can be wood. Permitted combustible building elements in noncombustible buildings are conveniently listed in Section 603 of the IBC. This list also includes structural elements that are constructed of fire retardant treated wood (FRTW) or heavy timber. Although FRTW is not considered noncombustible by the building code definition, it is often permitted to be used in place of noncombustible materials. For instance, FRTW can be used in place of noncombustible materials in exterior walls of Type III and IV buildings, and in roof structures of low-rise buildings of Types I and II construction. Untreated heavy timber can be used for roof structures of Types I and II buildings where the required rating does not exceed 1 hour.

 See Chart in this PDF Document