In certain unsprinklered one-storey buildings, the NBCC permits the use of a roof deck construction system, using FRTW, that meets the flame-spread performance standard originally developed for noncombustible roof assemblies.

The required fire-resistance rating of the roof assembly can be waived if the deck is constructed of FRTW and the assembly passes the requirements of CAN/ULC S126.

A roof deck system of FRTW may be supported by:

• metal and reinforced concrete beams or joists
• heavy timber supports
• FRTW joists or trusses

When supporting an FRTW roof system, unless the wood members are heavy timber, which has an inherent capacity to withstand fire exposure, they must be fire-retardant treated. Experience shows that both lumber and plywood decking must have a minimum actual thickness of 19mm and both should be tongue and groove. Plywood decking, if not tongue and groove, must also have unsupported joints solidly backed with FRTW or plywood.

The figure below shows that the construction of roof assemblies using FRTW is similar to that of other types of roof assemblies, using a metallic vapor barrier membrane between the decking and the insulation. Usually 0.05mm aluminum sheeting is attached with an approved adhesive, although steel foil is also acceptable. Galvanized roof nails may be used to fasten the insulation to the vapor barrier which is then stapled to the deck.

FRTW or noncombustible ceilings may be attached to the underside of the system, with the resulting concealed spaces appropriately fire stopped.

FRTW roof assemblies are permitted as an alternative to roof assemblies of noncombustible construction or ordinary wood-frame roof assemblies having a fire-resistance rating of 45 minutes. When used, however, the NBCC requires that, except for mercantile or light industrial occupancies, the area of the building be half that which would be permitted if either of the other two types of roof assembly were used.

As noted earlier, fire-retardant coated wood is not the same as FRTW and therefore is not permitted to be used under this requirement unless the system passes the extended 30-minute tunnel test (CAN/ULC-S126).