“Fire-retardant treated wood” (FRTW), as defined by the National Building Code of Canada (NBC), is ‘…wood or a wood product that has had its surface-burning characteristics, such as flame spread, rate of fuel contribution and density of smoke developed, reduced by impregnation with fire-retardant chemicals.’ FRTW must be pressure impregnated with fire-retardant chemicals in accordance with the CAN/CSA-O80 Series of Standards, Wood Preservation and when fire-tested for its surface flammability, must have a flame spread rating not more than 25.

Fire-retardant chemical treatments applied to FRTW retard the spread of flame and limit smoke production from wood in fire situations. FRTW products are harder to ignite than untreated wood products and preservative treated wood products.

Fire-retardant treatments applied to FRTW enhances the fire performance of the products by reducing the amount of heat released during the initial stages of fire. The treatments also reduce the amount of flammable volatiles released during fire exposure. This results in a reduction in the rate of flame spread over the surface. When the flame source is removed, FRTW ceases to char.

FRTW contains different chemicals than preservative treated wood. However, the same manufacturing process is used to apply the chemicals. FRTW must be kiln-dried after treatment to a moisture content of 19% for lumber and 15% for plywood.

The fire-retardant treatments used in FRTW do not generally interfere with the adhesion of surface paints and coatings unless the FRTW has an increased moisture content. The finishing characteristics of specific products should be discussed with the manufacturer.

Typical interior applications of FRTW include architectural millwork, paneling, roof assemblies/trusses, beams, interior load bearing and non-load bearing partitions. Exterior-type fire retardants use different chemical formulations from those used for interior applications, since they must pass an accelerated weathering test (ASTM D2898), which exposes FRTW to regular wetting and drying cycles to represent actual long-term outdoor conditions. Generally, exterior-type fire retardants are applied to shingles and shakes.

FRTW can be crosscut to length (not ripped) and drilled for holes following treatment without reducing its effectiveness. End cuts in the field, whether exposed or butt jointed, do not require treatment, since any untreated areas are relatively small compared to the total surface area and the flame spread rating remains unaffected. Plywood can be both crosscut and ripped without concern, since the chemical treatment has penetrated throughout the individual layers/plys.

FRTW is not excessively corrosive to metal fasteners and other hardware, even in areas of high relative humidity. In fact, testing has demonstrated that FRTW is no more corrosive than untreated wood.


Exterior use of FRTW
Fire retardant coatings
Fire-retardant-treated wood roof systems
Flame-spread rating


For more information on FRTW, visit the manufacture’s websites:

Arch Wood Protection, Lonza: www.wolmanizedwood.com

Viance LLC: www.treatedwood.com