Fire-retardant wood treatment (FRT) is the process by which pressure impregnates wood with fire-retardant chemicals. It is a protective treatment that reduces the combustible nature of wood, minimizes the effects of fire, and is suitable for interior and exterior wood, dimensional lumber, engineered lumber (excluding wood wafer products), and finish materials. Two types of FRT exist today: one involves a blend of a nitrogen-phosphorus organic compound and boric acid, and the other involves the use of ammonium polyphosphates with additives (boric acid, borax, moldicides, and others). Formulation compositions used to treat wood are proprietary and differ between manufacturers.
The FRT process occurs at wood treatment facilities and is similar to pressure treating wood for resistance to insects and decay. Air is removed from wood cells and the wood is placed in a horizontal chamber. The chamber is pressurized and the wood is impregnated with fire-retardant chemicals. After this, the chamber is depressurized and the lumber is kiln-dried to an appropriate moisture content. Fire-retardant treatment is typically colorless; however, some manufacturers' formulations leave wood with a distinct red or orange color. Whether colored or colorless, FRT wood is required to have a code-compliant label or stamp applied by a third-party inspection agency. The label identifies the product name, treating plant, drying method, treating standard, agency, flame spread, species, and applicable ASTM standard references for either interior or exterior applications.
When a building fire occurs, the chemicals in FRT wood react to the heat with the combustible properties of wood. This reaction creates carbon dioxide, water, and carbon char. Carbon char insulates the wood from the fire and slows the rate at which the fire spreads. To ensure this protection, FRT wood is tested in accordance with building code requirements. Three classes exist that define the flame spread index of FRT wood.
- Class A Ratings achieve a flame spread index of 25 or less with no evidence of progressive combustion after 30 minutes.
- Class B Ratings achieve a flame spread index of 26-75.
- Class C Ratings achieve a flame spread index of 76-200.
FRT Advantages and Disadvantages
FRT wood has advantages and disadvantages beyond fire performance characteristics, when compared to untreated wood. It can be cut, drilled, fastened, and overlayed (with carpet or plastic) in the same way as untreated wood, but it cannot be ripped or milled. FRT wood is light in weight and economical but heavier and more expensive than untreated wood as a result of its processing. Fire retardants have little effect on the properties of wood; however, the treatment and drying processes minimally reduce its strength. Refer to the manufacturer's conversion charts to determine what effect their formulation has on the structural capacity of wood and how that differs from untreated material. The use of FRT wood is typically specified in commercial structures due to building code requirements that include the use of a material classified as noncombustible. In other cases, FRT wood is specified for residential and commercial structures even when not required by building code. It can provide desired protection against the effects of fire or, in some cases, eliminate the need for a sprinkler system, as defined by the National Fire Protection Association (NFPA) 13, "Standard for the Installation of Sprinkler Systems."