Where flame-spread ratings of 75 or less are stipulated in building codes, most lumber and plywood must be treated or coated to reduce flame spread over the surface of the material. Two most common methods of reducing the flame spread rating are by pressure-impregnation with water-borne salts and by surface coating with fire-retardant chemicals.

This section describes the chemical means used to reduce flame spread on wood products. Information on the importance of flame-spread rating to building construction is covered in greater detail in our Wood Reference Handbook.

Fire-Retardant Treated Lumber and Plywood

Fire-retardant treated wood (FRTW) is material which has been pressure-impregnated with special chemicals which improve the performance of wood products in a fire. The pressure-treating process for injecting fire-retardant chemicals into wood and plywood products is essentially the same as that for preservative-treatment as shown in Figure 9.8 of the Preservative-Treated Wood section.

Fire-retardant chemical treatments retard the spread of flame and limit smoke production from wood in fire situations. FRTW can meet the most restrictive requirements for flame-spread rating and smoke developed-classification in the North American building codes.

Fire-retardant treatment of wood enhances the fire performance of the products by reducing the amount of heat released during the initial stages of fire. The treatments also reduced 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, treated wood ceases to char.

During a fire, fire-retardant chemicals begin to react when temperatures reach a point slightly below the point where wood will ignite. Nonflammable gases and water vapour are formed and released at a slow steady rate thereby insulating the wood fibres from temperatures that would cause them to burn.

To be an acceptable fire-retardant treated product in Canadian jurisdictions, wood products must be labelled by an accredited testing agency, such as Underwriters' Laboratories of Canada. A sample label is shown in Figure 9.11 below. In the US, similar labelling and certification requirements apply.

FRTW is suitable for indoor applications where the humidity is not expected to exceed 60 percent for long periods of time. FRTW should be protected from excessive moisture and weather during transit, storage, and erection. While some wetting might be expected during installation, frequent wetting or ponding is unacceptable. In general, FRTW requires more care in installation than would normally be considered good practice for non-FRTW products.

FRTW products are used in many interior applications, such as millwork and panelling, where the code requirements for flame spread are most restrictive. The building codes of North America also permit the use of fire-retardant treated lumber and plywood for roof and floor trusses, beams, interior roof decks, and for interior load-bearing and non-load bearing partitions.

FRTW is generally restricted to interior use because of the possibility that the protective salts will leach out of the materials if they are exposed to the weather. For exterior applications where codes specifically require FRTW products, exterior grade treatments are available for wood products which are capable of meeting accelerated weathering test requirements.

FRTW products used in areas where the material is exposed to weather or high humidity, are treated with special non-leaching chemicals similar to those used for FRTW shakes and shingles. An accelerated weathering test (ASTM D2898) exposes FRTW to regular wetting and drying cycles to represent actual long-term outdoor conditions. In order to qualify for exterior use, FRTW must still achieve a flame-spread rating of 25 after undergoing this accelerated weathering.

The fire-retardant treatment of wood does not generally interfere with the adhesion of decorative paint coatings unless the treated wood has an increased moisture content. The finish characteristics of particular products should be discussed with the manufacturers of the treated wood.

In the United States, where the use of FRTW products has been extensive, the combination of temperature, humidity, and certain fire-retardant chemical formulation has led to deterioration of the products in certain climatic zones, particularly in plywood used in roof assemblies.

This problem has been specific to certain chemical formulations. Some of the latest generation fire-retardant chemicals and processes have been formulated to prevent this degradation from taking place while continuing to meet the fire-safety requirements of the building codes. In selecting FRT wood products, the designer should review the documented performance of those products under consideration.

 Figure 9.11 Panel Markings for Fire-Retardant
Treated Lumber and Plywood.

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