Exterior Rated Fire Retardant Treated Wood

Article Written by:
Stephen L. Quarles, Senior Scientist, Insurance Institute for Business & Home Safety, Richburg, SC

Using fire-retardant, treated solid wood products that are rated for exterior use is an option for obtaining a wood product that will perform better under wildfire exposures. Products that can be treated with an exterior fire retardant include plywood and lumber siding, dimensional lumber (for use as decking or blocking, for example), and shakes and shingles. These products are available from building suppliers. Understanding how the fire-retardant chemicals are applied and the potential degradation mechanism will help you decide if these products will meet your fire prevention needs.

The fire-retardant (FR) chemical in treated wood rated for exterior use has been applied using a pressure impregnation process that results in greater penetration and chemical retention than when a fire retardant is applied by a brush or dip treatment. Exterior-rated FR wood has also been subjected to an accelerated weathering procedure that is intended to quickly leach any fire-retardant chemicals that may be leached out over time while the product is in service. After the accelerated weathering procedure, the treated wood is subjected to, and must pass, a standard flame spread test.

Wood shingles waiting to be loaded into the treating cylinder where fire retardant chemicals will be injected via a pressure impregnation process.

The horizontal tunnel that is used for the flame spread test. The material to be tested is placed in the upper portion of the tunnel. The progress of the flame spread down the tunnel is monitored by viewing the test through the windows.

There are two mechanisms that would result in the loss of effectiveness of the FR treated wood. These include: 1) leaching of the FR chemicals as a result of exposure to rain and other moisture, and 2) loss of wood fibers and FR chemicals as a result of in-service weathering.

After treatment in the cylinder, the FR treated wood is put into a dry kiln and exposed to elevated temperatures. One of the purposes of the post-treatment in the dry kiln is to “fix” the FR chemicals in the wood. The purpose of the accelerated weathering exposure prior to subjecting the FR treated wood to the flame spread test is to demonstrate the effectiveness of the dry kiln treatment in fixing the FR chemicals in the wood.

The now FR treated wood is at the inlet to the dry kiln.

The amount of weathering in wood will be variable, depending on exposure — that is, north-, south-, east-, or west-facing. Although it is generally a very slow process, weathering on southern and western exposures will be greater than that on northern and eastern exposures. The weathering process can be slowed down with a periodic application of a surface finish such as a 100-percent acrylic latex stain or semi-transparent and opaque paints. These film-forming finishes perform best over FR treated wood. Penetrating stains generally perform poorly because the small between- and within-cell openings in wood are filled with the polymerized fire retardant, minimizing the penetration of the stain. If the erosion of wood fiber is minimized, loss of the FR chemical via that mechanism will also be minimized.

Weathering on a piece of untreated plywood siding with a southern exposure. This siding originally had a high-quality thin veneer. Over time, the wood fibers comprising the thin veneer top surface eroded away as a result of exposure to sun, rain and wind. The lower-quality inner layer, with “core gap,” is now exposed. The reddish color is the adhesive used to bond together the two veneer surfaces. Wood loss could have been minimized with a regular application of a protective coating. Treated and untreated wood benefits from regular application of protective surface finishes.

Fire-retardant treated solid wood products can provide enhanced protection. The treated product must be maintained and inspected over time.