Glossary / fire-retardant-treated wood
Fire-retardant-treated wood
Wood treated to reduce flame spread, often listed as exterior FRT. Oregon R327 names it as one walking-surface path. It is not noncombustible and not a coating.
Fire-retardant-treated wood, FRT, is lumber or plywood pressure-treated with chemicals to reduce flame spread, sold for outdoor use as exterior FRT. It is a treated combustible material: not noncombustible, and not a coating.
Where it is a path. Oregon R327, where adopted, names exterior fire-retardant-treated wood as one walking-surface material for decks, porches and balconies 30 inches to 12 feet above grade, beside ASTM E2632/E2726 results, ignition-resistant material and noncombustible material (R327.3.5). Ignition-resistant in R327 is an extended ASTM E84 or UL 723 run, a separate path with its own test.
What the label does not do. A spray-applied or paint-on retardant is not the same product: presenters at the 25 June 2026 Central Oregon seminar said spray-applied retardant may need a jurisdiction-approved special inspector, and coatings are not an R327 wall path (guide). Pressure-treated wood for decay and insects is a different treatment with a different label; it is not fire-retardant-treated. California's OSFM handbook lists decking by model in category 8110, tested to SFM 12-7A-4; an FRT board earns a place there only through that test. Every book pairs the walking surface with an underfloor that is enclosed or protected, because the treatment reduces flame spread and does not remove the fuel.
Questions
Can pressure-treated wood be fire retardant?
Only if it is fire-retardant-treated: ordinary pressure treatment for decay and insects does not reduce flame spread, and the two labels are different. Ask for the FRT label and, outdoors, for exterior FRT.
Can wood be treated to make it fire resistant?
Yes, as fire-retardant-treated wood, and Oregon R327 accepts exterior FRT as one deck walking-surface path. It remains combustible and is neither noncombustible nor a Class A anything.
What is the best fire retardant for exterior wood?
The codes name a product class, exterior fire-retardant-treated wood, not a brand. Paint-on and spray-applied retardants are not that class and are not accepted as an Oregon R327 wall path.
Also called: FRT wood, exterior FRT, fire-resistant wood.
Do not use as a synonym: fireproof wood.
Related
Where this term is used
- Deck railings in the WUI
- Exterior FRT wood and weathering
- Spray-applied FRT and special inspection
- The 200 square foot 2-inch lumber deck exception