My Douglas Fir Decking Has a Smell

Resin Odor

• Cut Douglas fir exudes resin on the ends and sides, and there are sometimes streaks of resin on the grain of the wood. This resin, which is used commercially to make turpentine and varnish, is a source of the identifying odor of cut Douglas fir and may be what you smell on your deck. The odor should fade in time. Douglas fir is not a true fir and is the only species in the genus Pseudotsuga. Other species of conifer trees that yield resin in smaller amounts include pines, hemlocks, spruce, larch and true firs.

Odor and Beetles

• Douglas fir beetles locate a host tree by the odor of its resin. After finding a tree, a female bores a hole and emits a sexual pheromone to attract males. Males flying upwind find the females. Since males do not want competition, they use an acoustic signal to block the females from producing pheromones. They then emit an odor to block the ability of other beetles to smell the resin, thus preventing the tree from becoming overpopulated by the beetles. These borers and other insects that can smell Douglas fir resin is why the wood in your deck has likely been treated, perhaps with a chemical that emits an odor.

CC Odor

• If your deck smells like ammonia, it may have been treated with ammoniacal copper citrate (CC). CC was developed specifically for Douglas fir and some other species that are difficult to treat. The citric acid helps the copper penetrate the wood. The copper protects against boring beetles, insects and fungi. If your deck was treated with CC, its color may vary from light green to dark brown. The odor will dissipate with time. You can stain or paint wood treated with CC.

ACZA Odor

• A slight ammonia smell may also mean your deck was treated with ammoniacal copper zinc arsenate (ACZA), which is widely used as a preservative on the West Coast because of its ability to penetrate Douglas fir. Wood treated with ACZA is bluish green to olive. It protects against fungi that cause rot, plus borers and other insects. It is commonly used to treat piling, poles and timbers. The odor should go away. You can stain or paint wood treated with ACZA.