Pressure-treated wood is resistant to termites, but make sure that there’s no contact with soil.

Pressure-treated wood is wood that has had a chemical preservative forced into the pores to form a barrier that resists decay and wood-eating insects like termites and carpenter ants. Pressure-treated wood is used primarily where wood is in close contact with soil such as decking, fence posts, mailbox or light posts, trellises, or gazebos. Building codes may require that pressure-treated lumber be used in construction for below-ground installations and wherever wood will be touching soil.

“Termite-Resistant,” Not “Termite-Proof”

People mistakenly think that if their deck is made of treated wood, they don’t have to be as careful about the wood touching soil. It’s protected from termites, right? That may be true in the short term, but even pressure-treated wood is not fully protected from termites or decay. Construction codes in the U.S. state that pressure-treated wood is considered to be “termite-resistant.” It doesn’t say “termite-proof” and that is the difference. Pressure-treated wood may be protected for a while, but if it stays damp enough for long enough and starts to rot, termites can feed on it.

Also, when pressure-treated wood is used in construction, there will be new cut wood ends, as well as new notches and drill holes that are not protected and are supposed to be spot-treated with the preservative on site. That rarely happens. The problem here is that the chemicals leach out of the wood over a period of 7-10 years, so while the outer 1 inch of the boards seems to retain some protection, the inner wood does not. If there is any kind of a crack or hole where termites can bypass the outer protected surface and enter the wood, all bets are off. Termites will also build mud tubes over the surface of pressure-treated wood to reach untreated wood.

That’s not to say you should forget about buying pressure-treated wood. Some protection is certainly better than none. Much better, though, to take precautions and build your deck to last.

Construction Rule No. 1 – No Wood/Soil Contact!

Anytime you build with wood, pressure-treated or not, do not allow wood to make direct contact with soil. Soil contains moisture and fungi that rot wood. Termites live in soil and move through soil looking for embedded wood to feed on. Any wood, pressure-treated or not, that is directly in contact with soil will eventually rot and become susceptible to termite and carpenter ant attack.

• Wood members like door frames or lattice work should be at least 6 inches above the ground.
• Wooden support steps or posts should be mounted on a concrete base
• When pressure-treated wooden posts are buried in the ground, the portion below grade should be completely encapsulated with concrete.

Photo credit: ntm1909 / Foter / Creative Commons Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)