10 Tips on Buying and Selecting Deck Lumber

Buy Pressure-Treated Lumber With the Right Amount of Preservative for the Job

Use pressure-treated lumber for the structural part of your deck the posts, joists, beams and other members you normally don’t see. It’s the logical choice; PT lumber can support more weight and span longer distances than cedar, redwood, or other woods commonly used for building decks. It’s also much less expensive.

The lumber industry rates PT lumber according to the pounds of preservative retained per cubic foot of wood; the higher the number, the better the protection against fungi and insect attacks. Select decking boards with the preservative concentration suitable for their use. The wood used in situations where it’s more likely to rot more preservatives.

The three common ratings, which correspond to preservative concentration, are:

• Above-grade, use .25 (sometimes .15). This includes decking, fence and railing material.
• Ground-contact, use .40. This includes posts, beams, joists and decking.
• Below-grade, use .60. This includes support posts that are partially buried below grade and for permanent wood foundations and planters.

Note: Decking boards are tagged with the concentration and treating solution used. CCA (chromated copper arsenate) was once the most common preservative, but it was phased out in 2004 in favor of ACQ (alkaline copper quat) and other preservatives because of health concerns. “Modern preservatives are certified safe for plants, animals, people, ponds, and waterways,” says Kou.