As a result, early in the 21st-century, wood manufacturers began experimenting with other formulas, primarily Alkaline Copper Quartenary (or ACQ) lumber. While this new ACQ-Treated Lumber did not have the issues with toxicity of the traditional formula, the new pressure-treated lumber had its own problems. In particular, the high levels of copper in the material made it far more conductive, causing increased levels of corrosion in the material as it reacted with traditional nails and wood screws. To combat this problem, manufacturers began offering specially-treated screws designed for use with the new ACQ pressure-treated lumber.
How to Buy Pressure-Treated Lumber:
Very wet. Almost dripping wet.
This is because of the pressure-treating process. You'll find that wood on the outside edges of the bundles is dried (somewhat) on the outside edges, and likely you'll see that the wood is starting to warp outwards, away from the center of the bundle. This uneven shrinkage is because the outer edges have begun drying (and thus shrinking) while the inner-facing sides have yet to be exposed to enough air to begin drying. Since warped lumber doesn't sell, manufacturers want to get the wood to the lumberyard or home center as quickly as possible with all of the wood dripping wet so that it will sell before it begins to dry unevenly.
When wet, wood acts like a sponge, with the fibers (forming the grain along the long axis of the tree trunk) soaking up as much liquid as possible. As the wood begins to dry, it will shrink across the fibers considerably, but very little shrinkage will occur along the length of the boards.
With this fact in mind, when you buy pressure-treated lumber, you won't have time to wait for it to dry before you buy it to see what boards warp and twist and which boards stay straight. Instead, you'll need to find another way to judge what boards to buy. Inspect the long edges of the boards, looking for blemishes or weak spots that may show a tendency for the board to bend around the defect. Additionally, inspect the end grain of the wood. If possible, select wood whose grain lines cross the narrowest span of the end of the board. Boards that have arc-shaped end grain will likely cup more easily than boards whose end grain crosses the narrow span of the board.
Installing Pressure-Treated Lumber:
Additionally, to help resist cupping, position any boards with end grain in the shape of an arc so that the center of the arc-shaped grain is pointing away from the adjoining member. So, if you're installing decking onto a framework, the center of the arc on the end grain should point upwards. Be sure to place at least one deck screw through the center of the board to help minimize the bowing as the board dries. Pre-drilling the boards before driving the screws will help maintain the integrity of the wet wood as you drive the screws. A quick change drill driver set on the end of your cordless drill will make the job quick and easy.
Additionally, pressure-treated lumber should be used only on outside woodworking projects, and the wood should never be burned. Once again, this is less of a concern with newer formulations than the old arsenic-based varieties, but it is still a good rule-of-thumb by which to abide.