Pressure-treated lumber has been available for about 60 years. Most original formulas centered around arsenic as the main preservative, to help lumber withstand the extremes of outdoor use. As one might expect, the use of a potentially poisonous substance like arsenic was cause for considerable concern, particularly when children are exposed to such installations.
To address the pressure treated wood safety concerns, in 2002, the United States' Environmental Protection Agency convinced lumber manufacturers to find a non-arsenic based formula for treated. The result is ACQ (Alkaline Copper Quaternary) treated lumber.
The new ACQ treated lumber contains a very high level of copper to replace the arsenic. While this reduces the desired risk of poisoning, it does present a different, but very dangerous problem: increased levels of corrosion.
The problem stems from copper's very high levels of conductivity. Simply put, ACQ-treated lumber is extremely corrosive to metal fasteners such as nails, screws, deck hangers, etc. This problem has been complicit in a number of deck collapses in recent years, the worst of which occurred in Kentucky in 2003, where 13 people died and another 57 were injured.
Is There a Solution?
This increased level of corrosion should make the woodworker think more about the fasteners that they use on treated lumber woodworking projects. Just any wood screw or nail will not do; instead, look for fasteners that are specifically designed for use with ACQ-treated materials.
What types of fasteners are acceptable for use in this type of treated lumber?
Stainless Steel or Copper
Both stainless steel and copper fasteners are virtually immune to corrosion. However, unless you've got a sizable trust fund or have decided to trade in a car to finance your woodworking project, they aren't really practical.
Can you use galvanized nails and screws on the new ACQ-treated lumber? Yes and no. Plain old galvanized fasteners simply aren't designed for the increased level of corrosion and could fail in a very short period of time.
However, many fastener manufacturers now offer "hot-dipped galvanized" screws, nails, hangers and brackets under a variety of brand names. The key is to look on the package for a "G-185" designation (which refers to the thickness of the galvanization). Anything less than a G-185 level is inappropriate for ACQ lumber.
Other Acceptable Fasteners
Some manufacturers also offer deck screws (and other connectors) that have a non-galvanized coating that appears to be painted onto the fastener. These typically have a dark green or brown color, and the package is clearly marked for use on ACQ treated stock. Deck screws that offer a lifetime guarantee that the fastener will not corrode when used with ACQ lumber are also readily available at most home improvement centers. They often include a special square-head Robertson bit designed for use with these screws.
When your treated lumber woodworking plans call for a deck hanger or other type of lumber bracket, beware of aluminum hangers. These are extremely prone to corrosion and should be avoided for use with ACQ stock. Instead, look for G-185 rated brackets, and consider adding a protective barrier between the bracket and the lumber. A plastic or tar-based type of barrier would be perfectly acceptable.
Keep this corrosion problem in mind if your project requires flashing. Most flashing is aluminum and will corrode quickly if exposed to ACQ lumber.
Is Your Deck Safe?
If you've built a deck or other pressure-treated lumber project since 2002, how can you tell whether you have a corrosion problem?
Look at the areas where fasteners are in contact with the pressure-treated lumber. A certain amount of white-colored corrosion is normal and perfectly safe. However, if you notice any reddish, rusty discoloration in the areas of your fasteners or brackets, you may have a potentially serious problem to deal with. It would be a good idea to have your deck professionally inspected and have the offending connectors replaced as soon as possible.
Where to Learn More
By taking an overly-sensitive level of caution about the types of metal that will interact with your pressure-treated lumber projects, you greatly reduce the possibility of a problem. By making sure that no improper connectors find their way into your treated woodworking plans, you can have peace of mind that your projects will be safe.
One final note on this subject: a building inspector in Northern Utah has compiled a comprehensive website on the dangers of deck failures due to improper installation using pressure-treated lumber. Michael Leavitt's site may provide you with some help.