How Long Should Treated Lumber Last?

Back when I still wore a tool belt (and Bill Clinton was still in the White House), we didn’t give the PT lumber we used a lot of thought. As I recall, all of the CCA-treated (chromated copper arsenate) framing material that our local lumberyards carried was rated for ground contact, with a 0.40 pcf (pounds per cubic foot) retention level. They didn’t bother stocking 0.25 pcf CCA-treated lumber (the retention level for above-ground use), so we didn’t have to worry about inadvertently using tread stock for stair stringers, or make sure we had selected the right framing lumber for a deck built close to the ground.

It’s been 10 years since builders have been able to use CCA-treated lumber for decks. Much has been written about the EPA agreement that resulted in its voluntary withdrawal from the U.S. residential market in 2004, but it’s still unclear whether CCA-treated lumber actually poses a health risk. Are the decks being built now with the preservatives developed to replace it—such as ammoniacal copper quat (ACQ), copper azole (CA), micronized copper quat (MCQ), and micronized copper azole (MCA)—as durable as the decks built a decade or two ago? That’s a question that was raised recently by a deck builder who reports finding accelerated aging and rot in six-year-old CA-treated decking and framing. He’s not confident that the preservatives that have replaced CCA are as effective, and not happy about the cost and limited availability of ground-contact-rated lumber. He and other deck builders are worried about the possibility of widespread failures of low-level decks and children’s playsets that have mostly been built with lumber treated for above-ground retention levels.

Of course, there’s really no such thing as generic “PT” lumber anymore. Now, builders need to understand not only the differences between preservatives, but also differences in retention levels for each intended use. Or, they have to accept at face-value the AWPA’s use categories and follow the labeling on the lumber; in either case, they also have to hope that local lumberyards stock what they need, or make plans to place a special order. And to sleep well at night, they need to have confidence that the performance claims of the preservative manufacturers are accurate.

What has your experience been recently with treated lumber? We’d like to hear about it.