Until a few years ago, pressure-treated structures were pretty forgiving. We buried fence posts in soil; treating cuts with brush-on preservatives wasn’t required; and there weren’t any flashings, coatings, or tapes covering the tops of joists and beams. Yet, there never seemed to be problems.

When CCA (chromated copper arsenate–treated) southern yellow pine was taken off the residential market in 2004 because of toxicity concerns, my company had some reservations about the new treatments, but things seemed fine for years—until we had a handful of treated-wood failures, which made clear we needed to change our construction methods if we wanted the new outdoor lumber to last as long as the old stuff.

Changes in the chemical formulations of pressure-treated lumber beginning in 2004 have yielded lumber that is not as rot-resistant as it used to be. We have responded by using only ground-contact PT lumber, flashing the tops of joists and beams, treating field cuts, keeping lumber away from grade, and creating spaces for drainage and drying. So far our improvements seem to be working, but only time will tell.

Signposts of problems

One of the first indications that the newer treatments weren’t like the old was a call we received in 2020 about a deck we had built in 2013 that the client said felt springy and soft in areas. It was a pretty standard backyard deck, about 4 ft. above the ground with plastic cap-stock decking.

What I found when I crawled underneath was far from ordinary. Mushrooms were growing out of the joists, and black and white fungi covered their sides. There were a few places where the joists had crumbled, as well as many smaller areas of damage where water had gone between the deck boards. In total, we found 70 damaged joists, beams, and posts…