Q&A: Strength of PT Lumber

A. According to the National Design Specification for Wood Construction , pressure preservative treatment with CCA (chromated copper arsenate) does not affect the strength of lumber except in the case of impact loads (loads that last about a second). Fortunately, impact loads are not typical in residential construction.

The bigger concern with PT lumber is that in use it is typically exposed to the weather. Thus, its design strength is subject to a “wet service” reduction factor, and thus most fasteners have weaker values. Therefore, most lumber properties are lower and most connections are weaker. One way to avoid a moisture penalty for connections in PT lumber is to use threaded hardened-steel nails, which have been shown in testing to have full rated strength even in wet lumber. When working with pressure-treated lumber, choose fasteners that resist corrosion from CCA, such as hot-dipped galvanized.

— F.W.