Q. What is the moisture content of treated lumber? The treated lumber I use seems very wet, but it carries a grading stamp that lists the moisture content at 19%.

A.Gene Wengert responds: Most treated lumber carries two stamps: The grade stamp indicates the condition before treatment; the treatment stamp indicates the level and type of treatment.

Before treatment, the lumber (southern pine, in most cases) is typically dried to approximately 19% MC (moisture content). This initial drying removes the "free water" from the lumber, making room for the preservatives. After pressure-treating with chromated copper arsenate (CCA), the moisture content will be 75% or higher.

The treatment process causes the lumber to swell, so when you receive it at your job site, it will be oversized. The treated lumber will shrink when exposed to the air, just as it did when it was first cut from the tree and dried before treatment. Once installed, it will dry to an equilibrium moisture content that is controlled by the surrounding conditions. Treated lumber installed in a relatively dry location can shrink as much 4% in width and 2% in thickness: For a 2x12, for example, that’s nearly 1/2 inch in width and 1/16 inch in thickness.

Some areas offer treated lumber that has been "kiln dried after treatment" called KDAT. This material is considerably more expensive, and you should make sure that the final MC is specified.

Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forest Ecology and Management at the University of Wisconsin-Madison.