- Q. Can wood foundations built with pressure-treated wood be framed like conventional frame walls, or does the stud spacing need to be increased? How do you seal the outside to prevent water seepage?
A. The size and spacing of the studs in a treated wood foundation vary with the depth of fill against the foundation walls. An 8-foot foundation wall with 7 feet of fill, for example, will need to resist more weight than one with 4 feet of fill against it.
Depending upon the depth of fill and the soil characteristics, the studs may have to be as large as 2x8s, and the spacing as close as 12 inches. Before attempting a wood foundation you should get Publication A-400, Permanent Wood Foundation, free from the American Plywood Association, P.O. Box 11700, Tacoma, WA 98411. This publication contains all the structural design data needed to detail a "permanent" wood foundation.
To keep water out, a treated wood foundation must have a gravel drainage bed beneath the basement or crawlspace floor, and drainage material (gravel or a drainage fabric) which extends up the foundation wall. In addition, the wall is wrapped in plastic film, preferably vinyl, as shown in the APA design manual.
