A.David Bowyer responds: Our company has built many additions on wooden posts and concrete piers, and we’ve developed a simple technique to close off the open area between the floor system and the ground. We install a pressure-treated plywood skirt that matches the thickness of the wall sheathing above (see illustration). The top edge of the plywood skirt is nailed to the rim joist; the lower portion extends about 1 foot below grade and is fastened to a treated 2x4 nailer that is held a few inches above the ground. The plywood can be sided to match the walls, or parged (over wire lath) to simulate the look of a masonry foundation.

We’ve never had a problem using this approach in our area of Indiana, where the frost line is 3 feet and local soils drain well. In colder regions with heavy clay soils, it would be wise to surround the buried portion of the plywood skirt with gravel, and place a drain to daylight in the bottom of the trench.

No matter what your local conditions are, check in with your building official for approval of this post-and-skirt foundation system before breaking ground.

(For more on this topic, see "Post-and-Pier Room Additions," 7/94.)

David Bowyer is designer and sales manager for Peacock & Co., a South Bend, Ind., remodeling firm.

Plywood Skirt Board