Q. What’s the best way to build a skirt around an addition that’s built on piers or posts?
Plywood skirt board: This simple skirt works well for room additions on wood posts or concrete piers. In heavy soils, good drainage prevents frost heaves.
A. David Bowyer, designer and sales manager for Peacock & Co., a South Bend, Ind., remodeling firm, 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 2×4 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.
