- Q. In New York state, we built a pergola (a type of garden trellis) using 8x8 pressure-treated posts set 4 feet into the ground. During the first winter, the frost lifted the posts. What should we have done to keep the posts from heaving?
A.Ron Hamilton, owner of Hamilton General Contracting in Saylorsburg, Pa., responds: For structural reasons, pergola posts should be deeply buried. (In contrast, when building a deck, it’s better to set the pressure-treated posts on top of Sonotubes extending above grade.) In order to help resist wind uplift, pergola posts should be embedded in concrete, rather than in holes backfilled with dirt. But if the holes are backfilled with concrete, the concrete conforms to the irregular shape of the hole, allowing frost to grip the concrete and heave the post. I prefer to insert the buried section of the post into a Sonotube.
Check with your local building officials for information regarding the frost depth in your area. Your post holes should be dug to the frost depth plus 6 inches. Each hole should receive 6 inches of crushed stone for drainage. Drainage is important, since frost heaving is more likely in wet soil than dry soil.
The Sonotubes should extend from the crushed stone base to slightly above grade. After backfilling around the Sonotube with compacted dirt, insert the pressure-treated posts inside the Sonotube, holding the bottom of the posts about 6 inches above the crushed stone, to provide a space where concrete can flow under the posts. Then plumb and brace the posts and fill the Sonotubes with concrete.
