JLC Extra Q&A

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.

To resist wind uplift, pergola posts should be securely anchored to the ground. For a solid anchor, I prefer to embed pergola posts in concrete, by inserting each post into a Sonotube. If an irregular-shaped hole is used instead, frost will tend to grip the concrete and heave the post. (Another option is to extend the Sonotubes a little above grade and to use embedded post anchors with sufficient hold-down strength).

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 in order to provide a space where concrete can flow under the posts. Then plumb and brace the posts and fill the Sonotubes with concrete. -