SETTING FENCE POSTS WITH EXPANDING FOAM

Any fence builder knows you need strong posts for a strong fence, and that means backfilling the postholes with a dense, hard material other than dirt.

- CHRIS DEZIEL

While a lot can be said for well-draining gravel, the material of choice has long been concrete mix. It's available in 50-lb. bags at any home center, and you usually need two bags per post. For a 100-ft. fence with 10 posts, that means lugging half a ton or more of concrete mix to the site, mixing it with water and shoveling it into the holes.

If a lightweight alternative to concrete performed just as well, many fence builders would choose it just to save their backs. Fence post setting foam offers promise, but is it just as good as concrete?

We asked two experts who set posts with foam: Alex Capozzolo, cofounder of Brotherly Love Real Estate in Philadelphia, and Forrest McCall, co-owner of the home improvement blog "Mama Needs a Project."

The use of expanding foam to set fence posts is a recent innovation. We asked two experts how foam compares with concrete for strength and usability.

EXPANDING FOAM VS. CONCRETE

Post-setting foam is a two-part product, and before use, the parts must be mixed, which is done right in the bag. It's far lighter than concrete mix. Each bag contains enough material for a single post and weighs only 2 or 3 lbs.

Note: Unlike spray foam in an aerosol can, post-setting foam is a closed-cell formulation without air bubbles, making it more rigid. Most insulating foam is open cell.

BRACING IS KEY

Whether you backfill with concrete or foam, you dig the same size postholes to the same depth, which should be one-third the length of the entire post for stability.

Backfilling is more difficult with foam. Because the foam expands when it sets, McCall says the post tends to move if not braced properly. Capozzolo adds that there's little time to fix leveling errors. Bracing is also recommended with concrete, but it isn't as critical because concrete sets more slowly and provides plenty of time for readjustment.