Bigfoot Systems

Frost heave 101

The mechanics of frost heave are complex, but here's a quick primer. Water in the surrounding soil collects and freezes into thin layers of frost called ice lenses.

When water freezes, it expands about 9 percent - think of how ice cubes are domed above the original water level in the ice cube tray. Ice exerts a pressure of about 50,000 lbs. per square inch - enough force to lift even a large building. A small deck on inadequate footings doesn't stand a chance.

Buildings and decks don't always return to their original height. Surrounding dirt sometimes fills in under the footing while it's lifted.

Heavy clay soils don't drain well, so they tend to have more frost heave problems than sandy, well-drained ones. Even if footings are deep enough, ice lenses can latch onto the rough surfaces of wood and concrete and lift footings and posts from the side. That's why concrete piers poured in waxed cardboard tubes and smooth wooden posts work well for below-grade support.

When you contact your local building department for your building permit, ask what the frost depth requirement is and the required size and shape of the footings. Dig the footing holes to ensure the bottoms of the footings are at, or below, the frost depth.

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