Deck footings and frost depth in central Vermont

A deck that heaves, tilts, or pulls away from the house in the spring is almost always a footing problem. In central Vermont, where the frost line runs between 48 and 60 inches depending on the site, footings that do not reach that depth will move. It is not a question of if. Here is what you need to know before you build.

What frost heave actually does to a deck ¶

When water in the soil freezes, it expands. If a footing sits above the frost line, the expanding soil pushes it up. In spring, the soil thaws and the footing settles back down, but not always to exactly where it started. Over a few cycles, the deck shifts. The ledger connection to the house gets stressed. Boards gap or cup. Railings go out of plumb.

The fix is not to pour a bigger footing at the same depth. The fix is to get below the frost line so the footing is sitting in soil that does not freeze.

How deep is deep enough in Washington County ¶

Vermont's state building code references the frost depth map published by the American Society of Civil Engineers. For most of central Vermont, including Washington County, the design frost depth is 60 inches. Some municipalities adopt local amendments, so it is worth confirming with your town's zoning office before you dig.

We typically go to 60 inches on all our deck footings in this area unless the site has well-drained sandy soil and a southern exposure, in which case we discuss the local conditions with the inspector before finalizing the depth.

Footing diameter and load ¶

Depth is one part of the equation. Diameter is the other. A footing that is deep enough but too narrow will not distribute the load adequately, especially on softer soils. For a standard residential deck, we use a 12-inch diameter tube form for interior posts and a 16-inch form for corner posts carrying more load.

If you are getting quotes on a deck, ask each contractor what diameter footings they are planning and how they calculated the load. The answer will tell you whether they have actually done the math or are just going by habit.

Footings are the part of a deck you never see once the job is done. They are also the part that determines whether the deck is still level in ten years.