Frost Line (Frost Depth)
The maximum depth to which the ground freezes in winter. Footings for decks, fences, and outbuildings must extend below the frost line to prevent heaving.
In Detail
Frost line depth varies by location: 6–12" in the deep South, 36–48" in the upper Midwest and New England, 60"+ in parts of Alaska. Local building codes specify the required footing depth. Frost heave occurs when water in soil freezes and expands, lifting anything above it. Footings above the frost line will move annually, cracking foundations and racking structures. Contact your local building department for the frost depth requirement in your jurisdiction.