Frost line (frost depth)
The maximum depth to which soil freezes in a given location. Footings must bear below it so freezing ground cannot lift the structure.
What does frost line (frost depth) mean?
Frost depth is set by the local jurisdiction rather than by any national standard, and neighboring towns can require different depths. It ranges from essentially zero along the Gulf Coast to several feet in northern states. Building above the frost line is a leading cause of foundation movement in cold climates, particularly for porches, steps, and garages added later.
In practice
The first question a foundation contractor asks a new building department is what the local frost depth is, because it drives the whole excavation.