What Is a Frost Line?

The frost line is the maximum depth ground freezes in winter; footings are poured below it so frost heave can’t move the structure.

The frost line (frost depth) is how deep the soil freezes in a given climate. Footings poured below the frost line bear on unfrozen soil, so the repeated freeze-thaw cycles common in southeastern Pennsylvania cannot heave or tilt a pergola or pavilion. Local code sets the required depth.