Frost depth and frost heave

One-line orientation

In cold climates, footings generally extend below the local frost line. Frost heave can lift a shallow foundation when water migrates into freezing, frost-susceptible soil and forms expanding ice lenses.

Key points

• Frost depth (frost line): the depth at which soil freezes in a given climate. It governs how deep foundations must be placed to avoid frost damage.
• Frost heave: water migrates into a freezing zone in frost-susceptible soil and forms growing ice lenses. Their expansion pushes the soil and foundation upward.
• Design rule: in colder climates, foundations must extend below the frost depth so the bearing soil stays unfrozen.
• Shallow frost-protected footing (alternative): instead of digging below a deep frost line, place rigid foam insulation horizontally and vertically around the footing to keep the ground temperature stable. This permits a shallower foundation while still preventing heave, reducing excavation, material, and cost.
• Slab-on-grade works well in areas with minimal frost depth — i.e., where the frost line is shallow enough not to drive a deep footing.

Confusions / comparison

Depth	What it is	Driven by
Frost line (frost depth)	How deep soil freezes	Climate (cold regions = deeper)
Water table	Top of groundwater	Site hydrology / season
Bearing depth	Depth to adequate bearing soil	Soil strength / load

	Standard cold-climate footing	Shallow frost-protected footing
Strategy	Excavate below the frost line	Stay shallow, insulate the ground
How heave is avoided	Bearing soil never freezes	Insulation keeps soil from freezing
Trade-off	More excavation / cost	Less depth, material, and cost

Related

→ Settlement: differential vs uniform (this topic): the downward counterpart to heave’s uplift · Deep foundations: piles vs caissons (this topic) · Soil improvement methods (this topic).