Frost heave is the upward movement of soil and anything resting on it, caused by groundwater freezing and expanding within the soil during cold weather, capable of lifting a shallow foundation footing if it isn’t placed deep enough. This is why building codes require footings to be placed below the local frost line, the depth to which the ground typically freezes in winter.
Frost heave forces can be surprisingly strong, capable of lifting even substantial structures if a footing is placed shallower than local conditions require.
What Does Frost Heave Involve?
- Groundwater freezing and expanding within soil
- Upward pressure on shallow foundation elements
- Repeated freeze-thaw cycles worsening the effect over seasons
- Footing depth requirements set by local frost line data
Why Is Frost Heave Important?
A footing placed above the frost line can be lifted unevenly as the soil beneath it freezes and expands each winter, and then settle back down unevenly as it thaws, gradually cracking the foundation above it over successive seasons. This is precisely why frost depth requirements vary so much by region, with colder climates requiring significantly deeper footings than warmer areas where the ground rarely freezes at all.
Unheated structures like detached garages or porches are particularly vulnerable to frost heave, since they don’t benefit from the ground-warming effect of a heated building above, and their footings often need to go just as deep as the main house despite the lighter load involved. Insulating the soil around a shallow foundation is one alternative some builders use in certain climates, keeping the ground beneath warm enough to avoid freezing even when the footing itself sits above the traditional frost depth.
Frost Heave vs. Foundation Heave
Frost heave specifically refers to upward movement caused by freezing groundwater. Foundation heave is the broader category, which can also result from expansive clay soil swelling with moisture even in climates that never freeze.