How frost heave works
Frost heave is not simply water expanding as it freezes. In fine-grained soils such as silts and some clays, water is drawn up toward the freezing front and forms layers of ice, called ice lenses, that grow and push the soil upward. The result can be several inches of lift under a step, a slab or a shallow footing. When the ground thaws in spring, the soil above the melted lenses is saturated and weak, and whatever was lifted often settles back unevenly.
Three conditions have to come together: freezing temperatures reaching into the soil, frost-susceptible soil and a supply of water. Remove any one of them and heave is much less likely. That is why drainage and insulation are as important as depth.
Where it happens
Frost heave is a concern wherever the ground freezes each winter. Among the areas we serve, that includes Boston, Chicago, Long Island, North Jersey, the Philadelphia suburbs, Baltimore and Northern Virginia. Frost is shallower or rare in the Gulf states and Florida, where seasonal clay movement and water are the bigger concerns.
What building codes require
The International Residential Code, adopted with local changes in most states, requires foundation walls, piers and other permanent supports to be protected from frost. The usual way is to extend footings below the frost line set by the local building department. The code also allows frost-protected shallow foundations that use insulation to keep the ground under the footing from freezing, and footings on solid rock. Footings may not bear on frozen soil. Some states set their own frost depths: Minnesota, for example, adopted a state amendment specifying minimum footing depths by county zone.
Your local building department can tell you the frost depth that applies to your address. Older structures, porches added later and many decks and stairs were often built with shallower footings than today’s code would require.
What frost damage looks like
- Front steps or stoops that rise in winter and pull away from the house
- Porches whose floors tilt or whose posts lift and drop
- Garage slabs that crack and heave at the door
- Deck footings that jack upward over several winters
- Doors that stick only in the coldest months
- Cracks in foundation walls of unheated additions, garages or crawl spaces
The main house foundation is usually less affected, because a heated basement keeps the soil around it warmer and the footings are typically deeper. The trouble spots are shallow, unheated attachments.
Frost and basement walls
Frozen, saturated soil against a basement wall can add lateral pressure, and repeated cycles can contribute to horizontal cracks and inward bowing, especially where water is not drained away. See bowing basement walls.
Repair options
- Fix water first. Downspouts that dump next to steps and porches supply the water ice lenses need. Extend them, regrade and make sure water drains away.
- Reset or rebuild steps and stoops on footings that reach below the frost line.
- Support porches and additions with new footings, helical piers or push piers that extend below frost depth. Helical piers are common for porches and decks because they can be installed with small equipment. See pier types compared.
- Insulate where appropriate. Rigid insulation below and beside shallow footings can keep frost from reaching the soil underneath, following frost-protected shallow foundation design rules.
- Lift or replace slabs that have heaved and settled unevenly; see mudjacking vs. polyjacking.
Preventing frost problems
- Keep gutters clean and downspouts extended well away from steps, porches and walls.
- Do not pile snow against the foundation; move it where meltwater drains away.
- Slope soil and hardscape away from the house.
- When building a porch, stairs or addition, confirm the footing depth with the permit.
- Keep crawl spaces and attached garages from being colder than necessary where practical.
Frost heave or foundation settlement?
Seasonal timing is the clue. Frost heave appears in winter and often partly reverses in spring. Settlement tends to progress in one direction over time. If a crack or a sticking door gets worse every year regardless of season, the problem is more likely settlement, poor soil or water, and a professional evaluation makes sense. See foundation cracks: which are serious?
Insurance and frost damage
Standard homeowners policies commonly exclude damage from freezing, settling and earth movement to foundations and slabs, though burst pipes are handled differently. Frost heave damage to steps, porches and patios is usually a maintenance repair paid by the owner. See does insurance cover foundation repair?
Which soils heave the most?
Clean sand and gravel drain quickly and rarely heave, because water cannot be drawn up through them to feed ice lenses. Silts and silty clays are the most frost-susceptible: they hold water and let it move upward by capillary action. That is why builders often place a layer of crushed stone under slabs and steps and backfill around foundations with free-draining material. On sites with silty soil and poor drainage, even footings at code depth can be stressed by frost acting on the sides of a wall, a process called adfreeze, where frozen soil grips the wall and lifts it.
Unheated spaces need extra attention
The ground under a heated house rarely freezes, because heat from the basement or slab keeps it warm. That protection disappears under garages, porches, sunrooms that are closed off in winter and houses that are left unheated. Vacant homes are especially vulnerable: if the heat is turned off for a winter, frost can penetrate below footings that were never designed to resist it. If you winterize a seasonal home, ask a professional whether the foundation needs protection.
Signs to check in early spring
- Steps or stoops that have lifted and not returned
- New cracks in garage slabs or at the base of foundation walls
- Doors that stuck in winter and still stick after the thaw
- Gaps between porches and the house that have widened since fall
- Deck posts that have risen above their original position
Photograph these once in late winter and again in early summer. If the movement fully reverses, it is likely seasonal frost; if it stays, a repair may be needed. See chimney or porch pulling away from the house.
Frost and older stone foundations
Many older homes in the Northeast have fieldstone or rubble foundations with shallow footings, or none in the modern sense. Heated basements usually protect them, but water freezing in mortar joints and in soil against the exterior can loosen stones over time. Repointing, drainage and keeping the basement heated help. See stone and brick foundation repair.