Frost heave is water in soil freezing into layers of ice that push the ground up, lifting anything set above the frost line. Decks slope, fence posts rise and lean, gates stop latching, steps pull away from the house. The fix is footings below the frost line (5 feet around Duluth, at least 48 inches in Wisconsin) and drainage.
How frost heave works, in plain terms
Water expands when it freezes, but that alone doesn't explain how the ground can lift a deck by an inch or more. The bigger force is ice that keeps growing. University of Minnesota Extension describes heaving as an expansion of the soil caused by frozen water separating out and expanding in it, and notes that deep frost produces flat layers of ice, called ice lenses, running parallel to the surface (UMN Extension). A National Research Council of Canada building digest explains why the lenses keep growing: freezing at that plane acts like drying, so water in the unfrozen soil beneath moves toward it and joins the ice, which jacks up the ground above (NRC Canada, CBD-26).
The same digest lists three requirements for heave: a freezing plane in the soil, a fine-grained soil that moisture can move through, and a supply of water. Take one away and heave drops off. That's why silt and clay heave and clean gravel, which drains instead of wicking, heaves far less.
There are two ways heave grabs a structure. Ice lenses forming under a shallow footing lift it from below. And frozen soil can bond to the sides of a post or concrete pier and carry the lift up to it, which a second NRC digest calls adfreezing; that grip can move even a footing set below frost (NRC Canada, CBD-128). The digest also explains why heave worsens over time: soil drops into the cavity under the lifted piece, so it can't settle back, and each winter ratchets it higher.
Frost depth in Minnesota and Wisconsin
In Minnesota, the building code puts the minimum footing depth for frost at 5 feet in Zone I, which includes St. Louis and Carlton counties (Minn. R. 1303.1600). Duluth's deck page says the same thing in inches: footings 60 inches below grade, and the footing inspection checks the diameter, the depth, a belled bottom and that there's no water in the hole (City of Duluth). The belled bottom matters for adfreezing too: the NRC digest's answer to frost gripping a column is anchoring it to a footing below frost that resists the uplift, which is what a bell at the base of a pier does.
In Wisconsin, footings and foundations, including those for landings and stoops, must be placed below the frost penetration level or at least 48 inches below adjacent grade, whichever is deeper (Wis. Admin. Code SPS 321.16), and the code's deck section applies that frost rule to decks attached to a dwelling and to detached decks that serve as an exit (SPS 321.225). Decks, stoops and fences set on shallow blocks or short posts are the ones that heave.
What heave damage looks like
Heave is seasonal, so watch for things that change between January and July: a gate that drags in winter and swings fine in summer, a door onto the deck that sticks only when the ground is frozen, a step that rocks after the thaw. On a deck, the house side is often steady because it hangs on the ledger, while the outer posts ride up, so the deck can tilt toward the house and send water against the siding.
| Structure | Symptom | Fix |
|---|---|---|
| Deck posts and footings | Deck slopes or rocks, a post lifted off its bracket, beam no longer level | Shore the deck, replace the footing with one below frost depth, re-level |
| Deck stairs | Bottom step pulling away from its landing, stringers racked, a gap at the top | Set the landing on a frost footing or rebuild the stair base, then reset the stringers |
| Fence posts | Posts taller than their neighbors, a wavy fence line, posts leaning after thaw | Pull and reset deeper with gravel at the base, or replace rotted posts |
| Gates | Gate drags or won't latch in winter, then works again in summer | Reset the hinge and latch posts below frost depth; adjust the gate after |
| Concrete stoops and front steps | Steps tipping toward or away from the house, a widening gap at the wall | Rebuild the steps on frost footings; a sound slab can sometimes be lifted back into place instead |
| Low platforms and deck blocks | Seasonal rocking and gaps at a door threshold | Keep them detached from the house with room to move, or rebuild on frost footings |
The fixes that actually hold
Resetting versus replacing. A sound post that was simply set too shallow can sometimes be pulled, the hole dug to frost depth, and the post reset on a gravel base with drained backfill. A post that has rotted at the ground line gets replaced. For a deck, the fix is almost always a new footing: the deck is shored, the old shallow pad or block comes out, a new pier goes to 5 feet in Minnesota or at least 48 inches in Wisconsin, and the post and beam are re-leveled on it. Shimming a heaved post back to level only resets the clock, because next winter lifts the same footing again. See deck repair for footing and post work, and fences and gates for leaning posts and sagging gates.
Drainage. Less water in the soil means less ice. Point downspouts away from deck posts, stoops and fence lines, grade the soil so meltwater runs away from footings, and backfill around posts with gravel rather than the clay that came out of the hole. That gravel does double duty: the NRC digest's first defense against adfreezing is a drained gravel zone around the column, so the lift is lost in the gravel instead of reaching the post, provided the zone stays drained. The digest also notes that a slimmer column in the frozen layer gives frost less to grip.
Costs. Footing work is quoted after we see the deck, because the number of posts and access decide the price. Our planning range for board and railing repairs is $800–$3,500, and when a whole fence line is heaving, replacement runs $25–$60 per linear foot installed with posts below frost depth. Excavation waits for thawed ground, so book footing work for spring.
Why is my deck higher in winter than in summer?
Because the footings under it sit above the frost line. When the ground freezes, ice lenses form under and around those shallow footings and push them up, and when the soil thaws in spring they drop back part of the way. That seasonal rise and fall loosens connections and racks stairs. The fix is new footings below frost depth: 5 feet in the Minnesota counties around Duluth and at least 48 inches in Wisconsin.
Can a heaved fence post be pounded back down?
Not for long. Pounding a lifted post back down leaves it at the same shallow depth, so the next freeze lifts it again, and hammering can crack the post or its concrete collar. A post that keeps rising needs to be pulled, set deeper, bedded on gravel and backfilled with material that drains. If the post has rotted at the ground line, replace it rather than resetting it.
Do deck blocks work in Duluth?
Only for low, detached platforms that can move a little. Duluth treats a deck that isn't attached to a building, is no more than 30 inches above grade and has no frost footings as a platform, which needs no permit. Deck blocks sit on the surface, so frost will lift them. An attached deck needs footings 60 inches below grade, and a detached deck over 30 inches needs a permit, so ask Construction Services before building one on blocks.
Why are my front steps pulling away from the house?
Usually because the stoop or steps sit on shallow footings while the house foundation goes below frost. Each winter the steps heave and tilt independently of the house, and the gap at the wall opens. Wisconsin's code puts footings for landings and stoops below the frost penetration level or at least 48 inches below grade, whichever is deeper. Sealing the gap stops water from feeding the problem; lasting repair means lifting or rebuilding the steps.
Send photos of the heaved posts, stairs or steps and a note on what changes between seasons. Request a quote or call or text (715) 919-7956; every job gets a flat written quote before work starts, and our on-site service fee is waived when you approve the work.