In November the garage door closed tight. In January it stops a half inch short on one side and the opener backs off like it hit something. The front walk has a lip that catches the snowblower.
Come June, the garage door closes fine again. But that sidewalk lip is still there, maybe worse.
That's two different problems on one property: the ground pushing up for the winter, and the ground giving way for good. They need opposite responses. Mix them up and you either pay to fix something that would have fixed itself, or wait on something that never will.
How Frost Heave Actually Works
Most people picture water under the slab freezing and expanding. Water does grow about 9% when it turns to ice, but that isn't the main event. Frozen ground can rise far more than in-place expansion could explain.
The real cause is ice lenses: flat layers of nearly pure ice that grow in the soil just below the freezing line. As frost works downward, the freezing zone pulls water up from the wetter soil beneath it, the way a paper towel wicks up a spill. That water freezes onto the lens, the lens thickens, and everything above it, your slab included, gets pushed up.
It takes three things at once:
- 1Cold that reaches down into the soil
- 2Fine-grained soilSilts and clays draw water upward well and are the most frost-susceptible soils. Clean sand and gravel aren't.
- 3A water supplyA high water table, soaked ground, or water running in from above.
Take away any one and heave mostly stops. You can't change the weather or your soil. Water is the one you control.
The thaw brings its own problem. Ground thaws from the top down, and WisDOT notes that frost can linger 48 to 72 inches below a road while the soil right under the pavement has already thawed. Meltwater can't drain through the frozen layer, so the soil just under a slab turns soft for a few weeks. A thaw followed by a refreeze loosens it further, which is how slabs come down crooked.
Why Brown County Concrete Heaves
- The soil. Much of the Green Bay area sits on red, clay-rich glacial till left by the Green Bay Lobe, plus old glacial lake clays. Those are exactly the soils that grow ice lenses.
- The cold goes deep. WisDOT declares state highways frozen once frost reaches 18 to 20 inches below the pavement, around December 22 on average.
- Your concrete has no blanket. Snow insulates the ground under it, and even a thin layer damps soil temperature swings. The lawn keeps its snow. Your driveway and walks get plowed bare, so frost can drive deeper under them than under the yard next to them.
- Water shows up in winter too. January thaws, downspouts dumping onto the walk, sump discharge next to the driveway, and the snow pile at the edge of the slab all deliver water right where the frost is working.
And here's why the slab moves while the house doesn't. Wisconsin's dwelling code requires footings at least 48 inches below grade (or below frost depth, if deeper), and on newer homes that includes stoops and landings. Driveways, garage aprons, walks, and patios don't get footings. They sit on a few inches of gravel, right in the frost zone.
How Settling Is Different
Settling isn't frost. It's soil leaving, compressing, or giving way, and the slab following it down. The usual causes around here:
- Washout from downspouts, sump discharge, and spring melt, which carries soil out from under slab edges. See voids under concrete.
- Loose foundation backfill that keeps compacting for years under stoops, aprons, AC pads, and walks along the house.
- Rotting roots and burrows from removed trees, chipmunks, and woodchucks.
- Thaw-softened soil in early spring, when loaded driveway slabs can press down into it.
The big difference: settling only goes one way. A settled slab doesn't come back up in summer. It stays low or gets lower.
Heave or Settling? Side-by-Side Symptoms
| Clue | Frost heave | Settling |
|---|---|---|
| Direction | Up | Down |
| When it's worst | January through March, while the ground is frozen | Any time, often noticed after the thaw |
| By June | Mostly back where it was | Same as spring, or worse |
| Joint between slabs | Lip appears in winter, shrinks or disappears in spring | Lip stays all year and grows |
| Slab next to the house | Pushed up toward the siding, door, or threshold | Pulls away from the foundation or tips |
| Tap with a hammer | Usually sounds solid while frozen | Often sounds hollow over a void |
| Typical spots | Low spots, downspouts, shaded north sides, plowed edges | Along the foundation, at downspouts, over old backfill |
| Fix | Drainage, then wait | Lift after the thaw |
A slab near a tree that rises a little every year and never comes back down is probably roots, which is a different fix. See our tree roots article.
What the Garage Door, Stoop, and Apron Are Telling You
These spots give the clearest winter signals, because each one meets something that doesn't move: the house.
The garage door and apron. The door closes onto the garage floor, and the apron butts against that floor at the door line.
- Heave: If frost lifts the front edge of the garage floor, the door stops short, shows daylight at one corner, or the opener reverses before the door seals. If it lifts the apron, the apron rises above the garage floor and meltwater runs inside. By May it usually goes back to normal.
- Settling: A step down from the garage floor to the apron that's still there in August and grows every year. Water pools against the door and freezes the bottom seal to the slab.
Before blaming the concrete, check for ice under the bottom seal or a door that needs adjusting. And don't turn up the opener's force setting to make the door close. That setting is part of what makes it reverse when it hits something.
The front stoop.
- Heave: The storm door scrapes or won't swing over the stoop in January. The step up into the house gets shorter.
- Settling: The gap between stoop and foundation widens, the stoop tips, and the step up gets taller and stays that way.
Plenty of older stoops weren't built on frost footings, so they do both: heave every winter and settle over the years.
The apron's other edges. Where the apron meets the driveway, a lip that fades by May is heave. One that never fades is settling.
Mark It in February, Measure It in May
This costs nothing and settles the question. A fall baseline makes it even better, so if you're reading this in October, start now.
You need: a tape measure, a 4-foot level or straight 2x4, a paint marker or lumber crayon, and your phone.
- 1Pick three or four spots: the worst walk joint, the apron at the garage door, the stoop, and any slab along the house.
- 2Mark the exact spot so you measure the same place every time. Offsets change along a joint.
- 3Measure the offset. Lay the level across the joint on the higher slab and measure down to the lower one. Note which side is high.
- 4Check the slope along the house. The model building code calls for paved surfaces within 10 feet of the house to fall away at least 2%, about 1" over a 4-foot level.
- 5Photograph it from the same spot each time, with the tape in the shot.
- 6Repeat in mid-February (peak frost), late April, and late May or early June.
Why wait until late May? WisDOT's spring weight limits, which follow the thaw under roads, average about March 9 to May 9, and ground under lawns and shaded north sides can lag behind. Green Bay's normal April low is still about 35°F. By late May (May's normal low is about 46°F), the frost is out and the soil has usually drained. Measure on a dry day, not right after a soaking rain.
A sample log
| Spot | Feb 15 | Apr 25 | May 30 | What it means |
|---|---|---|---|---|
| Apron vs. garage floor | Apron 3/4" high | 1/4" high | Flush | Frost heave. Fix the water. Nothing to lift. |
| Front walk, 2nd joint | 1/2" low | 3/4" low | 7/8" low | Settling, and it dropped more in the thaw. A lifting job. |
| Walk slab at the stoop | 1" high | 3/8" high | 1/4" low | Heaved, then ended low. Likely heave-and-drop. Fix the water, then lift. |
| Patio slab by the maple | 5/8" high | 5/8" high | 5/8" high | Not seasonal. Likely roots. |
How to read it: A change of 1/8" or less is measuring error. If May comes back to flush, or to your fall baseline, it was heave. If it ends lower than flush, lower than last fall, or lower than last May, something is settling. If it never changes, it isn't frost. Keep the log a second year if you can: a slab that finishes each May a bit lower or more tilted than the last is the heave-and-drop pattern.
What to Do About Each One
Frost heave: fix the water, then wait
- Extend downspouts well past the slab onto ground that slopes away. See how to extend downspouts.
- Move the sump discharge away from driveways and walks. In winter it builds ice on top and soaks the soil that's freezing underneath.
- Fix the grade so the ground drops at least 6" in the first 10 feet from the foundation. See soil grading around concrete.
- Seal the joints and the gap at the house so runoff stays on top of the slab. Caulking needs a dry, frost-free joint and about 40°F, so it's a fall or late-spring job. See caulking where concrete meets the house.
- Pile snow on the lawn, away from slab edges and the foundation.
- Leave the slab alone until June. Don't grind a frost-heaved lip in February. Once the slab drops back, the ground-down edge sits low.
You don't need us for any of that. Most heave problems shrink a lot once the water is handled.
Settling: a lifting job, after the thaw
A slab that's still low in late May has lost support, and that's what foam lifting is for. We drill a few 5/8" holes, about the size of a dime, and inject high-density structural foam that fills the voids first, then raises the slab in small, controlled increments. Foam weighs a few pounds per cubic foot, so it doesn't load soil that already gave way, and it's waterproof once cured, so spring melt won't wash it out. Walk on it right away and drive on it the same day. Pair the lift with joint caulking and the water fixes above, or the water that dug the first void will start another. Our concrete lifting page has the details.
Heave-and-drop, every year
Some slabs ride up on ice each winter and drop back onto soft, loosened ground each spring, a little lower or more tipped every time. Stoops and aprons along the house are the classic case, and the tilt usually aims water back at the foundation, which feeds next winter's heave.
The fix is both: handle the water, then lift once the frost is out to fill the voids and restore the slope. Then seal the joints.
The honest part: lifting doesn't stop frost heave by itself. Foam fills voids and supports the slab, but the clay below is still clay. If water keeps reaching it, the slab can still move in winter. If it keeps heaving hard after the drainage is fixed, the long-term answer is replacement over a deep, free-draining gravel base. We don't do replacement, but our concrete replacement questions will help you hire it out.
Why We Won't Lift a Frozen, Heaved Slab
Frozen ground is borrowed height. A slab sitting on ice lenses in March isn't where it will be in June, and foam can only raise concrete, never lower it. Lift the neighbors to match a heaved slab and they'll all be too high once it settles. Lift a low slab over frozen ground and you're pushing against support that disappears in April.
So our lifting season runs from spring thaw to freeze-up. Send photos in February and we'll tell you what we see, then ask you to keep that log until the frost is out. More on timing in concrete lifting in winter.