"Uneven" covers a lot of ground. The driveway slab that catches the snowblower. The patio corner that rocks when you step on it. The garage floor where water runs toward the door instead of the drain. The stoop pulling away from the house.
They look alike from the kitchen window, but they're different problems with different fixes, and only some are lifting jobs.
Spend fifteen minutes with a few basic tools before you call anybody, us included, and you'll know which one you've got. (Public sidewalk? The rules on who fixes it are their own topic, covered in our uneven sidewalk fix guide.)
The 15-Minute Self-Check
What you need: a tape measure, a 4-foot level, a straightedge (a straight 8-foot 2x4 works) or a string line with a brick at each end, and your phone.
- 1Pick something that hasn't moved. The garage floor, the foundation, the top step, the curb. "Uneven" only tells you two things are different. A fixed reference tells you which one moved.
- 2Bridge the joint. Rest the straightedge on the higher slab so it hangs over the lower one, and measure the gap at both ends of the joint. Equal gaps mean the slab moved evenly. Different gaps mean it tipped.
- 3Run a string on long stretches. On a driveway or long walk, stretch a line tight between two bricks set on slabs that haven't moved. Measure down from the string to each slab in between. The biggest number sank. The smallest rose.
- 4Check the slope. Set the level on each slab next to the house, pointing straight out. The bubble should show it falling away from the foundation. A widely used model building code calls for paved surfaces within 10 feet of a foundation to slope away at least 2 percent, about 1/4" per foot.
- 5Tap and step. A dull, hollow sound under a hammer handle usually means a void. A corner that rocks underfoot has lost support.
- 6Look at the top. A slab at the right height with a flaking, pitted, or wavy surface is a different problem than a slab that moved.
- 7Photograph and date it, with the tape measure in the shot.
Measure Again After the Thaw
Between December and April, the ground may still be frozen. In fine-grained soils like clay and silt, water gets drawn toward the freezing layer and builds up as ice, pushing the ground up. That push varies with the soil, the moisture, and how much snow is insulating each spot, so one slab can ride noticeably higher than its neighbor in March and be back in line by late May.
Measure while it's frozen and again after the frost is out. If the number shrank, frost was part of the story. If it held, the change is permanent.
What Your Measurements Mean
It Sank
The signs: lower than your fixed reference, a gap or washed-out soil along the edge, a downspout or sump outlet nearby, a hollow sound, and a little worse after every wet spring.
The fix: lift it. Foam goes in through a few 5/8" holes (about the size of a dime) and raises the slab in small, controlled increments, packing the hollow underneath as it goes. Walk on it right away, drive on it the same day. Mudjacking also lifts; replacement only makes sense if the slab is broken up too. Our guide to how concrete lifting works compares the methods, and best uses for foam lifting covers where foam fits.
Then deal with the water that sank it, or you'll be measuring the same slab again in three springs.
It Heaved
Something pushed it up, and the two usual culprits need different approaches.
Tree roots: a tree within reach, a slow rise year after year, a hump or crack on the side facing the tree. No lifting method can push concrete down. Your options are grinding off a small lip, lifting the slabs around it into a gentle slope (fine on walks, rarely acceptable on patios or aprons), or replacing the high slab. Tree roots lifting concrete has the full rundown.
Frost: worst in late winter, often a whole slab, usually near a wet spot, a low area, or a downspout, and better by May. A classic Green Bay version: in February the garage door stops short or the opener reverses, because the concrete under the door rose.
Frost heave calls for patience first. Wait for the thaw and re-measure. If the slab settles back where it belongs, your job is keeping water out from under it: extend downspouts, fix grading, seal the joints. If it settles back lower or crooked, it's now a sunken slab and a lifting job.
And don't grind down a frost-heaved edge in March. You may be grinding off concrete that would have dropped back on its own by May.
It Tipped Toward the House
The signs: the edge at the foundation sits lower than the outer edge, the level's bubble runs toward the house, water puddles against the wall after rain, and the gap at the wall keeps widening.
What happened: the soil along your foundation was backfilled when the house went in, and it keeps settling. Runoff pouring into an open joint at the wall washes out more of it, and once the slab tips, it sends even more water into the gap.
The fix: lift the slab until it pitches away from the house, then seal the joint at the foundation with a flexible sealant over backer rod. Order matters. Seal a tipped slab where it sits and you lock in the slope that's aiming water at your wall. Details in caulking where concrete meets the house.
If water is already coming through the basement wall, add a waterproofing pro to the list.
It's Broken Into Pieces
The signs: several loose chunks that move separately, pieces missing, edges crumbling away.
The fix: replace it. Lifting moves concrete; it doesn't repair it. A slab with one crack, or two big pieces, can usually be lifted and the crack sealed. Rubble can't.
We don't do replacement, so this is one where you don't need us. Before hiring anyone, read our questions to ask before replacing concrete. New concrete on the same washed-out base can settle just like the old slab did.
Replacement timing is typical; a contractor's schedule and the weather can stretch it. New-concrete cure times from Concrete Network.
The Slab Is Level, but the Surface Isn't
The signs: the slab sits at the right height and slope, but the top is rough, flaking, pitted, or wavy. Usually that's road salt and freeze-thaw eating the surface (scaling and spalling), flaws from the original finishing, or a patch that never sat flush.
The fix: surface work, not lifting. Grinding takes down ridges and high spots. Resurfacing covers a worn top, though a thin outdoor overlay has to be done right to survive Wisconsin winters. Widespread damage may mean replacement. On concrete that's still sound, a penetrating sealer helps it shed water and salt, which slows further scaling. See concrete spalling causes.
In a garage, a floor coating covers a lot of cosmetic wear (filling cracks is part of the coating prep), but if the floor also sank, lift first and coat second.
Cause, Fix, and Who Does It
| What caused it | The fix | DIY or pro? |
|---|---|---|
| Downspout or sump line washing out soil | Redirect the water, then lift the slab | Water: DIY. Lift: pro. |
| Loose backfill along the foundation | Lift so it slopes away, then seal the house joint | Lift: pro. Sealing: DIY if it already slopes away. |
| Tree roots | Grind a small lip, lift the neighbors to blend it, or replace | Pro, plus an arborist before cutting roots |
| Frost heave | Wait for the thaw, re-measure, fix drainage | Mostly DIY until spring tells you more |
| Voids from rotted roots or burrowing animals | Evict the animals, then lift and fill | Wildlife control if needed, then a lift |
| Leaking pipe or broken drain tile under the slab | Fix the leak, then lift | Plumber or drain contractor first |
| Newer concrete settling on a poorly packed base | Call the original installer about their warranty | Installer first |
| Salt and freeze-thaw scaling | Seal sound concrete to slow it; resurface or replace if it's bad | Sealing: DIY or pro. Resurfacing: pro. |
| Slab broken into many pieces | Replace | Pro |
Most uneven slabs trace back to water, so check where your roof and sump water end up. Our guide to extending downspouts covers the cheapest fix of all.
When to Leave It Alone
Not every uneven slab needs fixing right away, and frost season is never the time to decide.
- The offset is tiny and off the walking path. A useful yardstick is the ADA standard for accessible walking surfaces, which allows a vertical change up to 1/4" without beveling. Under that, on a slab nobody crosses in the dark, it can wait.
- It moved once and stopped. If your dated measurements hold for a full year, the slab drains away from the house, and nobody's tripping, there's no clock running.
- It's cosmetic. Hairline cracks with both sides level, a slightly wavy finish, or stains and mildew aren't uneven concrete. Stains are a cleaning job (which we also do), not a leveling job.
- It's getting replaced soon anyway. Don't pay to lift a driveway you're tearing out next summer.
Don't wait if water runs toward the foundation, if there's a trip edge on a walk or stoop people use daily, if a slab rocks under the car, or if your numbers get worse every season.