You pull in on a January night, salty slush dripping off the wheel wells, and there it is: the crack running from the back corner out toward the door. Was it always that long? Was it always that wide?
Almost every garage floor has cracks. Most don't mean a thing. A few are telling you something is going on underneath.
Here's how to tell which is which.
The Six Cracks You'll Find in a Garage Floor
| Crack type | What it looks like | Usual cause | Worry level | Typical fix |
|---|---|---|---|---|
| Hairline shrinkage | Pencil-line thin, random or web-like, flat on both sides | Concrete shrinking as it cured | Low | Fill during coating prep |
| Control joint crack | Runs down the bottom of a straight saw cut or tooled groove | The joint doing its job | Low | Leave it, seal it, or handle it during coating |
| Off-joint crack | Wanders diagonally across a panel, often from a corner or drain | Joints spaced too far apart or cut too shallow | Low to medium | Fill during prep, keep an eye on it |
| Settling crack with an offset | One side sits lower; you can catch a fingernail or a toe on it | Soil under the slab compressed or washed out | Medium to high | Lift the low side, then repair and coat |
| Apron or door crack | At the door line, or the outside apron has dropped or tipped toward the garage | Frost, meltwater, salt brine, loose fill | Medium to high | Lift the apron, seal the joint |
| Widening or wall-related crack | Grows season to season, or lines up with cracks in the foundation wall | Bigger movement than a floor problem | High | Get it looked at before anyone coats or lifts |
Hairline shrinkage cracks
Concrete loses water as it cures and shrinks a little, and a big, thin garage slab relieves that stress by cracking. Hairlines that show up early and then stop aren't a structural problem. They do collect dirt and salt, which is why they get filled before a coating goes down.
Control joint and off-joint cracks
See those straight grooves sawn or tooled across the floor? They're control joints. The concrete is thinner at each groove, so when the slab shrinks and has to split somewhere, it splits there, out of sight in the bottom of the cut. A common rule of thumb is to space joints (in feet) about two to three times the slab thickness (in inches), so a 4-inch slab wants a joint roughly every 8–12 feet, cut about a quarter of the way through. A crack in the bottom of a joint means the joint worked.
When joints are too far apart, too shallow, or cut too late, the slab cracks wherever it wants. That's the diagonal crack that starts at a floor drain or the corner of a step. Usually still cosmetic.
Settling cracks with an offset
This is the one to pay attention to. If one side sits lower than the other, part of the slab lost the ground holding it up. Usually water is behind it: meltwater sneaking in at the door, or a wet spot along the house that keeps softening the dirt underneath until the concrete drops into the hollow.
In attached garages, a common suspect is the strip along the house. The ground next to a foundation is backfilled after the walls go up, often loosely, and the slab edge can sit right on top of it. Add a downspout at the corner or a sump line nearby, and that fill keeps settling.
Cracks at the apron and overhead door
The apron (the slab just outside the overhead door) lives outside. The garage floor lives inside. Up here, the apron rides through freeze-thaw all winter while the floor beside it stays somewhat sheltered, and the joint between them takes the beating. Every spring melt, water runs down the driveway, soaks in at that joint, and gets under the apron.
Give it a few winters and you get one of these:
- The apron sinks and tips back toward the garage, so meltwater runs toward the door instead of away.
- The apron heaves in winter and drops back in spring, a little more crooked each time.
- The joint opens up into a gap that swallows water and salt every time you drive in.
If water ponds at your threshold or the door seal no longer meets the concrete, the apron is the problem, not the floor.
Widening cracks and cracks that line up with the walls
A crack that keeps growing, cracks that continue into the foundation wall, stair-step cracks in block, or a slab pulling away from the wall all point to more than a floor problem. The garage slab is usually separate from the foundation, so a cracked floor alone doesn't mean the house is in trouble. But when the walls are cracking too, have a structural engineer look first. You don't need us for that part.
The 5-Minute Garage Floor Self-Check
Grab a tape measure, a credit card, a pencil, and your phone.
- 1Run your hand across the crackFlat on both sides? Probably cosmetic. Feel a step? The slab moved.
- 2Try the credit cardA crack wider than a credit card is thick, or one that's getting wider, is more likely movement than plain shrinkage.
- 3Check the doorIs there a gap between the apron and the floor? Does water pond at the threshold?
- 4Look at the wallsAny cracks in the foundation or block wall that line up with the floor crack?
- 5Mark it and date itPencil a mark at each end and photograph it with the tape across it. Check again after the spring thaw. A crack that hasn't changed through a Wisconsin winter is almost always done moving.
What Salt Does to a Bare Garage Floor
Every winter, the snow packed in your wheel wells melts on the floor, loaded with road salt, and finds every crack and joint. Deicer brine soaks into bare concrete and makes freeze-thaw damage worse. Some deicers (calcium and magnesium chloride especially) can also react with the cement paste itself and form an expansive compound that cracks it from inside, even above freezing. Cracks give that brine a head start, which is one more reason to fill them.
Matching the Fix to the Crack
Cosmetic cracks: repaired as part of coating prep
For hairlines and off-joint cracks that aren't moving, the repair is part of putting down a garage floor coating. Standard prep is to open the crack slightly so it's clean and has something to grip, vacuum it out, fill it with a rigid repair material, and grind it flush. Then the whole floor gets prepped so the coating bonds to the concrete and the patches alike.
Control joints get a decision: leave them as a coated groove, or fill them so the floor looks seamless. Each has tradeoffs, and we'll walk you through which makes sense for your slab.
Sunken sections and sunken aprons: a lifting job
If part of the floor or the apron has dropped, filling the crack is a bandage over a step. The low side needs to come back up.
That's what foam lifting does. We drill a few 5/8" holes (about the size of a dime), inject high-density structural polyurethane foam under the slab, and raise it in small, controlled increments. The foam fills the voids that let it sink, it's waterproof once cured so spring melt won't wash it out, and you can drive on it the same day. For an apron tipped toward the door, lifting it so water drains away again is usually the main fix.
After the lift, the joint between the apron and the garage floor (plus any crack that opened while it sank) should be sealed with a flexible caulk so meltwater and brine stop getting underneath. We can do that sealing right after the lift.
Planning a coating too? Lift first. Coating a floor that's about to move is backward.
Widening or structural cracks: a closer look first
If the crack is still growing after a winter of watching, or the walls are involved, find out what's moving and why before anyone coats or lifts. A foundation or drainage problem may have to come first.
Tearing the floor out only makes sense when it's broken into many pieces or badly heaved. New concrete shrinks and cracks too.
How a Coating Hides Cracks and Handles Salt
Once cracks are filled and ground flush, the coating goes over the whole floor as one continuous layer. A patched crack that would stick out on bare gray concrete mostly disappears under the finished coating.
The bigger deal in Wisconsin is salt. A coating puts a sealed surface between the brine and the concrete, so slush sits on top where you can squeegee it out the door instead of soaking in. Oil and other drips are far easier to clean up.
The honest part: a coating protects the surface. It doesn't hold a moving slab together, which is why we sort out movement first and coat second.