Every March it's the same puddle. Snow melts off the car and parks itself in the back corner by the house door instead of heading out under the overhead door. By April there's a white salt ring, and the concrete there is starting to flake.
Or it's the basement, and the vinyl plank installer says the floor needs leveling first.
Either way, somebody suggests a few bags of self-leveler. Sometimes that's right. Often it turns a small problem into a bigger one.
"Level" Is the Wrong Target in Most Garages and Basements
Garage floors are built to slope. The International Residential Code, a national model code, requires the parking area of a garage floor to be sloped so liquids move toward a drain or the main vehicle door. It doesn't set a number. The common rule of thumb is a minimum of about 1/8 inch per foot, roughly a 1 percent fall.
In a 22-foot-deep garage, 1/8 inch per foot puts the back wall about 2-3/4 inches higher than the overhead door threshold. Make that floor dead level and every drip of slush stays where it lands.
(New one- and two-family homes in Wisconsin fall under the state's Uniform Dwelling Code, which has its own garage floor section. Pouring new? Ask your building inspector.)
Many basement floors pitch toward a floor drain, at least in the area around it. The drain is supposed to be the low point. Flood that area with a product that flows out flat and the drain can end up sitting a hair high, with water stranded around it.
What you want instead is flat, with the right pitch. One vinyl plank install guide we checked wants the concrete flat within 3/16 inch over 10 feet, and separately allows it to slope up to 1 inch over 6 feet. Flooring cares about flat. It tolerates slope fine. (The terms are sorted out in concrete leveling types explained.)
Measure Before You Buy Anything
- A straightedge and a stack of pennies. Lay a straight board or aluminum screed (6–10 feet) across the floor in several directions. Where you see daylight, slide pennies under it. A penny is about 1/16 inch thick, two are about 1/8 inch, four are about 1/4 inch. Chalk a circle around each low spot and write the depth in it.
- A 4-foot level for slope. Point it toward the door or drain, raise the low end until the bubble centers, and measure the gap. A 1/2-inch gap is about 1/8 inch per foot. If the floor runs toward the back wall or the house, that's the most important reading you'll take.
- A laser level for the big picture. Read a tape at grid points every 4 feet and write each number on a sketch of the floor. A smooth slope with a few wobbles is a finishing issue. A sudden change at a joint or crack means a section moved.
- A bucket of water. Sweep, pour at the back wall, wait 20 minutes, and chalk around what's left. In a Green Bay winter, skip the bucket: the slush puddles are your map.
Also run your hand across every joint and crack (a step you can feel means one side moved) and tap suspicious areas with a hammer handle. Solid concrete rings; a dull thud means a void.
Four Reasons a Floor Is Uneven
A section sank: lift it
What it looks like: a step at a joint or crack, a panel that rocks or sounds hollow, a puddle that collects against the house wall or just inside the overhead door, and a problem that gets a little worse every spring.
Why it happens: something under that section compressed or washed out. In attached garages, the usual suspect is the strip along the house, where the dirt beside the foundation was backfilled loosely after the walls went up. A downspout at the corner or meltwater sneaking in at the overhead door keeps it settling. Under basement floors, it's often a plumbing trench that was never well compacted.
The fix: raise the section back where it belongs. Foam lifting works through 5/8-inch holes, about the size of a dime. High-density structural foam fills the voids, then lifts the slab in small, controlled increments, so a garage panel goes back to the pitch line instead of just "up." Walk on it right away; park on it the same day.
Basement sections can often be lifted too, with extra care around the floor drain, sump pit, and buried plumbing, since foam travels into open gaps. Send photos and we'll tell you if it's a job we'd take. Covering a sunken section only hides the dip; the void stays (see voids under concrete).
High spots, ridges, and small lips: grind them
What it looks like: a hump you can rock a straightedge on, a ridge left from the original finishing, or a small lip at a crack where both sides are solid.
The fix: diamond grinding, with a cup wheel on an angle grinder for a short ridge or a walk-behind floor grinder for anything bigger. It's the only method that takes concrete down, which makes it right for a high spot and wrong for a low one.
The dust is the serious part. Grinding concrete throws respirable crystalline silica, which OSHA links to silicosis, lung cancer, COPD, and kidney disease. Indoors, OSHA's construction rule has crews put a shroud and dust collector on handheld grinders, and run floor grinders with a water feed or dust collector plus a HEPA vacuum between passes. If you grind yourself, rent a shrouded grinder with a matching vacuum and wear a real respirator.
We grind garage floors as part of coating prep. For the bigger question of when grinding beats lifting, see concrete grinding vs. lifting.
Shallow dips and waves on a stable floor: patch or self-level
What it looks like: low spots from a sixteenth to a quarter inch or so, spread around, with no steps at the joints and no change from year to year. That's how the floor was finished, not something that happened to it.
The fix:
- Small, isolated dips: a feather-edge floor patch, troweled smooth.
- Wider areas under new indoor flooring: self-leveling underlayment, poured over a ground, vacuumed, and primed floor.
The catches: self-leveler seeks level by design, so flooding a sloped floor with it erases the pitch. Most bagged products are made for dry interior floors under flooring, not a garage full of salt slush. See self-leveling concrete problems and concrete floor prep.
Coating a garage? A coating follows the floor. A birdbath stays a birdbath, just a shinier one. Shallow waves don't matter much under a coating; a low spot that holds water does, and it's usually a sunken section that needs lifting.
The whole floor is out of plane: overlay or replace
What it looks like: your laser sketch shows the entire floor tilted toward the house, or wavy everywhere beyond what patching can handle, or the slab is broken into many pieces or crumbling.
The fix: a bonded topping or overlay from a concrete contractor, or tear-out and replacement. Both cost real money, and an overlay adds height everywhere.
First, double-check that it's really the whole floor. A garage that "slopes toward the house" is often one or two sunken panels along the house wall, and lifting them restores the drainage. When it truly is the whole slab, see can you pour concrete over existing concrete and when concrete is too damaged to lift.
What You Measured, What It Means, What Fixes It
| What you measure | Likely cause | Method | DIY or pro |
|---|---|---|---|
| Step of 1/4" or more at a joint or crack | A section settled | Lift the low side | Pro |
| Panel rocks or sounds hollow | Void underneath | Lift (foam fills the void as it lifts) | Pro |
| Floor runs toward the back wall or house; puddle against the wall | Panels along the house sank, or the floor was poured wrong | Lift if it sank; replace if it was built that way | Pro |
| Puddle just inside the overhead door | The apron outside sank and tips back toward the garage | Lift the apron, then seal the joint | Pro |
| Hump you can rock a straightedge on | Finishing ridge or a high spot | Grind | DIY with a rental for small spots; pro for big areas |
| Lip at a crack, both sides solid, under about 1/4" | Minor offset that isn't moving | Grind, or fill during coating prep | Either |
| Scattered dips 1/16"–1/4", no steps | Original finishing | Patch before flooring; often fine under a coating | DIY or flooring installer |
| Wider low area, stable, getting tile or plank | Original finishing or an old patch | Self-leveling underlayment | Flooring installer |
| Hump near the overhead door that's worse in February | Frost heave under the slab or apron | Wait for spring, fix the water, re-measure | Watch it |
| Wavy everywhere, many cracks, crumbling | The slab itself is worn out | Overlay or replacement | Concrete contractor |
A Worked Example: The Back-Corner Puddle
Say your garage is 22 feet deep. At 1/8 inch per foot, the back wall should sit about 2-3/4 inches above the overhead door threshold.
You shoot the laser. The back-right corner reads 2-5/8 inches above the threshold. Close enough. The back-left corner, where the house door is and a downspout empties just outside, reads 1-7/8 inches. That corner is about 7/8 inch low, there's a 1/2-inch step at the control joint, and the panel thuds when you tap it.
Option 1: self-leveler. You'd build up a layer about 7/8 inch thick at the wall and feather it out past the joint. The void stays, the panel keeps settling, the thin edge breaks up under tires and salt, and the step up to the house door gets shorter.
Option 2: lift the panel. Raise it about 7/8 inch, back to the pitch line. The step at the joint closes, water runs toward the door again, and the void gets filled. Then reroute that downspout.
Then the surface. Fill the cracks, grind the floor, and coat it. Grinding also takes care of any small leftover ridge at the joint. That's the order: position first, surface second.
Three Ways People Make It Worse
Leveling over a slab that's still moving
Anything bonded to a moving slab moves with it. Pour patch over a section that's still settling and it tends to crack right along the old step. Then you pay to lift the slab anyway. Mark and date the cracks and re-check after the spring thaw. A crack that hasn't changed through a Wisconsin winter is usually done moving.
Killing the drainage
The classic: filling the garage birdbath until it's perfectly level, so the water now sits against the house step instead. In basements, it's burying the pitch that fed the floor drain. Decide where the water will go before any material goes down, and test it with a bucket after.
Raising the floor under doors
Every eighth of an inch you add has to fit somewhere:
- The overhead door. Its bottom seal is set to the old floor height.
- The house entry door. If it swings out over the garage floor, a higher landing can make it drag.
- Steps. Building up the bottom of the garage steps shortens the first step, and an odd-height step trips people.
- Basement headroom. Older Wisconsin basements rarely have any to spare.
Lifting doesn't have this problem, because it puts a sunken slab back where it started instead of adding material on top.
Who does what: we lift sunken garage panels and aprons, caulk joints and cracks, and coat garage floors (crack filling and grinding are part of the prep). Self-leveling underlayment is a flooring installer's job, overlays and replacement are a concrete contractor's, and a few small high spots are a fine DIY grind with the right dust control. (Dealing with a lip on an outdoor walk instead? See uneven sidewalk fix.)