You found a crack. Maybe it's been there for years, or maybe it showed up this spring and looks like it's getting longer.
The hardware store wants to sell you a tube of something. But the fastest way to waste money on a crack is to fill it without knowing why it's there. The filler fails, the crack keeps working, and next year you're back where you started.
Here's how to do it in the right order.
Step 1: Figure Out What Kind of Crack You Have
A hairline from the day of the pour, a crack with one side dropped an inch, and a crack with crumbling edges each need a different fix. The quick version:
- Flush and thin: usually shrinkage. Cosmetic or close to it.
- One side lower than the other: the soil underneath moved. That's a support problem, not just a crack.
- Chipped, crumbling edges: freeze-thaw damage is already eating the concrete.
- In a foundation wall, especially horizontal, stair-step, or wider than about 1/4": stop here and talk to a structural engineer.
For the full breakdown with photos and a cheat sheet, see our field guide to the types of concrete cracks.
Step 2: Fix What Caused It
A crack is a symptom. If the cause is still active, the crack will keep moving and any repair will be fighting it.
| What's causing it | Signs to look for | What to do about it |
|---|---|---|
| Water | Crack near a downspout, sump discharge, or low spot where puddles sit | Extend downspouts, redirect sump discharge, fix grading so water runs away from the slab and house |
| Settling and voids | One side lower, slab rocks, gap under the edge | Fill the void and support the slab (foam lifting does both at once) |
| Frost heave | Slab higher in late winter, partly drops back in spring | Get water away from the slab; reassess after a full spring |
| Tree roots | Slow, steady rise near a tree, worse every year | Arborist-approved root pruning or a barrier (see tree roots lifting concrete) |
| Heavy loads | Cracks where a dumpster, delivery truck, or camper sat | Keep heavy loads off thin slabs; check for settling underneath |
In northeast Wisconsin, water is behind most of it: loose foundation backfill, downspouts dumping next to the house, and spring snowmelt soaking everything at once. Water under a slab softens or carries away the soil, and the concrete settles into the hollow. (More in voids under concrete.)
Step 3: Choose the Right Repair
This is where most DIY repairs go wrong. The right material for one crack is wrong for another.
Moving cracks: flexible seal
An outdoor slab is never quite still. It gets a hair bigger on a July afternoon, a hair smaller on a January night, and the frozen ground under it nudges it around all winter. Cracks and joints open and close with all that movement. A flexible sealant stretches and compresses with them. A rigid filler can't, so it cracks loose or pops out.
Outdoors around here, assume a crack moves unless you've watched it long enough to prove it doesn't. Control joints and expansion joints are built to move.
Wider, deeper cracks usually get a foam backer rod pushed in first. It keeps the sealant from bonding to the bottom of the crack, so it only grips the two sides and can flex, and it keeps you from burying a whole tube in one crack. Material choices are covered in our guide to the best concrete crack caulk.
Dormant cracks: rigid fill
If a crack has truly stopped moving, as with many indoor cracks and cracks in slabs that have long since finished settling, a rigid filler can bond the sides and restore a smooth surface. That's the usual approach for crack repair before a garage floor coating goes down.
Cracks with an offset: lift first, then seal
If one side has dropped, sealing it just fills a gap next to a trip lip. The hollow underneath is still there, and the crack keeps working.
The fix is to lift the low side level, then seal the crack. Our foam lifting process uses a few 5/8" holes, about the size of a dime. A lightweight structural foam fills the void and raises the slab in small, controlled increments, then the holes get patched. With the slab level and fully supported again, the seal that goes in afterward has a fighting chance.
Garage floor cracks: repair as part of a coating
A garage floor coating is one of the best fixes for a cracked, stained garage floor, because the cracks get cleaned out, filled, and leveled as part of the prep. The finished floor hides them and keeps salty slush from soaking in.
One caution: a crack that's still moving can show back through any coating over time. So we look at why the floor cracked first, and a dropped section gets lifted before it gets coated. See garage floor cracks for more.
Crumbling or broken concrete: patch or replace
Sealant needs solid edges to grip. If the crack edges are crumbling, the slab is broken into loose chunks, or a whole area is covered in cracks and flaking, sealing isn't a fix. Small damaged spots can be patched. Widespread damage means replacement, and that's a job for a concrete contractor, not us.
The Decision Table
| Your crack | Best repair | DIY or pro? |
|---|---|---|
| Hairline, flush, not growing | Leave it, or seal it if it holds water | DIY |
| Moving crack or open joint outdoors | Clean, backer rod if deep, flexible seal | DIY-able, or we can handle it |
| Stable crack indoors | Rigid fill | DIY |
| One side sunk, trip lip | Lift, then seal | Pro (lifting) |
| Cracked garage floor | Crack repair plus coating | Pro |
| Crumbling edges, loose chunks | Patch or replace | Concrete contractor |
| Foundation wall crack: horizontal, stair-step, wide, or growing | Professional evaluation | Structural engineer |
Why Sealing Matters in Wisconsin
An open crack is a funnel. Rain and snowmelt run straight through it to the soil under your slab.
Then winter hits. Water expands by about 9% when it freezes, and ice in a crack acts like a wedge, prying it a little wider. It thaws, more water runs in, and it freezes again, over and over between fall and spring. Deicers make the surface damage worse, and the salty slush dripping off your car rides that water right in.
The water that makes it through softens and washes out the soil underneath until the slab has nothing to sit on. A good seal stops both problems. The best time to do it is late summer or fall. The concrete needs to be dry, and sealants cure slowly in the cold, so don't wait until the first hard freeze.
How to Measure and Monitor a Crack
Not sure a crack is worth fixing? Watch it for a season. It takes five minutes and tells you whether it's moving or stable.
- 1Mark the endsDraw a short pencil or marker line across each end of the crack and write the date next to it. If the crack grows past the line, it's active.
- 2Mark a measuring spotPick one spot along the crack and mark it so you measure in the same place every time.
- 3Measure the width with what's in your pocketNo ruler that fine? Use these as gauges:
| What slides into the crack | Roughly how wide | What it usually means |
|---|---|---|
| Nothing, not even a card | Hairline | Cosmetic; leave it or seal it |
| A credit card | About 1/32" (0.76 mm) | Seal it to keep water out |
| A penny, edge-first | About 1/16" (1.5 mm) | Seal it; watch for growth |
| A stack of four pennies | Just under 1/4" | Find out why it's that wide before sealing |
- 1Check for offset. Lay a straightedge across the crack and measure any drop with a tape. For a sense of scale, accessibility standards for public walking surfaces call for beveling any change in level over 1/4" and ramping anything over 1/2". If you're past that, someone is going to trip.
- 2Take a photo with a coin next to the crack each time you check it.
- 3Compare like with like. Because concrete shrinks in the cold, a moving crack may read wider in January than in July. Compare readings taken in similar weather.
When You Can Leave a Crack Alone
You really don't need to fix every crack. Leave it be if it's:
- Hairline thin, both sides flush, and not growing past your pencil marks
- Crazing, the fine shallow web on the surface, which is cosmetic
- Indoors and dry, like a stable hairline in a garage floor you aren't coating
- Running down a control joint you're happy with (though sealing the joint is still a good idea outdoors)
A crack that hasn't moved after a full Wisconsin winter and spring is about as stable as concrete gets.