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Clean and dry the crack, grind it open if it's 1/8 inch wide or less, pack it with dry silica sand soaked in urethane, and scrape it flush once it sets. That works for floor cracks up to 1/4 inch with flush edges. If water comes up through the crack or the gap where the floor meets the wall, fix the drainage first, because filling the opening usually just moves the leak.
Before you start
Filling won't fix a floor that's still moving or wet. Check these first.
- One side of the crack sits higher than the other, or the floor has a hump
- The crack is wider than 1/4 inch or has gotten wider since you first saw it
- Water comes up through the crack or the joint at the wall when it rains
If any of these fit, find out who to call first: a structural engineer or a repair contractor.
What do you need to fix cracks in a basement floor?
A urethane slab crack kit, dry silica sand and a few hand tools. A basement floor stays out of the sun, so the standard indoor kit fits; the UV-stabilized version is for outdoor concrete.
- Straightedge and tape measure
- Tuckpoint grinder with a diamond blade, for cracks 1/8 inch and narrower
- Safety glasses, dust mask and gloves
- Shop vacuum and a stiff brush
- Dry silica sand
- Caulk gun and razor scraper
For one or two cracks, the small kit is enough. Longer cracks or a whole floor’s worth go faster with the medium kit; the kit sizing guide and the product chooser help you pick.
How do you repair cracks in a basement floor?
Open up thin cracks, clean and dry them, pack them with dry sand soaked in urethane, and scrape the repair flush once it sets. That’s the method in the indoor slab kit’s instructions, for cracks up to 1/4 inch wide with flush edges.
Here’s how to repair cracks in basement floor slabs, step by step:
Check the crack
Lay a straightedge across it. If one side is higher, or it’s wider than 1/4 inch, read the section on cracks you shouldn’t fill first.Open up thin cracks
For cracks 1/8 inch wide or less, the kit’s instructions say to grind the crack out to 1/4 inch wide and 1/4 inch deep so the filler has room to grip. Wider cracks skip this step.Clean and dry the crack
Vacuum out the dust and loose concrete. The instructions say it’s critical that the crack be dry, so run a dehumidifier or fan if the basement is damp.Prime with urethane
Wet the inside of the crack with the kit’s urethane. The instructions say just wet it; don’t fill it.Pour in sand
Slowly pour dry silica sand into the crack.Soak the sand
Saturate the sand with urethane, without overfilling the crack.Top up with sand
Go back to where you started and add more sand wherever it’s needed to fill the crack completely, then soak it until it sits in a slight hump. Trowel off any excess while it’s still wet.Scrape it flush
Once the urethane turns light gray and reaches its initial set, scrape or lightly sand the hump flush so nothing sticks up under flooring or catches a mop.
The same fill is also how to seal cracks in basement floor slabs before you paint, coat or cover them. The repair closes the crack flush with the floor, so dust, moisture from mopping and soil gas have one less path.
Why does water leak where the basement floor meets the wall?
That gap is the cove joint, and a cove joint leak usually means water is building up under and around the slab. The floor was poured after the footing and wall were already hard, so the floor and wall never bonded. When the soil outside is soaked, water pressure (hydrostatic pressure) pushes water up through the easiest opening, and the cove joint is usually it.
A home inspector writing for ASHI notes that basement floors often have gaps between the floor and the wall (opens in a new tab) or around the sump crock. The same article ties water pressure under a floor to a hampered drain tile system or an inoperative sump pump (opens in a new tab).
Should you seal a cove joint leak from inside?
Not as the first fix. Sealing the joint from inside doesn’t lower the water pressure behind it, so the water often finds the next weak spot: a floor crack, a wall crack or a gap around a pipe. The NRMCA says that before repairing leaking cracks, the drainage around the structure should be checked and corrected (opens in a new tab).
The slab kit isn’t the product for it either. Its data sheet says the urethane isn’t meant for expansion joints, and the cove joint is a separation between two pieces of concrete that move independently, the same job an expansion joint does.
Work from the outside in:
- Gutters and downspouts. Clean them and extend the outlets. For houses with basement water problems, a North Dakota emergency services guide adapted from Iowa State says downspout water should be dumped at least 5 to 10 feet from the house (opens in a new tab).
- Grading. The same guide says the soil should drop a minimum of 6 inches in the first 10 feet (opens in a new tab) away from the house. Backfill settles over the years and leaves a low strip along the wall, so add soil where water now collects.
- Sump pump. If you have one, test it. An inoperative or badly set pump lets water pressure build under the floor.
- Interior drain tile. If the outside is right and water still comes in, a perimeter drain under the edge of the floor that carries water to a sump is the usual next step. That’s a contractor job, so get more than one opinion.
Seal the cove joint after the water problem is fixed, if you want to, using a flexible joint sealant rather than a rigid patch. If the gap between floor and wall is wide and the wall looks tipped outward, that can mean the footing settled; ASHI’s inspector notes a wide gap can develop if the footing settles and the wall rotates outward (opens in a new tab). Have a structural engineer look at that before you seal anything.
Does sealing basement floor cracks stop radon?
No, not by itself. The EPA lists cracks in solid floors (opens in a new tab) as one way radon gets in, but its radon reduction guide does not recommend sealing alone to reduce radon (opens in a new tab). Sealed cracks help a mitigation system work; they don’t replace one.
Test your home before and after any repair. The radon and foundation cracks guide covers testing and what a mitigation system involves.
When shouldn’t you fill a basement floor crack?
When the floor is still moving or water is still coming through. A filler bridges a crack; it doesn’t hold a slab down or keep water pressure back.
- One side is higher. Offset edges mean the slab heaved or settled. The heaving cracks guide covers what pushes floors up.
- The crack keeps getting wider. The NRMCA says to seek professional advice (opens in a new tab) for active cracks that widen with time. Track it for a few months before you fill it.
- The crack runs into a wall crack. If a floor crack lines up with a crack in the foundation wall, look at the wall first. Wall cracks over 1/4 inch, horizontal ones and offset ones go to an engineer, not a kit.
- Water comes up through it. Fix the drainage first, as with the cove joint.
The when to worry guide walks through each warning sign.
Which basement floor crack fix do you need?
Match what you see to the fix before you buy anything. Most basement floor cracks are shrinkage cracks that only need filling, if that.
| What you see | What it usually means | Fix |
|---|---|---|
| Hairline crack, flush, dry | Shrinkage as the slab cured | Leave it, or fill it if you're covering the floor |
| Crack up to 1/4 inch, flush, dry | Shrinkage, opened wider | Urethane slab kit: dry sand soaked in urethane |
| Water at the joint where floor meets wall | Water pressure under the slab | Gutters, grading, sump, then drain tile if needed |
| Water coming up through a floor crack | Water pressure under the slab | Drainage first, then fill once it stays dry |
| Raised edge, hump or crack that keeps widening | Heave or settlement | Find the cause before filling |
| Floor crack lined up with a wall crack | Movement in the foundation | Check the wall; engineer if it's over 1/4 inch or offset |
How long before you can walk on the repair?
Under an hour. The urethane’s data sheet lists a 2 to 4 minute initial set and back in service within 20 minutes, and the kit’s product page says regular traffic in less than an hour.
| Material | Apply between | Light use | Full cure | Source |
|---|---|---|---|---|
| Urethane and sand | Dry crack, slab at least 28 days old | Initial set in 2 to 4 minutes; back in service within 20 minutes | Regular traffic in under an hour | Urethane data sheet and product page |
The data sheet says the urethane works even in cold conditions. Follow the temperature range on the cartridge label.
What mistakes make basement floor crack repairs fail?
Moisture and movement cause most of them. A crack filled while damp, or a floor that’s still being pushed up by water, opens again.
- Filling a damp crack. The urethane needs a dry crack. Dry the basement out first.
- Skipping the grinding on a thin crack. Filler can’t get into a crack 1/8 inch or narrower, so it sits on top and peels. Open it to 1/4 by 1/4 inch.
- Leaving paint, sealer or adhesive in the crack. The urethane’s data sheet says oils and sealers will inhibit bonding. Grind or scrape back to bare concrete.
- Sealing the leak instead of the water. Filling a leaking crack or cove joint without fixing drainage usually sends the water somewhere else.
- Filling a crack that’s still moving. It will open again along the repair.
The why crack repairs fail guide covers each of these in detail, and the basement floor cracks hub has the rest of the floor guides.
Should you seal the basement floor after filling the cracks?
It’s optional on a basement floor, but a sealer or coating over the whole slab cuts dust and makes the floor easier to clean. Let the crack repairs cure fully first.
The sealing after crack repair guide covers which sealers work over a filled crack and how long to wait.
Questions
Can I fill a basement floor crack that's damp?
No. The kit's instructions say it's critical that the crack be dry. If a crack never dries out, water is coming from under the slab, and that's a drainage problem to fix first.
How long before I can walk on the repaired floor?
Under an hour. The urethane's data sheet says the floor can be back in service within 20 minutes, and the kit's product page says regular traffic in less than an hour.
My house is new. Can I fill the floor cracks now?
Wait until the slab is at least 28 days old. The urethane's data sheet says the concrete needs a minimum of 28 days of curing before you apply it.
Should I fill floor cracks before putting down flooring?
Yes, if the cracks are stable and dry. A filled, flush crack is a better base than an open one, but some floor coverings need more than that. The before new flooring guide covers tile, vinyl plank and coatings.
Will the repair match my floor?
Not exactly. The kit's product page says the urethane cures to a tan color, so the repair reads as a thin tan line on a gray floor. Scraping it flush keeps the line narrow.