Basement floors
Basement Floor Cracks: Which Are Normal, Which Aren't, and How to Seal Them
Most basement floor cracks are normal shrinkage: hairline, dry, and level on both sides. Seal dry cracks about 1/8 inch or wider with a flexible polyurethane sealant to keep out moisture and soil gas. Cracks that leak, keep widening, or have one side higher than the other need the cause found first, because sealing alone won't hold.
Quick read
Width and what it usually means
- Under 1/8 in., dry and level: monitor.
- 1/8 to 3/16 in., dry and stable: seal it.
- Over 3/16 in., widening or offset: find the cause first.
- Wet after rain: a water problem, not a crack problem.
This page covers the floor slab only.
Short answer
Why basement floors crack (and why most don't matter)
A basement floor is usually a thin slab, about 3.5 to 4 inches of concrete, poured inside the foundation walls and often separated from them by an isolation joint. In most houses it holds up nothing but itself and what sits on it; the walls and footings carry the house. Concrete shrinks as it cures, and when a slab can't shrink freely it cracks. That's why most basements have a few thin, dry cracks within a year or two of construction.
What matters is what comes through the crack and whether it is moving: water, soil gas, or one side rising. This page covers the floor only. For cracks in the basement walls, start there instead.
Last reviewed: September 21, 2026
Read the crack
What your crack is telling you
| What you see | Likely cause | What to do | Where to go next |
|---|---|---|---|
| Hairline, dry; map or craze pattern on the surface | Surface drying and shrinkage | Nothing structural. Cosmetic filler if you like | This page: monitor |
| Straight dry cracks roughly 8–12 ft apart | Shrinkage where control joints were never cut (see the spacing check) | Expected. Seal if 1/8 in. or wider | This page: sealant table |
| Crack running from a sump pit, floor drain, column or inside corner | Re-entrant corner: stress concentrates where the slab changes shape | Usually stable. Seal once you've confirmed it isn't growing | This page: pencil-and-date |
| Dry crack 1/8 to 3/16 in., not growing | Shrinkage that opened further | Seal with flexible polyurethane | This page: how to seal |
| Crack wider than 3/16 in., or growing | Ongoing shrinkage, settlement or soil movement | Monitor monthly before sealing; get it looked at if it keeps growing | Handoff below |
| One side higher than the other | Settlement or uplift of part of the slab | Don't just fill it. Assess the movement first | Handoff below |
| Raised dome, or cracks radiating from a high spot | Heave: expansive soil, frost or water pressure below | Assessment on site | Handoff below |
| Crack wet or seeping after rain | Water under the slab pushed up by pressure | Sealing moves the water elsewhere. Fix drainage | water coming up through the floor |
| Crack damp in dry weather | Vapour rising through the slab, or condensation from humid air | Run the plastic-sheet test | This page: 10-minute check |
| Gap at the edge where floor meets wall | Cove or isolation joint, not a crack | Don't seal it shut if water drains through it | water at the wall-floor joint |
Width bands match the ones our foundation sister site uses, so the two sites never disagree. Surface spalling and pop-outs are surface defects, not cracks.
Two rows cover most basements: the straight, evenly spaced crack where a control joint should have been, and the crack running out from a corner of the sump pit or a column. Both are normal. They are also the two that contractor pages leave out.
Read the table in order of what matters. Moisture first: a wet crack is a water problem whatever its width. Movement second: any offset, dome or growth outranks width. Width last: it only decides between monitoring and sealing once the crack is dry and still. A crack that is 1/4 inch wide, dry and unchanged for a year is less of a concern than a 1/16-inch crack with one edge raised.
Measure it
Check your crack in 10 minutes
- Width. A credit card is about 1/32 in. thick and a US dime about 1/16 in. (0.053 in.). If a card won't go in, it's hairline. If a dime edge slips in, it is at least 1/16 in. A cheap plastic crack-width gauge from a hardware store is more precise. Record the widest point.
- Offset. Slide a card or coin across the crack, or run a straightedge over it. Any lip you can catch a fingernail on counts as displacement.
- Spacing. ACI guidance puts slab control joints at 24 to 36 times the slab thickness, up to 18 ft. A 4-inch basement slab would have joints every 8 to 12 ft, so parallel cracks about that far apart in a slab with no joints are the shrinkage the joints were meant to control.
- Pencil and date. Mark a tick across each end of the crack with the date, write the width at the widest point, and take a photo with a ruler. Recheck monthly and after heavy rain. Growth past a tick means the crack is active.
- Plastic sheet. Tape a clear polyethylene sheet about 18 × 18 in. over the crack on all four edges and leave it at least 16 hours (the ASTM D4263 method). Moisture under the sheet is rising through the slab. Moisture on top is condensation from humid basement air, which is a basement humidity issue rather than a crack problem. If it's under, find where the water starts.
Do the width and offset checks in a dry spell and repeat them after the next heavy rain; a crack that opens, darkens or lifts only when the ground is wet is telling you about water under the slab. Keep the photos in one phone album with the dates in the file names, so the comparison is easy months later.
| Measurement | Monitor | Seal | Investigate before sealing |
|---|---|---|---|
| Width | Under 1/8 in. | 1/8 to 3/16 in. | Over 3/16 in. |
| Offset | None | None | Any lip you can feel |
| Change over 3 months | None | None | Past the pencil ticks |
| Plastic sheet | Dry | Dry, or condensation on top | Wet underneath |
Materials
Which sealant fits which crack
| Material | Best for | Handles movement? | Works damp? | Soil-gas seal | Life expectancy |
|---|---|---|---|---|---|
| Flexible polyurethane sealant (self-leveling on floors) | Dry, stable cracks 1/8 in. and up; the default choice | Yes, some | Surface should be dry | Good; the type code radon provisions specify | Long |
| Epoxy | Dry cracks that will never move; under tile | No, it's rigid and fails on a moving crack | No | Good while intact | Long if nothing moves |
| Hydraulic cement | Plugging an active drip within minutes | No | Yes, it sets wet | Fair | Short; a stopgap |
| Penetrating / low-viscosity crack filler | Hairline cracks, cosmetic | Little | No | Limited | Moderate |
| Caulk or latex patch | Cosmetic touch-ups | Little | No | Poor | Short |
Generic material types; follow the product label for surface prep and temperature. Resurfacing or covering the floor is cosmetic and doesn't change what the crack is doing.
Flexibility matters because a slab keeps moving a little after it cures: it expands and contracts with temperature and dries out further for years. A rigid filler in a crack that still opens and closes seasonally cracks again beside the repair. Polyurethane stretches with it. If the sheet test showed moisture rising through the slab, pick a dry day, dry the crack with a fan, and apply the sealant then; if it never dries, that crack belongs in the wrong-fix section below, not in the sealant table.
Injection kits sold for foundation cracks are made for walls, where resin can be pushed through the full thickness toward the soil. A slab has gravel under it, so the resin just drains away. For walls, see sealing a leaking wall crack.
DIY
How to seal a dry floor crack
- Confirm it qualifies: dry under the plastic sheet, no offset, no growth past the pencil ticks.
- Vacuum the crack out with a crevice tool, and wire-brush loose or crumbling edges.
- If the edges are broken, widen them into a clean V-groove with a grinder or chisel so the sealant has sound concrete to grip. Wear eye and dust protection.
- Let the concrete dry. Most sealants want a dry surface; run a fan or dehumidifier first if needed.
- If the crack is deeper than about 1/4 inch, push in foam backer rod so the sealant stays near the surface and can flex.
- Fill with polyurethane sealant, slightly proud or self-leveling, and tool it flush. Most products list an application range of roughly 40–100°F; check the label.
- Let it cure as the label says before foot traffic or flooring, then keep checking the pencil ticks at the ends.
Radon
Floor cracks and radon
Cracks and the gap at the wall-floor joint are among the ways soil gas, including radon, gets into a basement. That same air can carry an earthy smell that is easy to mistake for a musty basement smell. Sealing them is worth doing, but the EPA is clear that sealing alone is not a radon fix. It has not been shown to lower radon significantly or consistently, and settling reopens cracks over time. Sealing makes an active mitigation system, usually sub-slab depressurization, work better.
Test first, seal as part of the mitigation plan, and test again afterwards. The residential building code's radon provisions (IRC Appendix F) call for polyurethane caulk or other elastomeric sealant at slab joints and cracks, the same material recommended above.
Know the limits
When sealing is the wrong fix
Water pushed up after rain. If the crack wets from below, the slab is sitting in water under pressure. Seal that crack and the water finds the next one, or the cove joint. The fix is lowering the water with sub-slab and perimeter drainage to a sump. See water coming up through the floor for how to confirm it.
A gap at the wall-floor joint. That joint is often where water that runs down the inside of the wall drains away. Sealing it can trap that water behind the joint. Read water at the wall-floor joint first.
A crack that is wet in dry weather. If the plastic sheet is wet underneath with no rain, rule out a leaking water or drain line under the slab before anything else; a plumber can pressure-test the lines. Sealing over a plumbing leak only moves where the water shows up.
Cost
What floor crack repair costs
| Scope | Planning range | Notes |
|---|---|---|
| DIY sealing of a dry crack | $15–$80 | Sealant, backer rod and a few tools. |
| Professional sealing of an isolated crack | $500–$2,500 | Grind-and-seal or similar; see professional crack repair. |
| Water pushed up through the slab | $5,000–$15,000 | The real fix is usually interior waterproofing with drainage and a sump. |
Planning estimates, not quotes, consistent with our basement waterproofing cost ranges. Finished floors, access and local labor move the number.
For the full range of scopes, see basement waterproofing cost.
Related pages
Where to go next
Water Coming Through the Basement Floor
When the crack is wet from below.
Read nextWater at the Wall-Floor Joint
The perimeter gap is a different problem.
Read nextBasement Wall Cracks Leaking
Cracks in the walls behave differently.
Read nextHow to Seal a Leaking Foundation Crack
Injection for wall cracks, step by step.
Read nextWhat Humidity Should a Basement Be?
If the sheet test showed condensation on top.
Read nextFAQs
Basement floor crack FAQs
When should I worry about a basement floor crack?
When it is wider than about 3/16 inch, grows past the pencil marks you put at its ends, has one side higher than the other, sits in a raised or domed area, or passes water after rain. Those point to movement or water pressure. A dry hairline crack that stays the same rarely matters beyond cosmetics.
Is it normal for a concrete basement floor to crack?
Yes. Concrete shrinks as it cures, and a slab that cannot shrink freely cracks. ACI guidance accepts some random cracking as normal even in slabs with properly spaced control joints, and many basement slabs were poured with few or no joints at all, so a handful of thin, dry cracks is expected.
How can you tell if a floor crack is structural?
In most houses the basement slab does not hold up the house; the walls and footings do. So the question is whether the crack shows movement: one edge higher than the other, a raised dome, or a crack that keeps widening. Those call for a foundation assessment on site. A web page cannot make that call remotely.
What is the best sealant for basement floor cracks?
For a dry, stable crack about 1/8 inch or wider, a flexible polyurethane sealant, self-leveling on a flat floor, is the usual choice; it is also what building-code radon provisions call for at slab cracks and joints. Epoxy suits dry cracks that will never move. Hydraulic cement only plugs an active drip for a while.
Should cracks in a basement floor be sealed?
Seal dry cracks of about 1/8 inch or more, and seal any floor crack before covering the floor or as part of radon mitigation. Hairline cracks can be left or cosmetically filled. Don't seal a crack to stop water pushed up from below; the water moves to the next weak spot, and the real fix is drainage.
Can I repair a basement floor crack myself?
Yes, if it is dry and stable: clean it, add backer rod if it is deeper than about 1/4 inch, and fill it with polyurethane sealant following the steps on this page. Not if water is pushing up through it, one side is higher, the area is heaving, or the crack keeps growing. Those need the cause diagnosed first.
How much does it cost to repair a crack in a basement floor?
DIY materials usually run about $15–$80. Professional sealing of an isolated crack is commonly around $500–$2,500 as a planning range. If the crack is only the visible end of water pressure under the slab, the real cost is drainage, often $5,000–$15,000 for an interior system. These are planning estimates, not quotes.
Will sealing floor cracks stop radon?
Not on its own. EPA does not recommend sealing alone as a radon fix, because sealing has not been shown to lower radon significantly or consistently, and settling can reopen cracks. Sealing does help an active mitigation system work better. Test before and after any radon work.
Sources
Sources
- EPA — Consumer's Guide to Radon Reduction (sealing alone not recommended)
- IRC Appendix F — Radon Control Methods (AF103.4, sealing slab joints and cracks)
- USBR — Joint spacing for concrete structures (cites ACI 302.1R: 24–36× slab thickness)
- IRC R506 — Concrete floors on ground (3.5 in. minimum, vapor retarder)
- ASTM D4263 — Indicating Moisture in Concrete by the Plastic Sheet Method
Standards summarised, not quoted. Code provisions are adopted with local amendments. This page is educational and not a structural or radon assessment.