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Yes. Most cracks in a basement floor are shrinkage cracks from the concrete drying, and they rarely affect the structure. The ones to take seriously have raised or sunken edges, run along a wall near the footing, line up with a wall crack, or are wider than 1/4 inch.
Are cracks in basement floor normal?
Yes. ASHI’s Tom Feiza puts it plainly: basement floor cracks are common, because concrete shrinks as it cures (opens in a new tab). Thin, random cracks with flush edges are the normal result.
Concrete shrinks as it dries, and a slab that’s held in place cracks to relieve the strain. That’s why builders cut control joints: the NRMCA describes joints as simply pre-planned cracks (opens in a new tab). If your floor has no joints, or they’re far apart, the slab picks its own crack lines.
The NRMCA also notes that most random cracks that show up early, although unsightly, rarely affect the structural integrity (opens in a new tab) or service life of the concrete. In a typical basement, the walls and footings carry the house. The floor slab is poured inside them.
What do cracks in basement floor mean?
The shape of the crack and whether the edges are level tell you the cause. Random, spiderweb and inside-corner cracks with flush edges are shrinkage. Cracks along a wall, cracks with raised or sunken edges and cracks that meet a wall crack point to movement under the slab or at the footing.
| Crack you see | Usual cause | How concerned to be | Next step |
|---|---|---|---|
| Thin, random or spiderweb, edges flush | Shrinkage as the slab dried | Low | Seal it if water or soil gas comes through |
| Runs from an inside corner, a post or the sump | Shrinkage at a point of restraint | Low | Same as random cracks |
| Gap between the floor and the wall | Slab shrank away from the wall | Low, unless it's widening | Usually a water path, not a structural sign. See how to fix floor cracks |
| Runs parallel to a wall, near the footing | Footing settlement | High | Engineer |
| One edge higher than the other, or a hump | Heave from water pressure, or settlement | High | Heaving cracks, then an engineer |
| Lines up with a crack in the wall | Wall or footing movement | High | Engineer |
| Wider than 1/4 inch, or growing | Ongoing movement | High | When to worry, then an engineer |
Sources: ASHI Reporter (opens in a new tab), InterNACHI (opens in a new tab), NRMCA CIP 4 (opens in a new tab).
Feiza says not to be concerned with random spiderweb-like cracks or cracks that occur from an inside corner (opens in a new tab), since these are usually shrinkage. He does flag cracks parallel to the wall and footings, vertical movement at a crack, and floor cracks that line up with wall cracks.
Why is my basement floor cracking?
Usually because the slab shrank as it dried. The NRMCA lists the job-site habits that make it worse: missing or poorly placed joints (opens in a new tab), a poorly prepared subgrade, too much water in the mix, and too little curing. None of those mean your house is moving.
Shrinkage keeps going for a while. As the concrete dries, the NRMCA says it will shrink further and either additional cracks may form or preexisting cracks may become wider (opens in a new tab). A new floor crack in a house that’s a year or two old is often just that.
The causes worth worrying about come from below:
- Settlement. Soil under the slab or footing compresses or washes out. Feiza notes that there often will be a crack in the floor where the footing has started to drop (opens in a new tab), and the floor may be raised or dropped along it.
- Heave. Water trapped under the slab pushes it up. Feiza links heaved, cracked floors to hydrostatic pressure from a hampered drain tile system (opens in a new tab) or a sump pump that isn’t working or is set wrong.
- Restraint. A slab poured tight against a wall, post or footing with no isolation joint can’t move freely, so it cracks there. The NRMCA calls for isolation joints where floors meet walls, columns, or footings (opens in a new tab).
Wire mesh doesn’t stop this. According to the NRMCA, wire mesh and reinforcement in slabs cannot prevent cracking (opens in a new tab), though it can keep the cracks tighter.
Is a cracked basement floor a structural problem?
Usually not. A flush crack under 1/4 inch is cosmetic in most basements. InterNACHI’s guidance on slab cracks says those less than 1/4” and which exhibit no significant vertical or horizontal displacement (opens in a new tab) are not regarded as structurally threatening.
The same article admits there is no absolute standard for judging cracks. That’s why the edges matter as much as the width: a crack where one side has risen or dropped is a sign the ground under the slab has changed.
The exception is a floor that carries a load. If a post or a wall stands on the cracked area, or the crack runs right next to a column, have an engineer look at it before you do anything else.
When should you worry about a crack in the basement floor?
Worry when the edges aren’t flush, when the crack runs along a wall or lines up with a wall crack, when it’s wider than 1/4 inch, or when it keeps growing. Feiza says plainly that floors that are heaved and cracked are a problem (opens in a new tab).
Look for other signs at the same time: doors that stick, a floor that slopes, a gap opening between the floor and the wall, or new cracks in the walls nearby. The when to worry guide walks through each sign, and the heaving cracks guide covers floors that are lifting.
If you’re unsure, monitor the crack. Mark the ends, measure one spot and check again each season.
Why does water come up through basement floor cracks?
Because there’s water under the slab with nowhere else to go. The crack is the exit, not the cause. Sealing it can stop the seep, but the water pressure stays.
Check the outside first. Gutters, downspouts and the slope of the soil decide how much water reaches the ground around your basement. If the floor is heaving too, look at the sump pump and drain tile, which Feiza names as the usual culprits.
Water at the edge of the floor is a different problem. Feiza notes that basement floors often have gaps between the floor and the wall (opens in a new tab), because the slab shrinks away from the wall as it cures. That joint is where water from outside usually gets in, and the basement floor crack repair guide covers how to deal with it.
Do cracks in the basement floor let in radon?
Yes, they can. The EPA lists cracks in solid floors (opens in a new tab) among the ways radon gets into a house. Test before you assume a crack is a problem, and test again after any repair.
Sealing helps but isn’t a fix on its own. The EPA says it does not recommend sealing alone to reduce radon (opens in a new tab), because by itself sealing hasn’t been shown to lower levels significantly or consistently.
Sealed cracks do make a mitigation system work better. The radon and cracks guide has more.
How do you fix a crack in a basement floor?
Fill a stable, flush crack with a flexible crack filler that bonds to concrete and gives a little as the slab moves. Clean the crack out, let it dry, fill it, and scrape it level. Leave raised, sunken or growing cracks alone until someone has found the cause.
Rigid patches, like mortar packed into the crack, tend to break loose because the slab still moves a little with moisture and temperature. A urethane filler stays flexible. The basement floor crack repair guide walks through the whole job.
For a lot of cracks or a large floor, the same kit comes in a medium size. If you’re putting flooring down afterward, read the before new flooring guide first. If you’re not sure which product fits, the crack repair product chooser narrows it down in a few questions.
Every guide in this section
Is it a problem?
How to fix it
Questions
Is it a crack or a control joint?
A control joint is a straight, even groove cut or tooled into the slab on purpose, usually in a grid, while a crack wanders. A crack along a joint is the joint doing its job. The crack or joint guide shows how to tell them apart.
Should I fix basement floor cracks before finishing the basement?
Yes, once you know the crack is stable. Flooring over a moving crack can split tile and open seams in plank floors. The before new flooring guide covers which floors hide a crack and which ones show it.
Will a basement floor crack get worse?
A shrinkage crack can get a little wider as the slab keeps drying, then stop. A crack that keeps growing, or whose edges start to rise or drop, has another cause. Mark the ends and measure it each season using the crack monitoring guide.
Do I need to fix a hairline crack in my basement floor?
Not for the structure. Seal it if water, soil gas or dirt comes through, or if you're putting down flooring that needs a sound base. Otherwise it's cosmetic.