Cracks in a Basement Floor: What They Mean

A crack across the basement floor can look worse than a crack in the wall, because it is wide open to view and often runs the length of the room. The good news is that the floor is usually the least important piece of concrete in the basement. Here's how to tell what your crack is telling you.

Why a basement floor is usually not structural

In most houses, the foundation walls stand on concrete footings poured below them, and the floor slab is poured afterwards between the walls. The slab keeps out dirt and moisture and gives you something to stand on. It doesn't carry the walls or the house.

There are exceptions. Steel posts and interior bearing walls often sit on their own footings under the slab, but in some houses they rest on a thickened part of the floor. If a crack runs right next to a post, or a post looks like it has sunk, treat that differently. The same goes for a few house types where the floor and walls are poured together. If you're unsure how yours was built, an engineer can tell you quickly.

The four kinds of basement floor crack

Shrinkage cracks

Concrete shrinks as it cures. A basement slab often cracks across the middle of the room, out from corners, or in a loose web pattern. These cracks are thin (usually under 1/8"), flat, and stop changing after the first year or two. They are the most common kind by far.

Settlement cracks

If the gravel or soil under the slab wasn't compacted well, parts of the floor can sink. Settlement cracks tend to be wider, and one side may sit a little lower. You might notice a dip near the walls where backfill was placed, or a gap opening between the floor and the bottom of the wall.

Heaving cracks

A heaving crack is one where part of the floor has been pushed up. Run your hand across it: if one edge stands proud of the other, or the floor rises into a hump along the crack, that is heave. Common causes are expansive clay soil swelling when wet, frost getting under a slab in an unheated space, and groundwater pressure pushing up from below. Heave is a sign that something under the floor is changing, which is why it deserves more attention than shrinkage.

Cracks along the wall-floor joint

The line where the floor meets the wall is a joint, not a crack. A thin gap there is normal. It matters when water comes through it or when the gap gets noticeably wider.

How to read your crack

What you see	Likely cause	What to do
Hairline cracks, flat, not changing	Shrinkage	Nothing. Mark and date if you want to be sure.
1/16" to 1/8", flat, dry	Shrinkage	Seal it if you plan to finish the floor, or if moisture comes through.
1/8" to 1/4", or growing	Possible settlement or heave	Monitor monthly, and have it inspected if it keeps changing.
Wider than 1/4", or one side raised or sunk	Settlement or heave	Professional assessment
Crack near a post that has dropped, or with wall cracks or bowing walls	Possible structural movement	Structural engineer

To check change over time, pencil a line across each end, write the date, and measure the widest point. Check again monthly for a few months and after heavy rain. If the crack is still the same width and still flat, it is very likely dormant.

When water or radon comes through

A crack that lets water in is a water problem first. Damp patches that appear after rain, or a crack that seeps when the ground is wet, point to groundwater pressure under the slab. A sealant may slow it, but it won't hold back water that is being pushed up from below. Look outside first: gutters, downspout extensions and grading that slopes away from the house. If that doesn't fix it, the usual next step is interior drainage.

Floor cracks can also let soil gas, including radon, into the house. Radon has no smell and you can't see it, so the only way to know is to test. Low-cost test kits are widely available. If levels are high, a radon mitigation system is the fix. Sealing cracks helps but is rarely enough on its own.

When a basement floor crack matters

Call a licensed structural engineer if a floor crack is wider than 1/4", one side is clearly higher than the other, it keeps growing, or it comes with other signs: a post that has dropped, horizontal or stair-step cracks in the walls, bowing walls, sticking doors upstairs, or sagging floors above. An independent engineer doesn't sell repairs, so they can tell you plainly whether the floor needs anything at all.

Look at the walls while you're down there. Wall cracks matter far more than floor cracks, and our foundation cracks guide explains what to look for. If you want a quick second opinion on the crack itself, try the Crack Checker.

For a stable crack that just needs sealing, the steps (cleaning, backer rod, a flexible sealant, and what to do before laying flooring) are in our basement crack repair guide.

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