Why is my concrete basement floor cracking, and should I be worried?

In many houses the basement floor is a concrete slab that sits inside the foundation walls, with a joint where the slab meets the wall. ^[1] The floor and the walls are different parts of the building, so a crack in one does not by itself tell you about the other.

This page is about cracks in that floor slab. If the crack is in a foundation wall, or you are not sure where it is, see Is this crack serious? If a basement wall is bowing inward, see Is my bowing basement wall serious?

Possible causes

Concrete changes size as it dries. A guide for people who build concrete floors puts it this way: concrete starts to lose volume as soon as it is placed, and that continues for as long as water, heat, or both are being released. ^[2] That is called drying shrinkage.

Several things can combine with it:

• Restraint. If something holds the slab back so it cannot shrink freely, the chance of cracking goes up. The guide says restraint “from any source, whether internal or external” increases the potential for random cracking. ^[2]
• Uneven drying and cooling. The top and bottom of a slab do not dry and cool at the same rate, so shrinkage varies through its thickness. ^[2] The same guide mentions curling, a change in the slab’s shape, and tells owners to expect some cracking and curling in concrete floors. ^[2] That tells you cracking is common. It does not tell you that any particular crack is unimportant.
• Movement of what is under the slab. The ground beneath a slab can also play a part. The sources we use do not let us say whether that applies to your floor, and nobody can judge it from a photo. (A reasonable possibility, not a finding.)

What a crack can mean for moisture and radon

A crack can be a pathway. That is different from showing that anything is coming through it.

Moisture. Building guidance from the U.S. Department of Energy’s Pacific Northwest National Laboratory says soil gas and moisture from the ground can enter a basement through slab cracks, holes, pipe openings and the joint between slab and wall. It adds that without an effective moisture barrier, water can pass through the pores of a concrete slab, which can potentially lead to moisture-related problems. ^[1]

Radon. EPA explains that air pressure inside a house is usually lower than in the soil around it, so the house can draw soil air in through foundation cracks and other openings. ^[3] A floor crack can be one of those openings. That is our reading of EPA’s wording, which does not single out any one crack. A crack does not show that radon is present. EPA says radon cannot be seen or smelled and that special equipment is needed to detect it, so testing is how you find out. ^[3]

How to identify the cause

You cannot identify the cause of a floor crack by looking at it or at a photograph. What you can do is notice what is visible, so that you know what to tell a professional:

• Is the floor still about level, or does one side of the crack sit higher than the other?
• Is there visible vertical movement, such as a part of the floor that looks lifted or sunken?
• Is water coming in, and is it coming in now or only after weather or water use?
• Does the crack run to where a support stands, for example a post or column resting on the floor?

These are screening observations. They are not tests, and they do not name a cause.

What a look cannot settle

• The exact cause. Shrinkage, restraint and ground movement can look alike on the surface.
• Whether radon is present. Only a radon test can answer that. ^[3]
• Where water comes from. Water at a crack shows that water is getting through. It does not show whether it comes from the ground, from a plumbing leak, or from somewhere else, and the sources we use do not let us say. The DOE/PNNL guidance notes that sealing alone is not enough where poor site grading, missing gutters, a high water table or other factors cause serious water problems. ^[1]

How serious is it?

We cannot rate your crack from here. This is a way to sort what you see:

• Nothing else you can seeThe floor looks level, there is no water, nothing has moved, and the crack does not reach a support. That does not establish the cause, or that nothing is wrong underneath. A radon test is still the only way to know about radon.
• Questions only a test or inspection can answerWhether radon is present, and whether moisture is passing through the slab. A crack can be a pathway for either, but it does not show that either is happening.
• Worth asking a professional to evaluateA crack that runs to where a support stands, visible vertical movement, or water coming in continuously. These are conditions that cannot reasonably be judged from a distance.

What can you do?

1. Write down what you can see: the four observations above. Photos can help you describe it later.
2. Test for radon if you have not. A crack is not a reason to test or not to test. EPA says testing is how you learn whether radon is present and at what level. ^[3]
3. If any of the three conditions above applies, arrange for a qualified building professional to inspect it in person. We do not set a deadline or a size at which a crack becomes a problem, because we found no source that supports one.
4. Leave the slab alone. Do not cut, drill or dig into it, and do not disturb a support or a utility. This page does not give repair instructions.
5. Do not rely on sealing alone. EPA does not recommend sealing alone to reduce radon, because sealing has not been shown to lower radon levels significantly or consistently. ^[3]

For cracks in a foundation wall, see Is this crack serious? For a damp wall or floor and where the water may come from, see Why is my wall damp?

Typical cost

This page does not give repair costs. We did not find cost evidence we trust for this situation, and what a floor crack costs to deal with depends on a cause that cannot be seen from a photo.

We do not have a sourced cost range for this yet.

Does fixing it affect property value?

“Return” is more than resale price. Where we have no reliable figure we say so instead of guessing. See how we think about value.

Related tools

We do not have a calculator for this yet.

Deeper technical explanation

Drying shrinkage. Concrete gets smaller as it loses water and heat. ^[2] A slab that could shrink freely would simply get smaller.

Restraint and tensile stress, in plain words. A slab is usually held back from shrinking, by its edges, by what it sits on, by anything cast into it. When shrinkage is held back, the slab is effectively pulled apart. Engineers call that pulling force tensile stress. When it grows larger than the concrete can resist at that point, the concrete can crack. This is our plain-language summary of how the ACI guide describes restraint and cracking. ^[2]

Joints. Floors are often built with joints so that shrinkage can open in planned places. The guide describes contraction and construction joints and warns against detailing that stops them from opening as the slab dries. ^[2] A crack can still form where no joint was planned.

Moisture and temperature. Both water loss and heat loss make concrete change size, and the top and bottom of a slab do not change at the same rate. ^[2] That is why slabs can curl as well as crack.

How water and soil air can get through. Without an effective moisture barrier, water can penetrate the pores of a concrete slab itself. Cracks, holes, pipe openings and the wall-to-slab joint are more direct routes. ^[1] Soil air moves for a different reason. Pressure inside a house is usually lower than in the soil outside, so the house can draw soil air inward through openings. ^[3]

What none of this establishes. A crack being a possible pathway does not show that water or radon is passing through it. Water at a crack does not show where the water comes from. A look at a floor does not show the cause of a crack.

Sources & methodology

This page rests on three sources, which are listed below with what each does and does not say. We cite them for what they say and mark where we are giving our own reading. We give no cost figures, no crack-width thresholds and no do-it-yourself tests here, because the sources do not support them. See how we work out costs and all sources.

1. Water Management of Existing Basement Floor — U.S. Department of Energy / Pacific Northwest National Laboratory (Building America Solution Center) Government agencyPractitioner guidance. Describes slab cracks, holes and wall-to-slab joints as places where moisture and soil gas can enter, and says sealing alone is not enough where grading, gutters or a high water table cause serious water problems. It does not say how to tell where water comes from. Scope: US national. Published or updated: Dec 2015. Accessed October 7, 2026.
2. Guide to Concrete Floor and Slab Construction (ACI 302.1R-15), Chapter 5 — American Concrete Institute Building-science / engineeringA guide for people who build concrete floors. Describes drying shrinkage, restraint and movement in slabs, and tells owners to expect some cracking and curling. It is written for construction, not for diagnosing a crack in an existing house. Published or updated: 2015. Accessed October 7, 2026.
3. A Consumer's Guide to Radon Reduction (EPA 402/K-10/005) — U.S. Environmental Protection Agency Government agencySays soil air, including radon, is drawn into a home through foundation cracks and other openings; that radon cannot be seen or smelled, so testing is needed; and that EPA does not recommend sealing alone to reduce radon. It does not say that any particular crack means radon is present. Scope: US national. Published or updated: 2016. Accessed October 7, 2026.