How to fix and repair
Basement floor cracks

Basement floor crackNon structural basement floor crack

A crack in your basement floor may have you wondering if there's anything wrong with your concrete floor.

I can tell you, most basement floors don't support the house, the foundation walls do. So the crack usually isn't a structural issue.

No question, having a cracked floor is unsightly and understandably cause for concern. 

But what I've found In most every case (or crack), there's usually a good reason you floor cracked and it certainly can be repaired.

The most common types of basement floor cracks

There's a few things I look for when I diagnose a crack in a concrete floor. The width and length of the crack can tell me what type of crack it is and why the floor cracked.

  1. Hairline Crack - About the width of a dime or less. The length of these cracks can be small or long. This crack is non-structural and quite common. Can be repaired if desired or left alone.
  2. Shrinkage Crack - About the width of a dime to 1/8". A freshly poured concrete floor shrinks as it cures. This causes stress on the concrete causing it to crack. Relief cuts are usually sawed in the floor to control these cracks. Considered non-structural and can easily be repaired.
  3. Settlement Crack - About 1/8" to 1/2" wide. Usually caused by the sub-base (gravel) under floor settling. When the gravel was being installed it wasn't properly compacted. Over time the gravel settles (could only be 1/4"). This causes stress on the concrete and a crack forms. One side of the crack is usually higher or lower than the other side. Could be a structural issue if there's a loly column nearby. Floor can be leveled and repaired or broken out and re-done.
  4. Heaved Crack - About 1/4" to 3/4" wide. Usually caused by water issue under the floor or something under the floor pushing upwards (large rock or stump). If the soil under the floor freezes it can cause the floor to heave and crack.  Can be structural issue if a bearing wall or loly column is sitting on the floor. Best to break out the concrete and fix the underlying issue, then re-pour the concrete floor.
  5. Perimeter Cracks - Where the basement floor meets the foundation walls. About 1/8" to 1/4" wide. This is caused by the concrete floor shrinking away from the walls. Not a structural issue but sometimes moisture may seep up through here. Can be sealed with caulking.
  6. Spalling, pop-outs, and flaking - the top 1/8 - 1/4 inch of the surface is peeling or delaminating in areas. This isn't really considered a crack but is damage and can be repaired. Usually not structural. 

why did my basement floor crack?

basement floor shrinkage cracksCommon basement floor shrinkage cracks

There's a few reasons that could explain why your basement floor cracked.

  1.  No sawed contraction joints were installed after the concrete floor was poured. A new concrete floor shrinks during the curing process. If no expansion / contraction relief cuts were made after troweling, there's a good chance the cracks are "shrinkage cracks".
  2.  The sub-base settled. If the gravel under the concrete floor wasn't properly installed and compacted, it may have settled. If a section of the concrete also settled it will cause the concrete to crack. This type of crack usually has one side of the crack a little higher than the other side. 
  3. The concrete was poured too wet. Adding too much mix water when pouring the concrete floor will weaken the concrete and cause it to crack. These cracks are also called shrinkage cracks.
  4. No drainage or improperly installed drainage pipes. If there's moisture under your concrete floor that has no way to drain, it will put pressure "upwards" on your concrete floor and crack it.

There are some other reasons like, too much calcium chloride was added to the mix and it dried too fast, or the concrete was improperly cured. All these will lead to shrinkage cracking at some point.

If you determine the cracks in your basement floor are due to settling or heaving, then that is a sub-base issue and the concrete may have to be removed to correct that issue.

Repairing these cellar floor cracks will only be temporary if the concrete continues to settle or heave.

If you're sure the concrete floor is done moving, you can repair those cracks and the shrinkage cracks with an epoxy material you inject into the crack and add a silica sand for a filler.

This will weld the crack together creating an excellent basement floor repair.

what if moisture is coming up through the crack

You can see moisture in a couple ways. 

First, if there's a white powdry substance on the floor along the crack, that's moisture. Moisture vapor is coming up through the crack, carrying lime and minerals from the concrete.

The moisture evaporates on the surface of the floor and leaves the lime and minerals behind (white powder called efflorescence).

Most likely no moisture vapor barrier was installed under the concrete floor when it was installed.

SOLUTION: You can spray or roll on a moisture blocking sealer called RADONSEAL. It penetrates into the concrete and blocks moisture vapor from coming up through the concrete. You need to repair the crack first, then apply the sealer.

Second, the floor is darker, damp looking, or actually wet along the crack. Actually seeing water coming up through the crack is a drainage problem.

Either no drainage piping was installed under the floor or the drain pipes are blocked or no gravel/crushed rock was installed under the concrete floor.


If you know drain pipes were installed under the floor, check the end where the water exists to make sure there's no blockage. If you have a sump pump pit, make sure water is draining properly and the pump is working.

The exterior of the foundation should have had drain pipe installed around the perimeter to collect ground water and move it away from the foundation. Make sure the end isn't clogged or crushed preventing the water from draining properly.

The landscaping around your house should slope AWAY from the house foundation. Sometimes the soil will settle causing pockets of lower soil. These lower areas will puddle rain water and cause moisture issues in your basement. 

If you have gutters, make sure the downspouts direct water AWAY from the foundation walls. A lot of water comes off the room when it rains. Extend the downspouts far enough away from the basement walls so the water isn't getting into the basement.

If no drainage pipes were installed during construction, you may need to install them now. You'd have to dig out around the foundation all the way to the footer and install the drainage pipes. 

Hire a PRO for this. The drainage has to be installed correctly and sloped away from the foundation at one intersecting point. This usually involves a lot of digging and then careful back filling.

can radon gas come up through a crack?

If you live in an area where radon gas is an issue, then yes. The best way to check for radon is with a radon gas detector

If you detect radon gas, then get the crack repaired and seal the floor with the RADONSEAL sealer. Wait a couple weeks and test for radon gas again. It should be a lot lower or gone altogether.

If not, then hire a pro to install a radon remediation system. This system will ventilate the radon gas out of the basement and keep it out of your house.

diy - how to repair a cracked basement floor.

Repairing basement floor cracksRepairing cracks in a basement concrete floor

Most cracks in basement floors can be repaired by yourself with a do it yourself epoxy concrete crack repair kit.

the 6 steps for repairing a basement floor crack.

STEP 1. Clean out the crack with a vacuum, remove any loose cement, rocks and debris.

STEP 2. Inject the crack weld resin into the crack to wet it, it will soon get tacky.

STEP 3. Push the dry silica sand into the crack with a putty knife to fill it.

STEP 4. Thoroughly saturate the sand with the crack weld resin and fill it to the surface.

STEP 5. In 10 - 15 minutes scrape the surface level with a putty knife.

STEP 6. Sand the surface smooth or lightly grind it with a small concrete floor grinder for a neat appearance.

Hairline concrete cracks can be done without the sand. Wide concrete cracks can first be filled partially with the sand then start with step 2.

This crack weld material sets up very quickly, only do 10 - 15 feet at one time.  

Radonseal makes a Do It Yourself basement floor crack repair kit that has all the repair materials included. They also have a couple of great videos that visually explain how to use this crack weld material.

Basement floor crack repair kit

Watch this video of how I fix a basement floor crack!

If you have a really wet basement with standing water and puddles click on BASEMENT FLOOR WATERPROOFING to learn how to fix having a wet basement.

If your basement floor is just damp or moist click on WATERPROOFING BASEMENT FLOORS to learn how to have a dry basement.

Click on BASEMENT FLOOR DRAIN to learn how to install a drain in your basement floor.

To learn how to install insulation on top of your basement floor click on BASEMENT FLOOR INSULATION.

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