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Causes of Basement Floor Cracks and What to Do About Them

Wednesday, August 28th by Kristina McGovern


Use this quick guide (with pictures) to determine whether or not the cracks in your concrete basement floor require attention.

Close-up of a crack in a concrete basement floor

Cracks in the basement floor can let in water, moisture, and radon gas.

Cracks in the basement floor can be a cause of panic for homeowners. The good news is most basement floor cracks are normal and don’t need to be repaired. However, some cracks in the basement floor can let in water, moisture, and dangerous soil gasses (including radon). This guide with visual examples can help you understand what the cracks in your basement floor mean and whether or not they should be repaired.

Why do basement floors crack?

It’s very common for cracks to form in a home’s foundation after it’s built. Most times the cracks are normal, non-structural settlement cracks. However, there are many reasons why a foundation cracks, such as settling of the home, concrete shrinkage and curing, stress, and poor construction.

How's your drainage?

If the soil around the foundation is poorly graded or you have improper drainage, water can build-up and put pressure (referred to as hydrostatic pressure) on the foundation, causing it to crack. There are simple steps you can take to relieve hydrostatic pressure and help prevent cracks in your basement.

Ways to improve drainage:

  • Make sure landscaping is properly graded and slopes away from the foundation.
  • Keep your gutters clean and in good working condition. Install gutters and downspouts if you don’t have them.
  • Extend your downspouts away from the house to keep them from dumping water close to the foundation walls.
  • Add interior perimeter drainage and a good sump pump system in the basement.

If you have a crack in your basement floor, determine the type of crack and what’s caused it to figure out how to best solve your problem.

6 types of basement floor cracks - and what they mean

  1. Cracks with heaving are signs of a bigger problem that needs to be addressed.

    Foundation heave crack in a concrete basement floor

    Heaving is common where there's clay soil.

    When a basement floor crack is combined with heaving, this is a cause for concern. Heaving suggests that the soil beneath the basement floor is expanding. Heaving is especially common in areas with clay soil. When the soil becomes wet, it creates enough pressure to lift and crack a concrete slab floor.

    What to Do: Call a professional foundation repair contractor. They can assess the situation and recommend the best solution.

  2. Concrete floors with cracks and sinking should be leveled.

    Close-up of a cracked, uneven concrete floor

    Settlement causes cracked, uneven concrete basement floors.(Photo: Supportworks)

    When a foundation settles, the concrete slab can crack and the part of the slab that's less supported sinks into the depression. The resulting uneven surface can create a tripping hazard.

    What to Do: Concrete basement floors that are uneven should be leveled. PolyLevel uses a high-density polyurethane formula to permanently lift and stabilize a sinking concrete slab. Penny-sized holes are drilled into the slab and a structural-grade polymer is injected into the void. The polyurethane material expands, lifting and leveling the slab.

  3. The gap between the basement wall and floor shouldn't be sealed.

    Close-up of a gap between the basement floor and bottom of the basement wall

    Water seepage at the cove joint is a common problem.

    The area in the basement where the walls and floor meet (called the cove joint) is a common problem area for water seepage. During construction, when the concrete walls and floor are poured, a small gap is left between the floor and walls. When the groundwater level rises and the soil becomes saturated, hydrostatic pressure forces water through the cove joint.

    What to Do: Sealing the gap isn't recommended since the sealant can crack. The most effective solution to keep water and moisture out of the cove joint is with a basement waterproofing system (interior drain tile and a sump pump), which helps relieve hydrostatic pressure.

  4. Hairline cracks don’t require repair.

    Hairline crack in a concrete basement floor

    Hairline cracks are caused by shrinkage as concrete dries.

    Small cracks in the basement floor commonly result from shrinkage as the concrete dries, which pulls the concrete apart. These types of cracks can show up as long as a year after the basement floor is poured, depending on how quickly the slab dries and how humid the basement is.

    What to Do: Nothing. Hairline cracks in the basement floor are superficial and don’t require repair. If you are worried about how they look, you can cover the concrete with basement flooring like Basement Systems’ ThermalDry wood-look plank flooring or carpeted flooring.

  5. 1/8-inch or wider cracks should be sealed.

    Large crack in a concrete basement floor

    Cracks 1/8-inch or wider should be sealed.(Photo: Supportworks)

    Like hairline cracks, these cracks that are slightly wider are likely the result of shrinkage and are not a sign of a serious foundation problem. However, even small cracks in the basement floor can allow water, moisture, and radon to enter into the basement.

    What to Do: Cracks wider than ⅛-inch should be sealed to help keep water, moisture, soil smells, and radon gas from seeping through the basement floor.

  6. Spalling or flaking is ugly but doesn’t require repair.

    Closeup of concrete spalling of a basement floor

    Spalling concrete is unattractive, but usually not cause for concern.

    It can occur when the concrete mix is too wet or hasn’t been properly cured. The water evaporates and loosens the top layer of the concrete, causing it to flake off. Concrete spalling is unattractive, but chances are the slab is still in good shape.

    What to Do: Don’t try to hide the problem with paint. The surface of the floor is likely to keep flaking and when it does, it will take the paint off too. Consider concrete resurfacing to make the concrete look like new and prevent future spalling. Another option is to install waterproof flooring over the concrete.

If you'd like a professional evaluation of your basement floor cracks or you are dealing with water intrusion in the basement, contact us for a free basement crack repair estimate!

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