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Community and Q&A

Separation of concrete slab and foundation

Joel Silverman | Posted in General Questions on

We removed carpeting in a condo that we own. We noticed by the sliding glass doors a crack in the slab. The slab has a layer of rigid insulation on the perimeter which seems to have seperated from the cinder block foundation. The cinder block wall does not appear to be bowing looking at it from the outside. Is this a big concern or is there a way to fix the problem without ripping everything up?

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  1. GBA Editor
    Martin Holladay | | #1

    It sounds like the rigid foam needs to be fastened to the CMU wall. This can be done with TapCon fasteners and large washers, or with special fasteners designed for this purpose (like those manufactured by Rodenhouse).

  2. Expert Member
    Malcolm Taylor | | #2

    Concrete slabs expand and contract. That's why they are often detailed with a flexible spacer. Foam also shrinks over time. there is a very good chance nothing is going on.

  3. Steve Knapp CZ 3A Georgia | | #3

    It wouldn't hurt to caulk the gap, correct? Joel. Is your house on an elevated slab with a concrete block stem wall?

  4. Joel Silverman | | #4

    Thank you Martin, Malcolm, and Steve for your answers, I think I left out some info which is that the condo is approx. 35 years old. So I couldn't use the tapcon fasteners if I wanted to, but thanks Martin for the info. Steve if I caulk the gap what type of caulk is best to use, also the condo is ground level with block foundation walls, and the slab is poured within the foundation so I guess it is not elevated. I am really not sure.

  5. Steve Knapp CZ 3A Georgia | | #5

    Joel. Usually you can use a polyurethane sealant from the big box stores. If the cracks are deep, you probably want to install a backer rod before caulking. If any cracks are wide, application can be easier if you use a plastic putty knife to spread the sealant. FYI. This is based on my personal experience DIYing similar issues. The pros here may have different or better advice.

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