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Community and Q&A

Leveling a Concrete Slab

moose_head27 | Posted in General Questions on

I’m planning to start a basement reno next week. I’ve decided on 1 inch XPS with 5/8 tongue and groove plywood for the subfloor. The assembly would be fastened with tapcons.

I’m just curious to know how level the concrete slab needs to be before I install the subfloor. My slab is fairly level however there are some low and high areas. Some difference is elevation is up to 1/4 inch. This difference is only in select areas. Is this acceptable before I put the subfloor? I was looking a grinding down the high spots. As for the low spots initially I was going to pour some self leveler but everyone who I talk to says that it will crack if I try to drill into it.

So really I’m just looking to see if a 1/4 inch differential in the concrete slab is ok or not, especially of the the low areas.


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  1. GBA Editor
    Kiley Jacques | | #1

    Check out this Q&A thread for some ideas.

  2. moose_head27 | | #2

    Thank you for the ideas.

    I did reach out to a few self levelers companies. They recommended to install the foam and plywood and to screw with tapcons on the existing slab floor. Then suggested to use the leveler on any low points to avoid screwing into the leveler.

    I may go this route and try to shim the plywood as it lay it on the worse points. I haven't been able to find much conversation on this kind of approach though.

  3. Expert Member
    Michael Maines | | #3

    Anyone who cares about our environment and climate change should think twice (or more) before going with XPS, which has blowing agents that are potent greenhouse gasses and persistent in the atmosphere. EPS works just as well in most situations and has significantly less impact; it's just a little harder to source. GPS outperforms XPS, with similar emissions as EPS.

    I would use floor leveler (or thin-set mortar, an unapproved but cheaper alternative, results may vary) to fill in the worst of the 1/4" dips, aiming for a surface within about 1/8 of level, and preferably closer to perfect. If you stand next to the hole when pre-drilling and screwing the sheets down with Tapcon (or similar) screws, you shouldn't have too much blowout.

  4. continentalsey | | #4

    Hmm, it's not that hard. Concrete floors can be leveled using self-leveling solutions, cement-sand screeds or glue for ceramic tiles. For linoleum and carpet, leveling concrete floors is considered a mandatory operation, for ceramic or stone tiles, the decision is made based on the actual condition of the bearing base. All floors mounted on logs can be laid without prior leveling of concrete bases. The first thing that is looked is the quality of the materials used. It works! I am sure you will succeed.

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