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Community and Q&A

Suspended concrete slab on OSB/joist floor

Regulajo | Posted in GBA Pro Help on

We are building a home with a 3″ regular concrete slab (not lightweight) with embedded radiant heat poured over a traditional TJI and 3/4″ T&G floor. The floor is engineered to hold the weight of the concrete of course. This will be the finished floor.

My question is, do we need to put any type of underlayment on top of the OSB (under the slab) before the pour. We want to be sure the concrete can dry appropriately. Do we just tape the seams of the OSB to keep the pour from leaking into the lower level or does it need a barrier of some sort?

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  1. GBA Editor
    Martin Holladay | | #1

    No matter what you do, most of the water that leaves this slab as it cures will be evaporating from the top of the slab. The OSB will block almost all evaporation from the bottom of the slab. So I don't think it matters much whether you include a layer of polyethylene under the slab. Include it if you want, or leave it out.

    If the concrete contractor wants to avoid slab curling, it's important to make sure that the concrete isn't too soupy. But of course that's a good rule for quality concrete in any case.

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