Question: What's the recommended way to prepare a concrete slab before installing wood flooring?

Answer: There are two options for prepping concrete before installing wood flooring. The one you should use depends on whether or not the concrete has been poured.

If the concrete hasn't been poured, place a 10 mil polyethylene vapor barrier down directly under the slab. Make sure the polyethylene is sealed at all plumbing and electrical penetrations and that the poly has been extended underneath the footers. The polyethylene will stop ground moisture from traveling into the concrete slab and being transferred to the wood floors, which can lead to buckling. For more information about installing a vapor barrier underneath a slab, download this step-by-step document.

The polyethylene should keep the concrete slab dry, but you should test it to make sure embodied water has dried before you install the wood flooring. To check the slab for moisture, use a calcium chloride test kit, which takes about 72 hours to conduct.

If the concrete has already been poured and polyethylene hasn't been installed under the slab, conduct a calcium chloride test to check the slab for moisture. If there's moisture, put polyethylene on top of the concrete slab. The polyethylene provides a barrier, keeping moisture from contacting the wood flooring. We recommend 6-mil polyethylene. We also recommend putting ¾ x 2-½ furring strips, which are narrow slats of wood, between the polyethylene and wood flooring to provide a nailing surface for floor installation.

Whichever option you use, we strongly recommend that you check for moisture before installing any flooring and implement water management strategies as a critical part of prepping the concrete slab.

- Anthony Grisolia, IBACOS
Building Services Manager