Ask the Building Scientists

Q. What is the recommended prep for a concrete slab before installation of wood flooring?

-Kathleen Stroupe, Morrison Homes

A. 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 has not been poured, put polyethylene down directly under the slab. The polyethylene will stop ground moisture from traveling into the concrete slab and being transferred to the wood floors, which can create buckling. 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 this test: simply tape down a 2’x2’ piece of polyethylene to the slab in several different areas, sealing all edges with moisture resistant tape. If after 24 hours there is no clouding or droplets of moisture on the underside of the poly, the slab can be considered dry enough to install wood flooring.

If the concrete has already been poured and there is no polyethylene under the slab, test the slab for moisture using the test mentioned previously. If there is moisture, put polyethylene on top of the concrete slab. The polyethylene provides a barrier, keeping moisture from the wood flooring. We recommend 6-mil polyethylene. We also recommend putting ¾ x 2-½ furring strips, 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 Performance Specialist