Concrete & Wood Flooring

Leveler installed.

Wood flooring can be a great option over concrete as long as the concrete substrate meets the requirements of the wood flooring. Concrete substrates must be blocked from transmitting moisture and they must be clean and flat. If the concrete needs leveling, then the concrete must be porous enough to absorb a liquid sealer. Then a liquid sealer can be applied to limit moisture vapor transmission. If the concrete is power trowelled so much that the surface is sealed, and then the surface must be abraded to open up the pores. If there are contaminates or adhesive residues then the surface of the concrete must also be abraded to open up the pores so the concrete will absorb a liquid sealer. After the concrete is sealed, then it must be flattened so it meets the flooring manufacturer’s specifications. This usually means flat within ¼” over 10’ or 3/16” over 8’ for glue down installations, but you must check the manufacturers installation directions. Concrete slabs are very rarely this flat when they are pored, so leveling compound is usually required. First the concrete is sealed, and then leveling compound mortar is pored over the surface to make it flat. The flatness is critical to ensure there are no voids or hollow spots under the product that is installed. These hollow spots will cause the flooring to fail.

When installing wood flooring over concrete there are several systems that can be used and each has its benefits and applications. The concrete must be flat enough to start with or it must be sealed and leveled before proceeding.

One option is to float the hardwood over the concrete. If the concrete is flat with no leveling needed, then a vapor barrier can be installed and the flooring can be installed.  There are numerous vapor barriers for floating applications: The concrete can be sealed with a liquid sealer, or 6 mil poly can be put down over the concrete with the seams sealed with a roof/tar compound, or a foam moisture barrier can be put down with seams taped with moisture proof tape. These floating installations have limitations on the amount of wood flooring that can be connected. Since the wood “sheet” expands and contracts as a whole, then the size of the sheet (rectangle) and the stress points in doorways or narrow openings must be considered. The floating floors typically have a limit on the size of a rectangle around 400 sf and they require T-moldings at doorway seams along the board edge. So for open floor plans, floating floors require moldings that break up the open floor plan and they are not a good option. But for single rooms, floating applications can be a great option.

A variation on floating the hardwood directly over the concrete is to install floating plywood before nailing the hardwood to the plywood. The moisture barrier is installed the same as floating the hardwood directly over the concrete. Then plywood is ripped into strips no more than 12” wide and loose layed over the moisture barrier. The plywood must lay flat or else it must be scored deep enough and often enough that it will lay flat. Then hardwood can then be nailed to the plywood. 

If the concrete is sealed and flat then another option is to glue engineered hardwood directly to the substrate. This works well when there is an open floor plan or the height of the floor must be kept to a minimum.

Another great option is to seal and level the concrete and then glue down plywood. The plywood must be cut into half sheets in either direction, and then scored in an 8” to 12” grid so that only the last ply remains to hold the sheet together. This plywood sheet conforms to slight irregularities and makes constant contact with the adhesive and the concrete substrate. After the plywood is glued down, then depending on the application (above or below grade) solid or engineered plywood can be nailed down. This method is more forgiving because a void of a couple inches on a piece of plywood does not affect the floor. But if you have a void on a floating floor or on direct glue down floor, then that board or boards will become loose and fail. If a house has the height available at the exterior doors, then this is the best option.