You can transform any room in your house by installing hardwood flooring, but you need to be especially mindful of moisture when the subfloor is concrete. Even after it has cured and dried, concrete retains enough moisture to ruin the flooring if you don't provide moisture protection. Concrete subfloors that are below grade -- which means below ground level -- are especially susceptible to moisture, and installing certain products on one can void the warranty.
Preparing for Installation
Before you install hardwood on a concrete slab, the entire slab must have cured completely and be dry, which means waiting a minimum of 60 days after you pour it. The relative humidity of the slab, as measured by a moisture meter, should be less than 75 percent for most products, and it should be uniform over the whole slab. Moreover, you need to unpack the flooring and spread it around the room for three to five days so it can come into equilibrium with the subfloor. The variation in moisture content between the flooring and the slab, as measured with the meter, should be within 2 and 4 percent, depending on the board width.
Plywood on Slab
One of the recommended methods for installing wood flooring on a concrete slab is to first lay a plywood or OSB subfloor to give you a substrate to which to nail the boards. If you choose this method, you should first apply a vapor retarder to the subfloor. To do this, you first paint the concrete with primer, then spread mastic, cover it with 15-pound asphalt or felt paper, and install a second coat of mastic and paper, staggering the joints. You can also simply use a single layer of 4- to 6-mil polyethylene film or, if moisture is a problem, a double layer reinforced with mastic.
Another installation method that allows you to nail down the boards is to use sleepers, which are lengths of two-by-fours laid across the floor perpendicular to the flooring direction. The sleepers, which should be six to seven inches apart, should be secured with asphalt mastic onto the subfloor, which you have first sealed with asphalt primer. Before laying the flooring, a single layer of 4- to 6-mil polyethylene sheeting goes over the sleepers. Before stapling the sheets to the sleepers, make sure the edges overlap by at least four inches and that the plastic isn't bunched or torn.
Some installers are loathe to glue flooring to concrete, because any residual moisture that happens to be present can interfere with adhesion. If you use a moisture-resistant product, such as a two-part epoxy or a moisture-curing urethane adhesive, on a properly cured and dry slab, however, the floor shouldn't lift. Even so, gluing down isn't recommended on below-grade subfloors, especially if you're installing solid -- as opposed to engineered -- flooring. Moisture that seeps from the ground may not affect the glue bond, but it can make the wood swell and cause defects such as cupping.