An option when installing hardwood floors over concrete involves the installation of screed over the concrete. Screeds, otherwise known as sleepers, are pressure-treated 1x4 or 2x4 boards that can be purchased in various lengths from a home improvement retailer. The pressure treatment prevents moisture and insects from destroying the wood, making for a sturdy floor base. There are several methods of fastening the screed to the concrete slab, but screws capable of penetrating concrete are the best option because they are hardier than nails.
Roll waterproof mastic onto the concrete slab, using a paint roller. Use an extension pole attached to the roller to cover larger areas when applying mastic.
Snap lines across the floor, using a chalk box, to represent the placement of the 1x4 screeds. The snapped lines, along with the screed, will run perpendicular to the finished floor material. The first snapped line is one inch away from the wall. This small gap between the installed screed and wall will prevent the build-up of moisture that could cause discolouration to walls over a period of time. Place the following snapped lines every 16 inches until the opposite wall is reached. Once the opposite wall is reached, leave another one inch gap between the wall and screed line.
Place the screed next to the snapped lines and screw them into place using a screw gun equipped with concrete screws. The concrete screws should be a minimum of one and a half inches long. Use your feet to firmly press the screed against the slab as the screw penetrates into the concrete, to ensure that the screed will not be pushed up, which would result in uneven floors.
Cut the screed to the appropriate length when necessary, using a circular saw equipped with a woodcutting blade. Leave a 1/8-inch gap between all joints to allow for wood expansion due to moisture and temperature fluctuations.
Place a vapour barrier over the installed 1x4 screeds. Polythene film, which comes in rolls, is the most widely used vapour barrier. In order to properly control vapours, the vapour barrier's seams should have a four to six inch overlap. Keep a minimum of four to six inches of extra vapour barrier around the entire perimeter of the floor to be placed behind installed wall base trim.
Overlap the installed screed with another layer of 1x4 screeds. Fasten them into place using wood screws and a screw gun. Place the screws 10 to 12 inches apart, sandwiching the vapour barrier between the screed layers. Finally, install the finish floor, such as hardwood or tile.
Some people prefer to use solid 2x4 pressure-treated wood instead of two 1x4 screeds. This requires the mastic application, followed by polythene film and then the fastening of 2x4 screeds. You will also need to use longer concrete screws or a hammer drill to pre-drill holes in the concrete slab for concrete anchors.