DEAR TIM: I really would like to have a solid hardwood floor but my existing house has a concrete slab. Is it possible to install a hardwood floor on top of concrete? How in the world do you attach the strips of hardwood to the concrete? What needs to be done to insure that the floor looks as good as one installed on a wood sub-floor? Stacey C., Boulder, CO

DEAR STACEY: If your slab surface is level with the outside earth or is an elevated concrete slab you can install traditional hardwood flooring over it. Concrete slabs that are below ground level are not candidates for solid hardwood flooring. If you have such a slab, I would consider installing an engineered hardwood flooring. This material is made by taking different plys or layers of wood and gluing them together to make a wood product that is more stable when subjected to elevated levels of humidity often found in basements or below grade slabs.

Wood is a hygroscopic material. In other words, wood changes its shape in response to changes in relative humidity or the presence of liquid water. As the humidity goes up or if wood gets wet, it swells. As the wood dries it shrinks or contracts. This type of movement can cause all sorts of problems with finished wood floors or even wood furniture. For this reason, it is important that the hardwood stays dry and at or near constant humidity levels before, during and after installation.

To test any slab for excess moisture, simply vacuum a small area first. Then tape a 15 inch square piece of clear polyethylene film to the slab using two strips of duct tape along each of the four edges. If no condensation or fog develops under the plastic after 48 hours, the slab is sufficiently dry. If the slab is wet, then heat the space and open windows or use a dehumidifier to draw moisture from the slab. Retest the floor for moisture content once you think it has dried.

The hardwood flooring is fastened to plywood or 2x4 strips that are securely attached to the concrete. Keep in mind that the height of your finished floor will end up at least one and one half to two and one quarter inches above the slab height depending upon which wood subfloor you decide to use. This raised floor can be a problem at doorways if you or the builder do not plan for this.

Before the plywood or 2x4 sleeper strips are attached to the floor, an additional vapor barrier must be installed on top of the dry concrete. Use the same high performance plastic that should have been used below the slab. The concrete floor needs to be level or in the same plane. Use a 6 foot long straightedge to check for high and low spots. Standard asphalt shingles make excellent shims to fill in low spots. The vapor barrier can be installed over the shingles and then the wood on top of that.

Fastening the plywood or 2x4 strips to the concrete can be done one of several ways. You can buy special hardened concrete screws that can bite into the sides of a pre-drilled hole in the concrete slab. Driving these screws requires a powerful screw gun to accomplish the task. A simple yet effective method is to drill a 1/4 inch hole through the wood and then directly into the concrete. The depth of the hole should be 3.5 inches. Take two 16 penny sinker nails and drive them side by side at the same time into the hole. Use a 20 ounce or larger hammer to drive the nails and strike them squarely. Believe it or not, the nails will bite into the concrete and hold the wood securely to the slab.

Before you actually install the hardwood flooring, make sure the room has been heated to room temperature for at least five days before the flooring is delivered. Once the wood is delivered, remove any packaging so it can acclimate. The wood needs to be exposed to the room temperature and humidity for at least an additional five days before you actually begin to install it.