How to Install a Hardwood Floor Over a Concrete Slab

By Corey Joe

Only a decade or two ago, most experts advised against installing a hardwood floor over a concrete surface. Wood reacts to changes in humidity and temperature by expanding or shrinking. Because concrete is porous, when a wood floor is laid directly over concrete, the wood is liable to act like a wick, drawing moisture up from the slab, resulting in cracks and warps in the wood. In the past, the most successful method was to lay a subfloor over the concrete before applying the hardwood floor. Today, new technologies and products are giving homeowners additional options.

Testing the Concrete

Before installing any type of wood floor over concrete, it's important to test the concrete slabs for excessive moisture, alkaline and calcium chloride. To prevent excess moisture, the building should be well-ventilated and the humidity of the building should be maintained at between 30 to 50 percent humidity, the range considered optimum for human health. Concrete can be tested for moisture by leaving a weighted, foot-square piece of rubber or plastic on the surface for 24 hours. If there is little or no condensation under the rubber or plastic after 24 hours, the concrete is likely dry enough for wood floor installations. Flooring supply distributors also offer vapor transmissions kits to test humidity, as well as kits that test the concrete's alkalinity and degree of calcium chloride.

Floating Engineered Hardwood Floors

Interlocking engineered hardwood floors are the easiest type of hardwood flooring to install over concrete. While only the surface veneer of these products is cut from hardwood, they have the look of solid wood once installed. Beneath the veneer are layers of wood that are oriented in different directions and glued together. This makes the finished product less subject to shrinkage and expansion than solid wood flooring.

 

 

 

While some engineered boards are designed to snap together without glue, for installation over concrete, the tongue-and-groove joints are typically glued together but not fastened to the floor, allowing the floor to "float" over the surface below. This allows the wood to expand and contract as needed.

As with any floor, it's important to ensure that the surface is level before installation. If the slab is too uneven or rough, it can be smoothed with a floor leveling compound or thin set. Most engineered flooring manufacturers offer 1/8-inch plastic foam insulation pads. These pads may be all that is needed to provide a moisture barrier between the slab and the floor. If not, 6-mil polyethylene film can be applied first. A 1/4-inch gap is left between the edges of the flooring and wall to allow for contraction and expansion. Baseboards will hide the gap.

Installing Engineered Hardwood Floors Directly onto Concrete

If a test of the concrete reveals an acceptable level of dryness and alkalinity, engineered hardwood flooring can also be directly installed onto concrete slabs, although this is not recommended for below-ground installations. For this technique, a high quality latex or urethane adhesive with moisture-cure properties is applied with a trowel to a clean, level, dry section of the slab. The hardwood flooring is then applied directly on the latex or adhesive.

Self-Adhesive Underlayment

Self-adhesive underlay products are available that can be used to install both solid and engineered hardwood flooring on concrete as well. This product consists of a sheet treated with adhesive that is covered with a top membrane made from silicon. If moisture in the concrete is a concern, a layer of 6-mil polyethylene film is applied first. The underlay is applied by rolling out the product, sticky side up, onto the concrete. The top membrane is peeled off and the flooring is applied directly onto the underlayment. Once the flooring comes into contact with the underlay, it becomes permanently attached to it and the adhesive sheet must be kept at a constant tension. Installation takes some practice and patience.

Hardwood Flooring Installed Over a Raised Subfloor

Raised subfloors are the traditional solution for installing solid wood floors over concrete. A moisture barrier is first applied to the concrete surface and allowed to dry. An adhesive is then applied. A grid of 2x4s is laid out over the floor, leaving a 1/2-inch gap between the ends of the 2x4s and the walls. The grid must be carefully checked to ensure it is level, using shims to adjust the level as needed. Plywood sheets are then nailed to the grid. A layer of 6-mil polyethylene film may be applied over the plywood. When installing the hardwood floor, it's important that nails reach through the plywood to the 2x4-grid below.

Hardwood Flooring Installed Over a Plywood Subfloor

In the past, some contractors have installed a plywood subfloor for hardwood flooring. This process involves layering asphalt felt or building paper with layers of urethane adhesive or asphalt mastic, followed by a final layer of 6 mil polyethylene film. Sheets of plywood are then fastened to the slab with power-activated fasteners, leaving a 1/2-inch space between the plywood and the walls.

Whatever method is used to prepare the concrete for flooring installation, the basic rules of installing hardwood flooring remain the same. The wood should be allowed to rest for 24 hours in the area where it will be installed to acclimate it to the environment. Planks should lie parallel with the longest wall in the room. Application should begin opposite the room's exit so that installer don't work themselves into a corner. Boards should be laid in overlapping patterns to improve stability and strength.

Perhaps surprisingly, the cost of installing engineered and solid hardwood flooring is comparable. Engineered flooring, however, is far easier and faster to apply. Engineered flooring also has a more established reputation for success in concrete floor installations. Traditional hardwood flooring can be installed over concrete, but consumers will want to carefully research the pros and cons of the available technologies and methodologies before making a final flooring installation decision.

For more information about hardwood flooring, contact World Floors Direct. They're sure to have the perfect floor for your home or office.