When installing hardwood flooring over a concrete slab, it is important to understand the keys to managing natural expansion and contraction in hardwood products. These include slab preparation, moisture management of the concrete slab and flooring materials, and subflooring options.

Slab Preparation The National Oak Flooring Manufacturers Association (NOFMA) recommends that concrete slabs be sound and flat. To prepare the slab, grind off any high spots, fill low spots, and remove grease, oil, stain, and dust.

Concrete Slab Moisture Management Concrete slabs must be tested for excessive moisture before installation. Testing options, which should be conducted in several spots on a slab, are the rubber mat method and the polyethylene film method.

Rubber mat testing involves laying a flat, non-corrugated rubber mat on the slab and weighting it to seal against the slab surface. If dark, wet marks appear on the weighted area after 24 hours, moisture evaporation levels from the slab are too high.

Polyethylene film testing involves taping a 12-inch square of clear polyethylene to the slab, sealing all four sides with moisture-resistant tape. If condensation collects under the film in 24 to 48 hours, the slab’s moisture content is too high for installation.

Managing Moisture in Hardwood Flooring Materials It is important to manage hardwood flooring’s tendency to expand and contract. Proper handling of hardwood flooring, coupled with job site preparation, will help ensure a superior end result.

Never truck, unload, or store unprotected hardwood flooring in rain, snow, or other wet conditions. Before taking delivery of hardwood materials, be sure that building structures are closed in, with all outside windows and doors in place. Maintain occupancy-level temperature and humidity for at least five days, and allow excess moisture anywhere in the structure to evaporate. Let sheetrock dry for at least two days, and plaster for a week or more, depending on weather conditions.

When hardwood flooring is delivered to the site, let the wood adjust to the home environment by dividing flooring pieces into small lots, and storing them in the rooms where they will be installed. It will take at least a week for their moisture content to reach a balance with their surroundings.

Dimensional changes on-site will vary with region, season, individual construction practices, and interior climate controls. For example, along the Gulf Coast, hardwoods will acclimate to an 11-13% moisture content; hardwoods installed in the Western Rockies may level off at 4-8%.

Subflooring Options for Installing Hardwood on a Concrete Slab NOFMA recommends two choices in concrete slab subflooring systems: plywood-on-slab and sleeper. Either subfloor system is satisfactory for ¾-inch flooring up to 4 inches wide. For plank flooring 4 inches and wider, use the plywood-on-slab system, or top the sleeper system with an additional nailing surface. The best materials for this surface are 5/8-inch or thicker plywood, or ¾-inch boards no wider than 6 inches. Allow ½-inch spacing between ¾-inch boards.

Plywood-On-Slab Begin by covering the slab with a vapor barrier of asphalt felt, building paper, or polyethylene. When using asphalt felt or building paper, first prime the slab and apply cold, cut-back asphalt mastic with a notched trowel (50-square-feet per gallon). Allow it to set for two hours. Next, unroll the15-pound asphalt felt or building paper, overlapping the edges 4 inches and butting the ends. Over this, apply a second, similar coating of mastic, and roll out a second layer of asphalt or paper in the same direction as the first, staggering the overlaps to achieve an even thickness.

To create a polyethylene vapor barrier, cover the entire slab with 4-6 mil polyethylene film, overlapping the edges 4-6 inches, and allowing enough film to extend under the baseboard on all sides. When moisture conditions are more severe, prime the slab and apply cold, cut-back mastic with a straight edge or fine-tooth trowel (100-square-feet per gallon). Allow it to dry for at least 90 minutes before laying 4-6 mil polyethylene film over the slab, remembering to overlap the edges 4-6 inches.

In either case, roll the film flat or “walk” it in, stepping on every square foot to ensure adhesion. Puncture bubbles to release trapped air.

Install the plywood after the vapor barrier is in place. Loosely lay a nailing surface of ¾-inch, 4-foot-by-8-foot exterior plywood panels over the entire area, leaving a ¾-inch space at the wall line, and ¼-inch to ½-inch between panels. Cut the plywood to fit within 1/8-inch near door jambs and other obstructions where finish trim will not be used. Lay the plywood diagonally across the direction of the finished floor to help prevent cracks along panel edges.

Fasten the plywood to the slab with power-actuated fasteners, securing the center of the panel first, then the edges, using nine or more fasteners. The plywood planks may be glued to the plastic with asphalt mastic. Spread it with a ¼”x ¼” notched trowel.

Sleepers Flat, dry, preservative-treated 2”x 4” sleepers in random lengths (18 inches to 48 inches) can also serve as a nailing base. Begin by sweeping the slab clean, applying an asphalt primer, and allowing it to dry. Next, embed the sleepers on their faces in rivers of hot (poured) or cold (cut-back) asphalt mastic, in rows 12 inches on center, at a right angle to the direction of the finished flooring. Stagger the end joints, overlapping the ends 4 inches, and leave a ¾-inch space between the sleepers and the wall.

Before installing the floor, loosely lay an additional vapor barrier of 4-6 mil polyethylene film over the sleepers, overlapping the edges on top of the two-by-fours. Avoid bunching or puncturing the film, especially between sleepers. Nail the finished flooring to the sleepers through the film.

For more guidelines on installing hardwood flooring over a concrete slab, visit the Hardwood Council Website at www.hardwoodcouncil.com.