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Hardwood Flooring Installations Over a Concrete Slab Pre-Installation Factors Experienced professionals pay close attention to managing moisture and minimizing wood floor expansion and contraction when preparing to install hardwood floors on concrete slabs. With dry concrete and the right subfloor and vapor barriers, hardwood flooring can be installed successfully on either on-grade or above-ground slabs. Below-grade installation is not recommended. The slab must be flat and level with a trowel finish, free of grease, oil, stains and dust. New concrete is heavy with moisture, so test for dryness before beginning the subfloor. Make tests in several spots in each room. When tests indicate excess moisture, wait for the slab to dry naturally or accelerate drying with heat and ventilation, then test again before installing floors. Concrete Slab Moisture Testing. Here are two simple procedures to check for the presence of excessive concrete slab moisture: Rubber mat method. Useful only on light-colored concrete. Lay a flat, non-corrugated rubber mat on the slab. Weight it to seal it against the surface. Look under the mat after 24 hours; if the covered area shows dark, wet marks, the slab is still too wet. Polyethylene film method. Tape a 15-inch square of clear polyethylene film to the slab with moisture-resistant tape, sealing all four edges. If no condensation collects under the film after 24-48 hours, the slab is dry enough for floor installation. Subfloor Options The National Oak Flooring Manufacturers Association (NOFMA) recommends two choices in sub-flooring systems for concrete slab installations: plywood-on-slab and sleeper. Either subfloor system is satisfactory for 3/4" inch flooring up to 4" wide. For plank flooring 4" and wider, use the plywood-on-slab system, or top the sleeper system with an additional nailing surface. The recommended materials for this surface are 5/8" or thicker plywood, or 3/4" boards no wider than 6 inches. Allow 1/2" spacing between 3/4" boards. Plywood-on-Slab Construction First, prime the slab and apply a cold, cut-back asphalt mastic with a notched trowel (50 sq. ft. per gallon). Allow to set for two hours. Unroll and layout 15-lb. asphalt felt or building paper, lapping the edges and ends a 4 to 6 inches. Over this, apply a second similar coating of mastic and roll out a second layer of 15-lb asphalt in the same direction as the first, staggering the overlaps to achieve an even thickness. Cover the entire slab with 4 to 6 mil polyethylene film, overlapping the edges 4" to 6", allowing enough film to extend under the baseboard on all sides. 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 3/4" 4' x 8' exterior plywood panels over the entire area, leaving a 3/4" space at the wall line and 1/4" to 1/2" between panels. Cut plywood to fit within 1/8" near door jambs and other obstructions where finish trim will not be used. Lay plywood diagonally across the direction of the finished floor to help prevent cracks along panel edges. Fasten plywood to the slab with power-actuated fasteners, securing the center of the panel first, then the edges, using nine or more fasteners. Do not use power-actuated fasteners or concrete nails when radiant heat pipes are embedded in the slab. Instead, cut the plywood into 2' by 8' planks and score the backs 3/8" deep on a 12" grid. Lay panels in a staggered pattern with at least 2' long planks along starting and finishing walls. For systems other than radiant heat, the plywood planks may be glued to the plastic with asphalt mastic. Spread using a 1/4" by 1/4" notched trowel. Sleepers Flat, dry, preservative-treated 2" x 4" sleepers in random lengths (18" to 48") 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 flat faces in rivers of hot (poured) or cold (cut-back) asphalt mastic, in rows 12" on center, at a right angle to the direction of the finished flooring. Stagger the end joints, overlapping the ends 4", with 1/4" space between. Before installing the floor, loosely lay an additional vapor barrier of 4 to 6 mil polyethylene film over the sleepers, overlapping the edges on top of the 2 x 4s. Avoid bunching or puncturing the film, especially between sleepers. Nail the finished flooring to the sleepers through the film.
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