Hardwood flooring is one of the hallmarks of quality construction. Craftsmanship and materials are clearly exposed to inspection, and few aspects of the home are as high in consumer appeal. That's why experienced builders pay close attention to minimizing expansion and contraction when preparing to install this traditional material, especially in concrete slab construction.

With the right subfloors, hardwood flooring can be installed successfully on either on-grade or above-grade slabs. Below-grade installation is not recommended. The slab must be flat and level with a trowel finish, and free of grease, oil, stains and dust. New concrete is heavy with moisture, so test for dryness before beginning the subfloor.

There are two subflooring systems that are generally recommended: plywood-on-slab and sleeper. Either system is satisfactory for 1/4-inch flooring up to 4-inches wide. For plank flooring 4 inches and wider, use the plywood-on-slab system, or the top sleeper system with an additional nailing surface. The best materials for this surface are 5/8-inch or thicker plywood, or 3/4-inch boards no wider than 6 inches. Be sure to allow 1/2-inch spacing between 3/4-inch boards.

Plywood on slab


Begin by covering the slab with a vapor barrier. For asphalt felt or building paper, prime the slab and apply cold, cut-back mastic with a notched trowel (50 sq. ft. per gallon) and allow it to set for two hours. Unroll the 15-pound asphalt felt or building paper, lapping 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.

For polyethylene, cover the entire slab with 4- to 6-mil polyethylene film, overlapping the edges 4 to 6 inches, 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 straightedge or fine-tooth trowel (100 sq. ft. per gallon) and lay 4- to 6-mil polyethylene film over the slab, overlapping the edges 4 to 6 inches. In either case, roll the film flat or "walk" it in, stepping on every square foot to insure 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-inch, 4-by-8-foot exterior plywood panels over the entire area, leaving a 3/4-inch space at the wall line and a 1/4-to-1/2-inch between panels. Cut the plywood to fit within 1/8-inch near doorjambs 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 the 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.

Do not use power-actuated fasteners or concrete nails when radiant heat pipes are imbedded in the slab. Instead, cut the plywood into 2-by-8-inch planks and score the backs 3/8-inch deep on a 12-foot grid. Lay the panels in a staggered pattern with at least 2-foot long planks along the starting and finishing walls.

For systems other than radiant heat, the plywood planks may be glued to the plastic with asphalt mastic. Spread the mastic with a 1/4-by-1/4-inch notched trowel.


Flat, dry, preservative-treated 2-by-4-inch sleepers in random lengths can also serve as a nailing base. Begin by sweeping the slab clean, then apply an asphalt primer and allow 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-inches on center, at a right angle to the direction of the finished flooring. Stagger the end joints, overlapping the ends 4 inches, with a 1/4-inch space between.

Before installing the floor, loosely lay an additional vapor barrier to 4- to 6-mil polyethylene film over the sleepers, overlapping the edges on top of the 2-by-4s. Avoid bunching or puncturing the film, especially between sleepers. Finally, nail the finished flooring to the sleepers through the film.

For more information, contact The Hardwood Council at P.O. Box 525, Oakmont, PA 15139, or go to www.hardwoodcouncil.com