At or Above Grade
With dry concrete and the right subfloors and vapor retarders, solid hardwood floors can be installed on slabs that are either on-grade (at ground level) or above-grade (above ground level). Moisture can pose problems for solid hardwood floors below ground level, so engineered wood floors (made of layers of wood glued together) are the best choice there.
Protect the Flooring

Wood floors should not be subjected to very wet conditions. Before delivery to the site, the building should be closed-in with outside windows and doors in place. All concrete, masonry and sheet-rock should be completely dry. In warm weather, the building should be wellventilated. In winter, heating should be maintained near occupancy levels at least five days before the flooring is delivered and maintained from that point on.

Dry Slab
Concrete slabs, whether new or existing, must be dry before the subfloor is installed. For best results, several areas of each room should be tested for excess moisture. If tests indicate too much moisture is in the slab, do not install hardwood floors. A new moist slab should be allowed to dry naturally or the process can be hastened with heat and ventilation. Slabs less than 60 days old are generally too wet to install floors.

Test for Dryness
New concrete is heavy with moisture, and it must be a minimum of 30 days old before moisture testing begins. Determining moisture content of both the wood flooring and the subfloor is an important part of quality control in the flooring installation process. Detailed qualitative and quantitative testing methods are provided by the National Wood Flooring Association.

National Wood Flooring Association – Moisture Guidelines and Moisture Testing*

Flat, Clean Slab
The slab must be flat with a trowel finish, free of grease, oil, stains and dust. Grind off any high spots and fill any low spots.

Vapor Retarder
With solid hardwood floors, a proper vapor retarder must be installed on top of the slab to make sure normal slab moisture does not reach the finished floor. Layers of asphalt felt with asphalt mastic or 6 mil polyethylene sheathing are some of the choices.

Subfloors
The National Wood Flooring Association recommends two choices in sub-flooring systems for concrete installations: plywood-on-slab and sleeper. Either subfloor system is satisfactory for three-quarter inch flooring.

Plank Flooring
Plank flooring four inches and wider requires either the plywood-on-slab system or the sleeper system topped with a minimum of 5/8” inch thick plywood or ¾” OSB.

Plywood on slab
Begin by covering the slab with a vapor retarder.

Asphalt felt or building paper: first, prime the slab and apply cold, cut-back asphalt mastic with a notched trowel (50 sq. ft. per gallon). Allow to set for two hours. Unroll 15-lb. asphalt felt or building paper, lapping the edges 4″ 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.

Polyethylene: Cover the entire slab with 4 to 6 mil polyethylene film, overlapping the edges 4″ to 6″, 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 straightedge or fine-tooth trowel (100 sq. ft. per gallon). Lay 4 to 6 mil polyethylene film over the slab, overlapping the edges 4 to 6″.

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 retarder 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, spacing fasteners 2” from the edges every 6-8” along the perimeter and 1 fastener spaced approximately 12” apart within the interior of the panel.

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.

Floating subfloor of 2 layers of plywood secured together over a vapor retarder. No fasteners penetrate the retarder or the concrete.