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Q. What is the best way to install a wood floor over a concrete slab?

A.Howard Brickman responds: Concrete is a good substrate for installing wood flooring if proper precautions are taken to ensure that excessive moisture conditions are detected prior to installation,and controlled during the life of the floor. In addition, traditional nail-down, solid 3/4-inch strip or plank flooring must have an adequate wood substrate for proper nailing.

Concrete substrates. Even though concrete was used to build Hoover Dam, it doesn’t qualify as a "waterproof" material. Actually, the opposite is true. Concrete is quite porous.

To avoid excessive moisture problems, new slabs must be detailed properly. Place at least 6 inches of gravel or crushed stone on the ground, then install a 6-mil polyethylene vapor barrier. Make sure that this vapor barrier is not destroyed when the concrete is poured. The exterior of the concrete slab should also be dampproofed prior to backfilling. Before laying the floor, the slab should be allowed to dry sufficiently.

Always check for moisture in concrete prior to installing a wood floor by taping polyethylene over a clean place on the slab and allow 12 to 24 hours for signs of moisture to develop.

Solid 3/4-inch tongue and groove strip and plank flooring cannot be installed directly to concrete. You must install a wood subfloor for nailing. Two recommended methods are a 3/4-inch "plywood-on-slab" and a "floating plywood" system.

Plywood-on-slab. Place a 6-mil polyethylene vapor retarder directly over the concrete. Lay 3/4-inch plywood sheets opposite to the direction of the finish flooring, leaving a 1/4-inch gap between panels to eliminate squeaking. Fasten the plywood to the concrete using powder-actuated fasteners or concrete nails. Standard 2-inch powercleats or pneumatic staples will contact the concrete surface beneath the plywood unless the machines are tilted forward by placing a 5/16-inch-thick spacer at the back edge of the nailer’s faceplate. The alternative is to purchase 1 3/ 4-inch powercleats specifically designed for this application.

Floating plywood system. Place a 6-mil polyethylene vapor barrier over the concrete. Lay 1/2-inch plywood sheets along the long axis of the room. Place a second layer of 1/2-inch plywood at 45 degrees over the first layer. Again, leave 1/4-inch gaps between sheets of plywood to eliminate squeaks. Then fasten the two layers of plywood together using 7/8-inch pneumatic staples. Leave a minimum of 1/2-inch expansion space at the perimeter for normal-sized areas. It is a good idea to increase expansion for large areas. When similar systems are used in large spaces, such as gymnasiums, a 2-inch expansion space is required.

Laying the floor. When laying any wood floor, be sure to: ­ Use 15-pound asphalt-saturated building paper or felt under all nail-down floors ­ Start the flooring straight using a chalk line or string ­ Use plenty of nails ­ Make use of the tongue and groove or splines when changing direction and or working from one room to another.

— Flooring contractor Howard Brickman is the owner of Brickman Flooring Co., which sells, installs, and finishes wood floors. He also conducts training seminars at his shop for flooring contractors and finishers.