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
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
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
using a chalk line or string Use plenty of nails
Make use of the tongue and
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