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Re: Wood flooring over concrete
From: info@builderswebsource.com
Category: Flooring
Remote Name: 63.198.181.43
Date: 28 Oct 2002
Time: 02:53 AM
Comments
You should never lay OSB or plywood directly over a concrete slab. First,
before you do anything, check the slab's moisture with a moisture meter to be
sure that it's thoroughly dry. If the concrete is new, let it sit for at least
1-2 months to fully hydrate. Assuming the slab is dry, cover it with a
heavy-duty vapor barrier to prevent any residual moisture from wicking up into
the wood. Then install plywood or OSB over the vapor barrier. You'll need to
shoot nails into the concrete through the wood. Now you have a good clean
surface for installing hardwood flooring using traditional processes. Our
preference is generally to use 3/4" plywood with an exterior-grade glue.
The following excerpt from the article Installing Hardwood Flooring Over a
Concrete Slab by Sandra McVeigh (see Floor Covering Installer Magazine at http://www.fcimag.com
provides additional tips: "Plywood-On-Slab: Begin by covering the slab with
a vapor barrier of asphalt felt, building paper, or polyethylene. When using
asphalt felt or building paper, first prime the slab and apply cold, cut-back
asphalt mastic with a notched trowel (50-square-feet per gallon). Allow it to
set for two hours. Next, unroll the15-pound asphalt felt or building paper,
overlapping 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. To create a polyethylene vapor barrier, cover the entire slab with
4-6 mil polyethylene film, overlapping the edges 4-6 inches, 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 straight edge
or fine-tooth trowel (100-square-feet per gallon). Allow it to dry for at least
90 minutes before laying 4-6 mil polyethylene film over the slab, remembering to
overlap the edges 4-6 inches. 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 barrier is in place.
Loosely lay a nailing surface of ¾-inch, 4-foot-by-8-foot exterior plywood
panels over the entire area, leaving a ¾-inch space at the wall line, and
¼-inch to ½-inch between panels. Cut the plywood to fit within 1/8-inch near
door jambs 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 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. The plywood planks may be glued to the plastic with asphalt
mastic. Spread it with a ¼”x ¼” notched trowel."
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