A.Howard Brickman responds: The best way
I’ve found is to build a floating plywood floor over
foam, then apply strip flooring over this (see "Laying Wood
Floors Over Concrete Slabs," 10/94). Depending on how much foam
you use, you will raise the floor level several inches.
Under any floor on a slab, I first lay down a double 6-mil
poly vapor barrier, taping the seams at the overlaps and
extending the edges up the walls. Next I lay down the foam,
keeping it as flat and firmly supported as possible.
Cut the foam about 1/4 inch shy of the wall and seal the gap
with a squirt of spray foam. If you prevent the foam from
moving, you’ll avoid squeaks in the finished floor later.
The concrete should be flat with no variation greater than 1/4
inch over 10 feet when checked with a straightedge. This means
that if you’re retrofitting an old garage into new living
space, you’ll have to deal with the slope in the floor,
preferably before installing the vapor barrier. Pouring a thin
layer of concrete to fill in the low spots is probably the best
Also, use a compression-grade extruded polystyrene foam that
will support the weight of the floor, furniture, and occupants.
One type of compression-grade foam is Styrofoam "greyboard."
Tape the joints between the tongue-and-groove foam panels to
minimize any movement or shifting that could cause noise.
When you have a stable foam base, place a layer of 1/2-inch
plywood parallel to the long dimension of the room. Space the
panels about 1/4 inch apart, and leave at least a 1/2-inch
space around the perimeter (see illustration at right). Do not
glue the plywood-to the foam. Then lay a second layer of
1/2-inch plywood over the first, orienting the panels 45
degrees to the first layer. Space this layer the same distance
apart as the first layer, and screw or staple the two layers
Over this floating wood "slab" and the usual layer of
15-pound building felt, you can lay down your strip
Howard Brickman is a hardwood flooring contractor in