Installing Hardwood Flooring Over Concrete Slabs

Sara Miller
Posted in:

Even if your residential or commercial property currently has concrete floors, it is still possible to fulfill your dream of a hardwood floor. Installing hardwood over concrete will take considerable work, but it can be done.

Steps for Installing Hardwood Over Concrete:

1.) Checking For Moisture: Installing hardwood over moist concrete  will cause problems later on. To check for moisture on light colored slab, place a rubber mat on the concrete floor and place a weight over it. After 24 hours, if the concrete under the rubber mat shows dark spots, too much moisture is present. For darker slabs, you should tape a square foot of clear polyethylene film over the slab, making sure to seal off the edges. If condensation collects under the film during the 24-hour wait, too much moisture is present.

2.) Remove Moisture: If moisture on your flooring exists, remove it using a industrial-grade humidifier and heat. If possible, turn up the heat in the space to 80 to 85 degrees for 48 hours. Repeat the test (see #1) after this period to validate if there's still moisture. 

3.) Repeat Until Dry: Do these tests in several areas of the room, multiple times.

Installing Vapor Retarders:

1.) Prime the Surface: Prime the slab and apply cold, cut-back asphalt mastic with a notched trowel at the rate of 1 square meter per liter. Allow to set for two hours.

2.) Purchase and Roll the Underlayment: Unroll 15 lb. asphalt felt or building paper, lapping the edges 4" and butting the ends.

3.) Rinse and Repeat: Apply a second similar coating of mastic and roll out a second layer of asphalt felt or paper in the same direction as the first, staggering the overlaps to achieve an even thickness.

If Using Polyethylene:

1.) Cover the Concrete: Where humidity and temperature are steady, cover the entire slab with 4 to 6 mm of polyethylene film, overlapping the edges 4 to 6", and allowing enough film to extend under the baseboard on all sides.

2.) Remove Moisture: Where moisture conditions are higher than average, prime the slab and apply cold, cut-back mastic with a straightedge or fine-tooth trowel (2 square meters per liter). Allow to dry for at least 90 minutes.

3.) Lay the Poly: Unroll the polyethylene film over the slab, overlapping the edges 4 to 6 inches (10 to 15 cm).

After Installing Vapor Retarders:

1.) Add the Vapor Retarders: Roll the film flat on every square centimeter to insure proper adhesion. Bubbles should be punctured to release trapped air.

2.) Install Plywood: Install the plywood after the asphalt felt or plyethylene is in place. Loosely lay a nailing surface of 3/4" x 4' x 8' exterior plywood panels over the entire area, but be sure to leave a 3/4" space at the wall line, and 1/5" - 1/2" between panels. Where a finish trim will not be used (doorjambs, etc) it is recommended to cut plywood to fit within 1/8". The best way to prevent cracks along panel edges is to lay the plywood diagonally, across the direction of the finished floor .

3.) Consider the Room's Layout: If the slab does not have radiant heat pipes installed, then fasten plywood to the slab with power-actuated fasteners. Secure the center of the panel first and then do edges, using nine or more nails. If the slab does have radiant heat pipes, cut the ply- wood into 2'"by 8" planks, score the backs (3/8" deep on a 12" grid is recommended), and lay the panels in a staggered pattern. Plywood planks may be glued to the plastic with asphalt mastic if the system being used is not radiant heat.

Now your floor is prepped for hardwood flooring! Now you'll want to consult a hardwood flooring installation expert or learn how to DIY hardwood floors!