The method used to lay hardwood floors over a concrete slab depends on the thickness of the floorboards and the condition of the concrete. If the floorboards are less than 3/4-inch thick, they must be laid over a solid subfloor--either the slab or wood sheets installed over wood strips. Wood can only be laid directly on concrete that is dry and will remain dry, so you must test the moisture content of the slab. If the floorboards are more than 3/4 inches thick, they are laid over wood strips installed on the concrete.
- Skill level:
Things you need
- Clear plastic sheeting, 2-foot to 3- foot square
- Adhesive tape
- Concrete cleaner
- Epoxy patching material
- Building mastic
- Vapour barrier, 6-mil polythene or 15-pound builder’s felt
- Screeds (lengths of wood 2 inches by 4 inches)
- Tape measure
- Hand saw or circular saw
- Flooring adhesive
- Subflooring material (hardboard or plywood)
- Annular nails
- Nail punch
Test the dampness of the concrete if you're installing floorboards less than 3/4-inch thick. (Continue directly to Step 2 if you're installing floorboards 3/4-inch thick or more). To test for moisture, tape a 2-foot or 3-foot square of clear plastic sheeting to the concrete and leave in place for 24 hours. If the plastic has misted up or any moisture has collected under it, then the concrete slab is too damp to install floorboards less than 3/4-inch thick directly onto its surface; you will need to install a solid subfloor.
Remove any sealant that may have been applied to the concrete by sanding its surface. Clean the slab with a concrete cleaner and let it dry completely.
Spread epoxy patching material with a trowel to fill any minor imperfections in the concrete. Level the patching material as you go along.
Cover the concrete with a 3/16-inch layer of building mastic, using your trowel. The mastic should set in about two hours, but check your specific product for setting times. After it has set, measure and cut your vapour barrier to cover the area of the floor, overlapping any seams by about 4 inches.
Measure the length of each wall. Measure and cut screeds to fit these lengths. Lay them around the perimeter of the floor, about 3/4 inch from the walls, and butt their ends. Trowel a 1/4-inch layer of building mastic to each screed and set it on the concrete to bond.
Lay more screeds across the room in the same manner, running them perpendicular to the direction that the floorboards will be installed. Place the screeds with their centres 12 inches apart. Butt their ends against the perimeter screeds.
Lay another sheet of vapour barrier over the screeds; it will be secured when the subflooring or flooring is nailed to the screeds.
Skip to Step 10 if you are installing floorboards that are 3/4-inch thick or thicker. Continue with Step 9 for floorboards that are less than 3/4-inch thick.
Lay sheets of your solid subflooring material over the screeds, keeping their long edges perpendicular to the screeds and staggering their joints across the room. Allow a 1/2-inch to 3/4-inch gap at the wall. Leave a 1/8-inch gap between the panel edges and a 1/16-inch gap between panel ends. Measure and cut panels with a saw to fit as necessary. Nail the sheets of subflooring to the screeds.
Install your hardwood flooring over the subfloor according to the manufacturer's instructions, using adhesive or nails.
Tips and warnings
- The best way to clean concrete is with a concrete degreasing solution.
- If the concrete slab has high areas, grind them down by machine, or apply a liquid concrete underlay over the entire slab to even the surface.
- If your concrete slab is completely dry, you can bond the vapour barrier directly to the slab, then apply your subflooring material to the concrete using concrete fasteners. Allow a 3/4-inch gap between the subflooring panels and the walls.
- Always wear safety equipment and clothing when working with power tools and chemicals such as epoxy patching material and adhesives: respirator, safety goggles and protective clothing.
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