If you're hoping to transform a concrete slab into beautiful hardwood floors, you're in luck. There are several methods of construction. Before beginning the process, be sure to keep a few things in mind. Concrete should be completely dry before installation, the slab must be flat with a trowel finish and vapor retarder must be installed on top of the slab to make sure moisture doesn't reach the finished layer.
Floated Subfloor System
To begin, you'll need two layers of 3/8-inch minimum CD Exposure 1 Plywood subfloor panels (CDX) 4-foot by 8-foot sheets. First, position the plywood layer with its edges parallel to the wall. Do not fasten and leave 3/4 of an inch of space between the wall and plywood. Plywood panels should be set down with 1/8-inch gaps between sheets. Place the second layer perpendicular or at a 45 degree angle to the first layer. The second layer should also be set down with 1/8-inch gaps between sheets and 3/4 of an inch of space between the wall and plywood. Staple or screw and glue – using urethane or construction adhesive– the second layer to the first on a 12-inch interior grid pattern, with 6 inches on the perimeter. Be careful not to penetrate the vapor retarder.
In on-grade and below-grade applications, add vapor retarder before applying underlayment.
Alternate Subfloor System
Use a minimum 3/4-inch CD Exposure 1 Plywood sheathing, 4-foot by 8-foot sheets. Begin by cutting sheets to 16 inches by 8 feet or smaller panels, marked 3/8 of an inch deep on back and a minimum of every 12 inches across in width. Place 16-inch planks perpendicular or diagonal to direction of flooring. Stagger panels every 2 feet and space 1/8 of an inch between ends with 3/4-inch minimum expansion space at walls and all vertical obstructions.
Glue-Down Subfloor System
Materials include a minimum of 3/4-inch CD Exposure 1 Plywood subfloor panels, 4-foot by 8-foot sheets. Cut plywood panels by 2-foot by 8-foot or 4-foot by 4-foot sections. Score the back of the panels half of the thickness on a 12-inch by 12-inch grid. Apply an adhesive approved for plywood installation and recommended by the plywood manufacturer. Stagger sections on the adhesive with 1/8-inch spacing between sheets and a minimum of 3/4 of an inch of expansion space at walls and all vertical obstructions.
If necessary, add vapor retarder before applying underlayment, especially recommended if 3/4-inch wood flooring is installed over concrete.
Nail-Down Subfloor System
Get 3/4-inch minimum CD Exposure 1 Plywood subfloor panels, 4-foot by 8-foot sheets. First, stagger panel joints, allowing approximately 1/8-inch expansion space around panels to prevent edge peaking. Allow a minimum of 3/4 of an inch of expansion space at walls and all vertical obstructions. Panels should be mechanically fastened.
Fasteners may be powder-driven pins, pneumatic driven nails, screws, deformed pins or other fasteners suitable for concrete applications. Areas with higher humidity may require additional fasteners. Nail a minimum of 32 shots per 4-foot by 8-foot panel. Also, use a vapor retarder before applying underlayment.