How to Install a Plywood Subfloor on a Concrete Slab

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.

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How to Install a Plywood Subfloor on a Concrete Slab

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.