How to Install a Basement Subfloor

Build a subfloor over concrete in basements before adding engineered wood flooring.

Basement subflooring is necessary for some types of flooring material, such as engineered hardwood and other materials specifically designed for below-grade applications. Basements are often damp, and moisture migration through the slab is an important consideration. When building the subfloor, you must take steps to minimize moisture. Once the subfloor installation is complete, the basement floor will actually be raised approximately 1-1/2 to 2 inches, a factor that could affect doors and trim that may already be installed.

Things You'll Need

• 6-mil plastic
• Duct tape
• Pressure-treated 1-inch by 2-inch wood strips
• Circular saw
• Safety glasses
• Power drill
• Hammer drill
• Driver/concrete bit combo
• Concrete screws
• 3/4-inch exterior-grade plywood
• 1-1/4-inch decking screws

Instructions

1. Spread sheets of 6-mil plastic across the floor, overlapping each course by at least 12 inches. The plastic is available in rolls and is sold at home centers and hardware stores. The material makes a good vapor barrier, and the overlap ensures excellent coverage. Duct-tape the seams together for added durability.

2. Lay down 1-inch by 2-inch pressure-treated wood strips. These strips will keep plywood subflooring off the concrete and will simulate floor joists. Make sure you have enough 1-inch by 2-inch boards to run the length or width of the basement, 16 inches on center, just as joists would be set. Cut the strips with a circular saw to fit the room dimensions and butt them together to create long runs. Wear safety glasses when using power tools.

3. Attach a hammer drill and a special combination bit to the power drill. These attachments are available at home centers and make the task of drilling through wood and concrete faster and more accurate. Drill pilot holes through the strips and into the basement floor, spacing the holes approximately 16 inches apart. Drive concrete screws through the pilot holes with a driver/concrete bit combo. The strips need to be well-secured to the basement floor to help make the subfloor solid.

4. Place exterior-grade plywood sheets over the 1-inch by 2-inch strips, anchoring the sheets in place with decking screws. Exterior-grade is more moisture-resistant than common plywood, a consideration for basements even with the moisture barrier in place. Stagger seams between plywood sheets so they do not all run in the same direction. Depending on the size and layout of the basement, some sheets may need to be trimmed to fit, using a circular saw.

References

• The Family Handyman: Installing Wood Flooring Over Concrete
• Ask the Builder: Wood Flooring
• Mother Earth News: Installing Hardwood Flooring