While concrete creates a durable flooring surface, it is not very appealing or attractive if left bare. Homeowners who plan to install decorative flooring materials, such as hardwood floors, will need to lay a plywood subfloor directly on top of the existing concrete slab. When exterior and interior temperature and humidity levels change, the floor will expand and contract, necessitating a buffer between the decorative flooring and concrete slab. Nailing plywood over concrete is a relatively simple project that will create a flat, smooth surface for the decorative floor installation.
Remove all materials, tools or other objects on the concrete floor that would get in the way of the installation project. Wear a dust mask and clean the concrete floor with a broom to sweep away loose concrete and other particulates, such as rocks or nails. The dust mask will protect you against dust inhalation.
Skim a 6- to 8-foot level across the concrete material to find any low areas. Mix and pour self-leveling compound over the sunken areas. Even the edges of the leveling compound flat with the concrete floor using a flat-edged trowel. Allow the leveling compound 12 to 24 hours to fully cure and set.
Lay moisture barrier over the concrete floor, such as polyethylene sheeting. Leave an excess of 3 inches of polyethylene sheeting running up the sides of each wall. Joint any edges of sheeting together with duct tape. Use a utility knife to cut this excess material off after the plywood goes down.
Place 5/8 inch plywood on top of the polyurethane sheeting, running diagonally from the intended direction of the decorative flooring. Mark all areas on the plywood boards requiring cuts with a marker.
Cut the marked plywood boards with a jigsaw and lay them in their intended place. Maintain a ¾-inch gap between the plywood boards and the wall perimeters. Keep a 1/4-inch gap between the edges of the plywood panels themselves.
Fasten the plywood to the concrete with concrete nails. Hammer the concrete nails down the center of the plywood with a ball peen hammer, spacing them at least 16 inches apart. Secure the edges of the plywood using the hammer to sink concrete nails into the boards, spaced every 8 to 12 inches around the perimeter of each plywood board.
Slice the excess polyethylene sheeting off, level with the floor, using a utility knife.