How to Build a Wooden Subfloor on a Preexisting Concrete Slab

Apply a layer of six mil thick polyethylene plastic over the entire concrete slab to help protect the wooden subfloor from moisture. Run the plastic up the walls about 4 to 6 inches, securing it to the wall with duct tape on the back of the plastic. Where seams occur, overlap the plastic by about 6 inches and seal the seams with duct tape.

Secure pressure-treated 2-inch by 4-inch boards, flat side down, around the perimeter of the concrete floor, using a powder-actuated nail gun to nail them to the floor. Begin at the highest point of the floor so wood shims can be placed in areas that are lower. Regularly check the level of the 2-inch by 4-inch wood to ensure that none are higher or lower than the others.

Build a grid work of 2-inch by 4-inch boards inside the perimeter boundary, with the boards lying on their flat sides. Four-foot grid squares will accommodate the plywood sheets perfectly without having to cut them. Use the powder-actuated nail gun to secure the wood to the concrete base.

Lay sheets of rigid foam insulation between the 2-inch by 4-inch wood to add additional insulation to the subfloor, if you wish. This step is not required but will help reduce heating and cooling costs of the area. Cut the foam insulation to the correct size using a razor knife.

Position sheets of 3/4-inch pressure-treated plywood on top of the grid work you have in place. Secure the plywood sheets to the 2-inch by 4-inch wood with the nail gun. If you laid your grid work out properly, you should not have to cut the plywood at all. However, if you made an error and need to cut the plywood, use a circular saw or handsaw to cut it to the proper length.