How to Build a Wooden Floor Joist Addition on a Concrete Slab

Covering the Floor

• Before building a wooden floor joist addition on a concrete slab, you must install a waterproof barrier. First, clean the surface of the concrete slab to remove all dust and dirt and cut enough strips of the waterproof barrier to cover the entire floor with the edges of the strips overlapping by about 6 inches. Remove the backing from the strips of the waterproof barrier and press them into place on the floor so that the adhesive sticks to the floor. Place plastic tape along all of the seams where the barrier strips meet.

Building the Border

• Once you have the floor waterproofed, you can begin building the wooden floor joist addition. To do this, measure the slab along each edge and cut 2-by-3 inch boards down to these lengths. Place these 2-by-3 boards so that the sit on their edges along the base of the walls around the slab and attach the boards to the floor with 4 ½-inch concrete screws. Make sure the screws sink fully into the boards and don’t sit above the top of the wood.

Joists

• After you make the wood perimeter for the floor joist addition, you can cut the actual joists. Measure between the inside edge of a board at one side of the room to the inside edge of the board directly opposite it. Divide the number by 12 to get the number of joists you need to make. Now, measure between the two boards in the opposite direction the room to get the length that you need to cut those joists.

Complete the Floor

• Set the joists 12 inches apart inside the wood perimeter so that they stretch between the two boards on opposite sides of the room. Drive 4 ½-inch concrete screws into the joists every 6 inches to affix them to the floor’s surface. Once the joists are all secured, cover the wood structure in ¾-inch plywood panels, starting in one corner and using a circular saw to trim panels down when needed. Use wood screws to attach the plywood panels to the floor joists.