How to Reinforce the Floor Before the Tile

Place the first row of cement backerboards onto the floor in a line along one of the walls of the room. Leave a space 1/4-inch in width between the edge of the boards and the walls. Leave a 1/4-inch between the boards to allow room for the expansion of the floor beneath from temperature changes.

Measure the partial space left between the last full board in the row and the wall with a tape measure. Mark the length needed to cover the space, minus the gap space, onto a full-sized board with a pencil.

Score a line at the marked length on the board with a utility knife, using a straightedge to keep the scored line running straight across the board. Snap the board at the scored line by pulling one end of the board towards you while holding the other end stationary. The board will break along the scored line. Place the partial board into the space you cut it to cover.

Snap the board at the scored line by pulling one end of the board towards you while holding the other end stationary. The board will break along the scored line. Place the partial board into the space you cut it to cover.

Move back to the beginning of the placed row to start the next row of boards. Place the other end of the cut board onto the floor as the first board of the next row so that the boards of the new row will have their joints staggered to the boards of the first. Set the rest of the boards in the row following the same method that you used with the first, maintaining the 1/8-inch gap between rows and the 1/4-inch gap along the row ends and walls. Continue to place each new row like this until you reach the wall opposite to the starting wall.

Snap the boards lengthwise along the end wall to fit the space if necessary.

Return to the first board placed, and begin the process of securing the boards in place with a layer of thinset mortar. Lift the board from its placed position, and spread the mortar over the floor beneath the board using a notched trowel. Tilt the trowel on its edge after spreading the mortar to run the notched end over the mortar to leave ridges in the mortar. Press the board back into place, and remove any mortar that squeezes from beneath the board with a damp sponge. Follow the same adhering process with all of the boards on the floor.

Secure the boards further by screwing them into place with corrosion-resistant screws. Use a drill with a screw bit attached to drive the screws into the subfloor through the cement boards every eight inches around the perimeter of the boards. Position the screws about an inch from the board edges.

Fill a grout bag with thinset mortar. Squeeze the mortar into the gaps between the boards until the mortar is slightly overflowing the edges of each gap. Use a putty knife to smooth the overflow onto the surface of the adjacent boards until you create a strip of mortar over the gap that is 3 inches wide. Place a strip of fiberglass tape over the mortared gap, and then use a trowel to cove the tape with a thin layer of the mortar. Allow the mortar to dry overnight before tiling over the backerboard.