Installing New Backerboard

If you are not finishing the floor, protect it with a tarp. Backerboard particles will easily scratch a floor. Mark the line to be cut and position a drywall square or metal T-square on the line. Using a carbide backerboard scriber and firm pressure, scribe the cut line. Make several passes.

Stand the sheet on edge or turn the sheet over; working from the side opposite the scored line, snap the board.

Using a utility knife and keeping the pieces at an angle, cut through the board to separate the two pieces. Depending on how deeply you made your first cut, you may have to make several passes with the knife to separate the pieces.

Backerboard cuts are rough, whether made with a carbide scriber or a utility knife. Pieces being joined should have as smooth an edge as possible. Use a contour plane with a serrated blade, a rasp, or a masonry stone to smooth out the edge. Keep the tool perpendicular to the edge of the board and pass over the board until edge is flat.

Mix and pour thinset mortar. Hold the smooth side of a notched trowel at a 30-degree angle and spread the mortar in a thick, even coat, forcing it into the subfloor. Then, keeping the notched side of the trowel in contact with the floor and at a 45- to 75-degree angle, work the mortar into ridges.

While the mortar is still wet, tip the board on a long edge and hinge it toward the floor. Line the first board on a joist and keep a gap of 1/8 inch between boards, 1/4 inch at walls. Manufacturers' directions may vary, but typically you should stagger the joints. Walk on the board to set it in the mortar.

Using a cordless drill and Phillips bit, drive backerboard screws through the board and into the subfloor at about 8-inch intervals. Use 2-inch backerboard screws at the joists and 11/4-inch screws in the field. Set the screws so they are flush with the surface of the board.

Apply 2-inch gummed fiberglass mesh tape over each joint, pressing the tape firmly on the backerboard. The tape cuts easily with a utility knife. Use 4-inch tape (if available) for increased strength. Alternatively, you can embed ungummed tape in a thin coat of mortar applied to the joints. Use this method where strong joints are required--in stone tile installations, for example.

Finish the joint by applying a thin coat of thinset mortar over the tape. Use a margin trowel to scoop mortar from the bucket. Apply the mortar so it levels the recess in the joint from side to side. Feather the edges to avoid creating high spots under the tiled surface.

Nail blocking between studs to support joints, if necessary. Apply construction adhesive to the studs. Screw the board to the studs and blocking. Use 1/8-inch spacers (8d nails) between pieces.

Use 2- or 4-inch gummed fiberglass mesh tape over each joint. Press the tape into the joint and unroll it as you go. Use a utility knife to cut the tape at the end of the joint.

Apply a thin coat of thinset mortar to the taped joint with a margin trowel. Trowel on enough mortar to fill the joint and level it with the backerboard. Feather the edges smooth.