Installing New Backerboard

This story shows how to work with and install new backerboard for floors or walls.


Because you must install backerboard with its edges centered on joists and studs, mark the joist and stud locations before you start. Because you won't be able to see marks after you have troweled on the thinset, mark joist locations on the wall and stud locations on the ceiling.

When possible, offset by half a sheet the joints where the sheets meet. Leave at least a 1/8-inch gap between sheets (use an 8d nail) and a 1/4-inch gap at the walls (about the diameter of a pencil).

Scoop thinset out of the bucket with a margin trowel, then spread it with the notched trowel recommended by the adhesive manufacturer.


About 30 to 45 minutes per square foot of surface

Cutting backerboard: drywall square, carbide scriber, utility knife, rasp Cutting holes: tape measure, cordless drill, carbide hole saw, compass, utility knife, and hammer for large holes Installing backerboard: trowel, margin trowel, corner drywall knife, cordless drill, utility knife

Precise measuring and cutting, driving fasteners with cordless drill, troweling

Prepare, vacuum, and damp-clean surfaces; install waterproofing membrane in wet locations.

Thinset mortar, backerboard, 11/4- and 2-inch backerboard screws, 2- or 4-inch gummed fiber mesh tape, 2x4 lumber for blocking (walls only), 8d nails

Cutting Backerboard: Step 1

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.

Cutting Backerboard: Step 2

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

Cutting Backerboard: Step 3

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.

Cutting Backerboard: Step 4

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.

Installing Backerboard on Floors: Step 1

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.

Installing Backerboard on Floors: Step 2

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.

Installing Backerboard on Floors: Step 3

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.

Installing Backerboard on Floors: Step 4

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.

Installing Backerboard on Floors: Step 5

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.

Installing Backerboard on Walls: Step 1

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.

Installing Backerboard on Walls: Step 2

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.

Installing Backerboard on Walls: Step 3

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.

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