Cement board is a type of underlayment that is now standard for most tile installations, but particularly for floor tile. The primary benefit of cement board is that it doesn't break down or expand or warp if it gets wet. This is a distinct advantage over plywood and drywall. Cement board also provides an additional layer over a plywood or particleboard subfloor, adding stiffness to the tile installation to help prevent cracks.
Cement board often is called "waterproof" or "water-resistant," but neither description is accurate. Water and vapor can pass right through it, just like with other cement-based materials. For this reason, cement board should be installed over a moisture barrier, such as heavy plastic sheeting, in wet areas like showers and bathtub alcoves.
There are various cement board products available, but most are installed in a similar fashion, starting with mortaring and screwing the panels to the subfloor, then taping and "mudding" the joints between panels, much like you do with drywall. When the mudded joints are dry, it's time to lay tile!
Equipment / Tools
- Margin trowel or putty knife
- 1/4-inch notched trowel
- Drywall T-square
- Utility knife
- 4- or 6-inch drywall knife
- Cement board panels
- Thinset mortar
- 1 1/4-inch cement board screws
- Self-adhesive cement board mesh joint tape
Apply the Mortar Bed
Mix a batch thinset mortar, following the manufacturer's directions, using a bucket and margin trowel or putty knife. Spread the mortar onto the subfloor with a 1/4-inch notched trowel, starting at the corner of the longest wall in the room. Apply enough mortar for a single cement board sheet at a time.
Place the First Cement Board Sheet
Lay the first sheet of cement board onto the mortar so the rough side of the board is facing up. Leave a 1/4-inch gap between the board and any adjacent walls.
The edges of the cement board should be no closer than 8 inches to the joints in the subflooring. This is called "staggering" the joints and makes for a stronger tile base.
Fasten the First Sheet
Fasten the cement board sheet to the subfloor with 1 1/4-inch cement board screws. These are specially coated, corrosion-resistant screws that have notches under their heads that help the screws burrow into the board so the heads sit flush with the panel. Don't use regular drywall or wood screws, which corrode in cement board. Drive the screws every 8 inches along the edges and in the field of the panel.
Install More Sheets
Repeat the same process to install the remaining panels—one at a time—along with the first row, leaving a 1/4-inch gap between panels and staggering the joints on the subfloor by at least 8 inches.
Trim the Last Panel
Cut the last panel in the row (and others, as needed), using a drywall T-square and a utility knife: Hold the T-square on your cutting line, and score the panel with the knife, running it along the edge of the square. Make two or three deep scores to cut through the fiberglass layer just under the surface of the panel.
Lift the panel up onto its edge and snap the panel backward along the cutting line until it breaks the core of the material. Reach behind the panel with the knife and cut through the fiberglass layer on the back of the panel to complete the cut.
Install the panels in the remaining rows, leaving a 1/4-inch gap between all panel edges.
Apply Joint Tape
Cover the joints between all of the panels with self-adhesive cement board joint tape. This is a special alkali-resistant tape that can withstand contact with cement. Do not use standard drywall joint tape. Make sure the tape fully adheres to the cement board.
Mud the Taped Joints
Cover the joint tape with a thin layer of thinset mortar, using a 4- or 6-inch drywall knife. Smooth the mortar so it is flush with the surrounding surfaces. This is similar to mudding drywall joints but doesn't have to look as nice; it just has to be smooth and flat. Let the thinset fully cure as directed before beginning the tile installation.