How to Install Cement Board on Floors

1. Plan the Installation

   • Make a rough plan for the cement board sheet layout based on the area to be covered and the size of the cement board sheets.
   • Offset the joints between the cement board sheets by the joints in the plywood (or other) subflooring by at least 8 inches to provide maximum floor stiffness.
   • Create an offset (if necessary) by starting the first row of sheets with a partial sheet.

   Tip

   There is no need to install cement backer board on a concrete substrate—ceramic tile can be applied directly over concrete, provided the surface is flat, smooth, and free of significant cracks. Add a decoupling membrane to prevent concrete cracks from transferring to the tile.

2. Cut the Cement Board

   Cement boards can be cut with a utility knife and a T-square. To make round or curved cuts, use a jigsaw with a carbide blade, drill, and hole saw bit.

   • Score the cement board deeply with two or three passes with the knife, using the square to ensure a straight line.
   • Snap the waste area of the sheet back to break the board along the cutting line.
   • Cut the fiberglass mesh along the backside of the board, as applicable, to complete the cut.

3. Mix and Apply the Mortar

   • Mix a small batch of thin-set mortar as the manufacturer directs, mixing the dry powder with water in a bucket.
   • Apply a mound of mortar to the subfloor, then spread it out in an even layer using a 1/4-inch ​notched trowel.
   • Cover an area just large enough for the first cement board sheet.

4. Install the First Sheet

   • Place the first sheet over the mortar bed. If applicable, the rough or embossed surface of the sheet should face up.
   • Allow a 1/4-inch gap between the edges of the sheets and all surrounding walls.
   • Fasten the sheet with cement board screws, using a drill-driver, making sure the screws are flush with the surface of the sheet or are slightly recessed.
   • Space the screws 8 inches apart across the entire sheet.

   Tip

   Use only cement board screws and alkali-resistant cement board joint tape. Standard screws and drywall joint tape will corrode or deteriorate from contact with the cement board.

5. Install the Remaining Sheets

   • Apply mortar using the same techniques, lay and fasten the remaining sheets in the first row, and continue installing sheets to cover the entire area.
   • Space them 1/4 inch apart and leave a 1/4-inch gap at all walls.
   • Keep offsetting the joints between the cement board and any underlying plywood and to gap the cement board sheets by 1/4 inch.
   • Mix fresh mortar only as needed to prevent it from drying before you can use it.

6. Tape and Mud the Joints

   • Apply cement board joint tape to all of the joints between sheets.
   • Mix more thin-set mortar and, using a drywall knife, cover the taped joints with a thin layer, similar to mudding drywall joints.
   • Tool the mortar with the knife so the tape is covered and the joints are flat and smooth.
   • Allow the mortar to cure completely, following the manufacturer’s recommended drying time.

How to Install Cement Board on Walls

Affixing cement boards on walls uses the same techniques as flooring, but the biggest difference is it does not need a layer of mortar behind the cement board. It is screwed directly into the wall studs. Consult the cement board manufacturer for recommendations on using a moisture barrier between the board and the wall studs, such as thick plastic sheeting or another type of water-resistant membrane.

• Place the cement boards: Stagger the cement boards, ensuring that four corners never meet. Avoid forcing the boards together. Leave a slight gap for the expected expansion and contraction of the base materials and studs. Similarly, avoid forcing the edges of cement boards against floors, ceilings, or other surrounding areas.
• Screw the boards in place: Screw the cement boards onto the wall, placing the screws every 8 inches. Every stud that runs behind or along the perimeter of a cement board must be fastened to the board.
• Tape the joints: Apply fiberglass mesh tape to all the seams. Wherever tape strips meet, be sure to overlap them slightly. Avoid doubling up the tape.
• Fill the joints: Use the drywall knife to apply thin-set mortar to all taped joints. Force the mortar through the holes in the mesh tape. Run the knife along the top of the seam to even it out on each side.
• Feather the joints: If there are humps of thin-set mortar, feather the mortar out across the board. The cement boards must be as flat as possible for correct tile installation. To feather, begin the knife on the highest area, then drag the knife, gradually reducing the amount of mortar.

When to Call a Professional

Installing cement boards in shower stalls and bathtub surrounds can be more difficult because the areas need to remain watertight. For wet areas, you may want to call tile installers.