Cementboard is an underlayment material that building professionals recommend you use if you are installing a floor made from materials such as natural stone tile and some ceramic tile. These types of tile need this kind of material to draw moisture away from the tile and grout. Cementboard is a denser material than plywood, and it is essential to use under tile floors in rooms where chances are good the floor will get wet, such as bathrooms.

Tools and Materials You Will Need:

  • Cementboard
  • Thin-set mortar
  • 1 ¼ inch cementboard screws
  • ¼ inch notched trowel
  • Power drill
  • Tape measure
  • Utility knife
  • Level
  • Fiberglass mesh wallboard tape
  • Jigsaw with carbide-tipped blade

Step 1 – Mix Mortar

Follow the manufacturer’s instructions to mix the amount of thin-set mortar you will need, depending on the size of the floor area. Start at the longest wall and use your ¼ inch notched trowel in a figure-eight motion to apply the mortar to the subfloor. Apply only enough mortar for one sheet of cementboard at a time. Place the cementboard on the mortar with the rough side up, and check that all edges are offset with the seams of the subfloor.

Step 2 – Trim Cementboard to Fit

Before securing it down, cut all cementboard pieces to fit as needed. Leave a ¼ inch gap around the entire room perimeter, and also leave a 1/8 inch gap at all joints. Use a utility knife to make straight cuts and score a line through the fiber mesh layer just underneath the surface. Snap the cementboard along the scored line. To cut round holes or irregular shapes, use your jigsaw with the carbide-tipped blade.

Step 3 – Secure Cementboard

Drive 1 ¼ inch cementboard screws every 6 inches along the edges of the cementboard. Make sure the screw heads are flush with the surface of the cementboard. Continue applying cementboard sheets this way on the entire floor. If you are using a fiber cementboard underlayment, you will need a 3/16 inch notched trowel to spread the mortar. You will also need to drill pilot holes for the cementboard screws.

Step 4 – Apply Wallboard Tape

Secure wallboard tape along the seams of the cementboard. Apply thin-set mortar to the seams with your wallboard knife; fill all gaps between the sheets and spread a thin layer of mortar over the tape. Allow the mortar to set for 48 hours.