How to Install Floor Cement Board Underlayment for Tile

By Joseph Lewitin
Updated on 12/21/22
Reviewed by
Johnathan Brewer
Johnathan Brewer
Reviewed by Johnathan Brewer
Johnathan C. Brewer II is a licensed general contractor specializing in kitchen, bath remodels, and general construction with two decades of professional experience.
Learn more about The Spruce's Review Board
Fact checked by
Emily Estep
Emily Estep
Fact checked by Emily Estep

Emily Estep is a plant biologist and journalist who has worked for a variety of online news and media outlets, writing about and editing topics including environmental science and houseplants.

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Thin-set mortar being spread over particleboard with trowel

The Spruce / Kevin Norris

Project Overview
  • Working Time: 2 - 4 hrs
  • Total Time: 1 - 2 days
  • Yield: 10 square feet
  • Skill Level: Intermediate
  • Estimated Cost: $140

Cement board is a type of underlayment that is now standard for most tile installations, especially for floor tile. The primary benefit of cement board is that it doesn't break down, 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 membranes designed for tile floors or a paint-on membrane like RedGuard or AquaDefense, especially 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 your tile!

Tip

Cement board can be installed over a plywood subfloor using an appropriate construction adhesive and screws for a quicker project that's less messy than taping and mudding.

How to Install Ceramic Tile Flooring
3:01

Click Play to Learn How to Properly Install Cement Board

What You'll Need

Equipment / Tools

  • Bucket
  • Margin trowel or putty knife
  • 1/4-inch notched trowel
  • Drill-driver
  • Drywall T-square
  • Utility knife
  • 4- or 6-inch drywall knife

Materials

  • Cement board panels
  • Thinset mortar
  • 1 1/4-inch cement board screws
  • Self-adhesive cement board mesh joint tape

Instructions

Materials and tools to install cement board

The Spruce / Kevin Norris

  1. 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 (length, not height) in the room. Apply enough mortar for a single cement board sheet at a time. 

    Thinset mortar spread with trowel over particleboard

    The Spruce / Kevin Norris

  2. 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. ​

    Tip

    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. 

    Measuring tape on cement board with gap from wall

    The Spruce / Kevin Norris

  3. 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. ​​​

    Cement board secured to floor with electric drill

    The Spruce / Kevin Norris

  4. 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. 

    Cement board placed next to installed board

    The Spruce / Kevin Norris

  5. 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. 

    Utility knife cutting cement board panel with drywall T-square

    The Spruce / Kevin Norris

  6. 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. 

    Joint tape rolled out over cement board panels

    The Spruce / Kevin Norris

  7. 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. 

    Thinset mortar spread with trowel over cement board panels

    The Spruce / Kevin Norris

FAQ
  • How do you secure cement board to the floor?

    You’ll first place the cement board on a layer of mortar. Then, you’ll screw it into the subfloor using special cement board screws that withstand corrosion.

  • Can I use 1/4 cement board on floors?

    Yes, a 1/4-inch thickness at minimum is generally fine when installing cement boards. Thicker cement boards also can be used.

  • Do you have to waterproof cement board?

    While water won’t warp or break down cement board, it’s still not a waterproof material. So it should have a moisture barrier under it, especially in wet spaces, such as a bathroom.