Installing Cement Board Underlayment for Tile

cement board mortar
© PebbleArt
  • 01 of 08

    Why Use Cement Board?

    Cement board (and other types of tile underlayment) 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 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. What...MORE it does do is stay together and not fall apart if water reaches it. Cement board also provides an additional layer over a plywood or particleboard subfloor, adding stiffness to the tiled surface to help prevent cracks. 

    There are various cement board products available, but most are installed in a similar fashion. The steps here show the basic process, which involves 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!

    Supplies Needed:

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  • 02 of 08

    How to Install Cement Board: Step 1

    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. 

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  • 03 of 08

    How to Install Cement Board: Step 2

    Lay the first sheet of cement board onto the mortar so the rough side if the board is facing up. Leave a 1/4-inch gap between the board and any adjacent walls. ​

    Note: 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. 

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  • 04 of 08

    How to Install Cement Board: Step 3

    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 screw burrow into the board so the head sits 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. 

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  • 05 of 08

    How to Install Cement Board: Step 4

    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.  

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  • 06 of 08

    How to Install Cement Board: Step 5

    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...MORE 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. 

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  • 07 of 08

    How to Install Cement Board: Step 6

    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.  

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  • 08 of 08

    How to Install Cement Board: Step 7

    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.