How to Install Cement Board Underlayment for Ceramic Tile

Tile work
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Cement board, commonly sold under trade names such as Durock, Hardiebacker, and WonderBoard, is now the standard underlayment used for ceramic or stone tile laid with thinset mortar adhesive. Generically, it is sometimes called cementitious backer unit (CBU). Not only does cement board form a more durable, stable base for ceramic tile, but it has no organic materials such as those found in drywall, greenboard, or plywood, and therefore is not prone to mold, rot, shrinkage, or decomposition. Also, because there is no cellular material in cement board, it does not draw in moisture as cellulose materials do. Ceramic tile laid over cement board is one of the most durable floor or wall surfaces you can install.

The Surface Below the Cement Board

Because cement board can be used either for floors or walls, the surface below the cement board will vary:

  • Flooring: For ceramic tile floors, the cement board usually rests on a plywood subfloor (minimum 5/8" exterior-grade plywood or even OSB). If new ceramic tile is being laid on an existing floor, the cement board can usually be laid right over the existing floor finish, provided it is in good shape and the extra floor thickness is not a problem. There is no need to install backer board on a concrete substrate—ceramic tile can be applied right over concrete, provided the surface is smooth and level.
  • Walls: For highly wet areas such as showers and bathtub surrounds, the cement board should be installed directly on studs that have been covered with an intervening waterproof membrane. In such wet areas, never install cement board directly over drywall or greenboard—instead, first remove the existing surface, then install the cement board over studs covered with membrane. Walls that you expect to stay dry need no membrane; cement board can be applied directly to the studs. 

Cement Board Sizes

Cement board, regardless of brand, is most commonly used in 3 x 5-foot sheets, and 1/4-inch, 1/2-inch, and 5/8-inch thicknesses are available. A 3 x 5 panel of 1/4-inch cement board weights about 30 pounds; a 1/2-inch sheet, 36 to 44 pounds. Larger, 4 x 8-foot panels are also available, but the heavy weight makes them hard to manage for DIYers.

Use gloves when handling cement board panels, as the edges are sharp. Be cautious when moving or transporting the panels, since it is easy to break or crack them. Bring a helper along when buying cement board to prevent damage. 

Choose thicknesses based on the application: 

  • Floors: A 1/4 -nch minimum thickness is needed, but thicker panels are also fine. 
  • Walls: Use 1/2-inch or 5/8-inch-thick cement board, not 1/4-inch.  The extra thickness is needed to span the studs and provide a solid base for the tile.
  • Countertops: 1/4-inch, 1/2-inch, or 5/8-inch-thick panels are all suitable for countertops. 

Cement Board Brands

All available at major home improvement stores, popular brands of cement board include:

  • HardieBacker: Imprinted with a 1 x 1 -inch grid for easier tile-setting, James Hardie Industries' HardieBacker is 90% Portland cement and sand, with a patented MoldBlock additive.
  • Durock:  USG's Durock tends to run lighter than equally sized HardieBacker.
  • DenShield:  From Georgia-Pacific, DenShield is a two-in-one product, combining cement board and waterproof membrane.  Although DenSheild works in dry applications, it is more expensive so should be confined to wet applications.
  • Wonderboard: Wonderboard is a somewhat cheaper cement board product that can be crumbly, though it works fine if handled carefully. 

How to Install Cement Board on Floors

What You Need:

  • Cement backer board (CBU)
  • Thinset mortar
  • Notched trowel
  • Fiberglass seam tape
  • Cordless drill
  • Cement board screws
  • Safety glasses
  • Utility knife (optional)
  • Contractor's paper
  • Contractor's pencil
  1. Test-fit the panels. Lay out the full-sized panels on the floor, but do not affix them to the subfloor yet. Mark partial boards by drawing cut lines on the surface of the cement board using a carpenter's pencil. Where the board must go around irregular obstructions, such as plumbing pipes, toilets, etc., draw a template on contractor's paper, cut out the template with scissors, then transfer the pattern to the cement board with a pencil.
  2. Make cuts. If you have any obstructions to work around, cut the cement board with a circular saw or jig saw outfitted with a carbide blade. Make sure to wear safety glasses—cutting cement board is a messy process. An alternative, cleaner way is to score the mesh side with a utility knife and snap off the panels along the scored lines.
  3. Apply thinset mortar to the subfloor surface where the cement board will be affixed, using a notched trowel to "comb' it.
  4. Position the cement board panels on the floor, maintaining a 1/8- to 1/4-inch gap between panels.
  5. Screw the panels down to the subfloor with the cement board screws driven every 8 inches along the seams and throughout the field. Screw heads should be driven slightly below the cement board surface so they don't protrude. 
  1. Tape the seams. Cover the seams with fiberglass seam tape. Fill in and smooth with thinset mortar. Let dry fully before laying tile.