How to Install Backer Board to a Sub Floor


Backer board is also referred to as cement backer board or cement board. These boards are composed of cement and fiberglass and are used to provide a strong, stable base for the application of floor or wall tiles. Backer board is usually purchased in sheets ranging from 1/4-inch to 1/2-inch thick. Before you begin to install backer board, make sure that the subfloor is in good condition and does not need to be replaced.

Things You'll Need

  • Screwdriver
  • Backer board
  • Power saw
  • Dust mask
  • Pre-mixed thinset mortar
  • Notched trowel
  • Board (2 x4)
  • Rubber mallet
  • Cement backer board screws
  • Inspect the subfloor and make sure that the surface of the floor is smooth and level. Drive any protruding nails or screws further into the floor until they are flush with the surface of the floor.

  • Lay sheets of backer board on the floor, leaving a 1/8-inch to 1/4-inch gap between sheets.

  • Cut pieces of backer board to fit as needed, with a power saw or by scoring the backer board with a utility knife several times and then breaking the board along the scored line manually. Keep in mind that cutting backer board with a power saw generates a great deal of dust; you will need to wear a dust mask to prevent inhalation of hazardous dust.

  • Remove two panels of the backer board from the floor and spread thinset mortar onto the subfloor, using the flat side of a notched trowel. Turn the trowel and use the notched side, which is similar to a large comb in appearance, to spread the thinset and create lines in the spread thinset.

  • Press the backer board into place on top of the thinset mortar. Place a one-foot length of 2 x 4 board on top of the backer board, and strike the board with a rubber mallet. Continue to do this at different spots on the backer board to press it firmly into the mortar.

  • Install cement backer board screws (1 1/4 inch) through the backer board and into the subfloor every six to eight inches around the perimeter of each board. Keep the screws at least two inches away from the edge of the board to prevent the screws from tearing through the side of the board.

  • Continue to spread enough mortar for two panels of backer board at a time, and attach the panels to the floor until you have covered the entire room. Allow the thinset to dry according to the directions on the packaging before you begin tile installation.

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