How to Secure Cement Backer Board to a Plywood Subfloor

Our builder installed our tile floor on ¼” thick cement board, and the tiles are moving and popping up. Is there any way to fix this easily? -Karen

It sounds like the cement backer board was not attached properly to the subfloor using thin-set adhesive, screws, and fiberglass tape on the joints. If that’s the case, unfortunately, there’s no quick fix for this problem other than removing the tile and cement backer board and starting over.

While using 1/4” thick—rather than 1/2” thick—cement backer board is acceptable, how you install the backer board and attach it to the plywood subfloor is as important as how you lay the tile on top of it.

Here’s the right way to attach cement backer board to a plywood subfloor:

  1. Make sure the joists are wide enough for the span and spaced on 16” centers to provide a sturdy floor with no bounce.
  2. The plywood subfloor should be at least 5/8” thick on 16” centers (3/4″ preferred) and securely attached to the joists preferably with screws.
  3. Layout the cement backer board at right angles to the plywood subfloor so the joints in the backer board are staggered and don’t fall on joints in the plywood. Leave a 1/4″ gap between the backer board and walls and 1/16” gap between the sheets of backer board.
  4. Mix up and apply a modified thinset to a large enough area of the plywood subfloor to cover each piece of backer board using a 1/4″ x 1/4″ notched trowel.
  5. Position each piece of backer board in the thin-set, press in place, and attach using special backer board screws on the spacing recommended by the backer board manufacturer.
  6. Cover the joints in the backer board with fiberglass tape embedded in thin-set adhesive and allow to set before tiling.

Good luck with your project,

Danny

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