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,


Further Information


  1. I am putting down a Nuheat pad under my ceramic tile. I was planning to staple the cemnet board to the subfloor instead of screwing it. Is there a problem with screws in the cement back board and under tile heating…? Reading this article makes me think I am going to have trouble with tile shifting if there are only staples instead of screws holding the cement board to the subfloor plywood. I plan to use thinset between the cement board and plywood. Can you comment/advise me…? Thanks.

  2. hello, why does my bucket of pre-mixed thinset morter say do not use to install backer board to sub floor? thanks

  3. Can I install backer board using thinset only? Grout lines have cracked. I have pulled up the tile and smoothed out the old thinset. I have a heated floor and cannot use screws. Is there anything that will adhere backer board to the old thinset? Or is my only option to take up everything down to the subfloor and start from scratch<

  4. I’m installing a prefab shower pan (36″x48″) and while I have ensured that the plywood subfloor is nailed down and level with my super solid floor joists, is there a need for a thinset mortar between the plywood subfloor and the backerboard? I’m putting cement mortar on top of the backerboard to serve as “base” to set in my prefab pan (as suggested by the manufacturer). What’s the purpose of such a step before laying the cement mortar as foundation for my prefab shower pan? Most research I’ve done does not address my kind of situation. Any response is highly appreciated. Thank you!

  5. I’ve installed 1/2″ cement board (wonder board) over 3/4″ hardwood floor. A very sturdy and strong substrate. I started by placing 30# roofing paper to act as bond break to avoid squeaks before installing the cement board being sure to stagger joints. Is thinset truly necessary?

  6. I have two questions for you.
    1. We are in the process of gathering supplies for a basement bathroom redo. The walls are currently papered. What we want to do is tile everywhere! Floors, walls and ceiling within the shower. My question is do we need to put up backer board onto the walls that are block? I know we need to do inside the shower regardless of wall construction, but, in the rest of the bathroom can we just put the tile directly on the block walls? Or will it be too hard to get a level job done due to the strike lines? Would it be best to use backer board everywhere? Also, how do we attach the backer board to block walls? Screws and thinset?
    2. Now for the real stickler! We live in a 1950’s brick ranch with a hip roof which has a very low incline. Over the years we have replacwd all the windows and exterior doors, remodeled the kitchen and bath, insulating where we could get to the studs. When the house was built they didn’t believe in insulation, so consequently, we only have studs, Celotex and brick in the bedroom walls. We have insulated the attic. Over the last 10 or so years I have noticed mildew growing in the corners of the outside walls of the bedrooms, where the walls meet the ceiling but mostly on the ceiling. We have a ridge vent that runs the length of the roof and have installed soffit vents every two feet for more air circulation. Any suggestions on what we can do to fix this issue? We are planning on a new roof late winter or esrly spring. Not that it’s leaking, it’s just time to replace. Also planning to blow insulation into the bedroom walls. Even though they are papered, I thought I could cut the wallpaper larger than the hose, drill out the plaster, blow it in the replace that piece of wallpaper, hoping it won’t be too noticeable. Do you think this would solve the mildew problems? I am getting desperate to find an answer. I hate mildew! It’s freaking me out everytime I see it! I think I’m driving my husband crazy! I would really appreciate any input you may have. Right now we a pulled in two directions, should we start the bath remodel or insulate? I know I’m all for the insulation if it will get rid of the mildew!

  7. Hello,

    Will be purchasing supplies at Home Depot

    Will MULTISET suffice to get the job done on securing cement board to plywood.

    Thank you!

    • Hi, Crystal!
      Custom Building Products’ Multiset is made from a blend of Portland cement, inorganic aggregates, copolymers and chemicals and you can use it to install most cement backer boards. Good luck with your project!

  8. I had a tile man show up, lay moisture barrier, then backer board, then tape the seams with duct tape! He said it sealed the seams, but before he could lay any tile most of the tape came lose and was hanging in many-many places. I began questioning his knowledge of tile prep. After questioning other tile setters I learned many tile setters use duct tape. I never heard this as an industry standard. Do I trust this guy to continue tile work for me?

    • Hi, Quentin,
      Need more information about this topic? Connect one-on-one with a home improvement pro immediately through JustAnswer, a Today’s Homeowner partner:
      Good luck with that tile installation! 


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