I have an old house that's built on pillars and has shifted a little over time. We are remodeling the kitchen and just ripped up many layers of creatively installed flooring. We are down to the bottom layer of plywood and it dips on one side of the room (8"x 15" room) and then kind of comes back up.

We aren't sure how to tell how sloped it is or how much leveling is really necessary. We have heard that if we don't level it the tiles will eventually crack so we want to prevent that.

We are planning on putting down backer board in between the plywood and the tiles, will that be enough support or is it important to do some sort of leveling first? If we level with something like cement will we need another layer of plywood or can we put the backer board right on top?

Any advice appreciated, thanks so much!

  • 1
    don't use mortar nor cement on the floor. Any floor patched with either will not last. Use a product designed for this type of repair. Any brand labeled as a "floor leveler" or a "floor patch" will do. But be advised they have limits to how large a repair the can cover. Read the bag label for product performance.
    – ojait
    Aug 29, 2015 at 20:49
  • 1
    it needs to be FLAT not LEVEL Dec 28, 2015 at 15:41
  • backerboard is NOT considered "support" in any way for a floor Dec 28, 2015 at 15:43

2 Answers 2


You did hear correctly: the floor must be as flat as possible (and as rigid) in order to support the tile. You mentioned the floor being supported on posts. If there is enough head room a better repair is to shim-up the low spot from below. Depending on how severe the floor depression is this may entail hammering some store bought shims under the joists or installing concrete footings so as to support new posts.

The quick easy solution is to outline the low-spot of floor , mix-up a bag of self leveling floor mix, and trowel it into the depression feathering the edges.

It would also be a smart idea to check the other sections of floor that will get covered by tile for any squeaks, bounce or shifting. Nail or screw to secure them to joists.

When you are ready to begin installing the cement board it would be wise to embed the 3' x 5' x 1/2" cement boards in a thin slurry of thinset for complete support. Use cement board screws to affix them to the floor.

  • 1
    Great thanks for the solution! After adding the self leveling mix to the depression can I put the cement backer board right on top and then screw down through the leveling compound? I've heard the leveling compound can crack, is that ok or will it create an unstable surface for the tile over time?
    – Dewey
    Aug 30, 2015 at 17:22
  • I've also heard that I can just put the backer board down and use extra thinset on the tiles where there is a depression to set them up a little higher. Is this a good solve?
    – Dewey
    Aug 30, 2015 at 17:23
  • Let the floor leveler cure completely before installing the backer board. As for adding a thicker layer of thinset to adjust for the low spot.... Depending how much you need to add.... yea it should be ok. I'm reluctant to 100 % agree only because you could have "issues" from the differing thinset thicknesses. I'm remembering from concrete slab experience were the dirt surface has to be relatively flat with minimal divots. This ensure a uniform thickness and creates a stronger slab.
    – ojait
    Aug 30, 2015 at 18:27
  • Don't try to span the depression with only cement board it will eventually flex and the tile will fail. If shimming the floor up to level isn't an option than at the minimum fill the void with floor leveler first, let cure, and than install cement board.
    – ojait
    Aug 30, 2015 at 18:30
  • As for the durability and performance of cured floor leveler: yea you heard correctly. It usually compacts and crushed overtime, (more the reason to shim!) but it's better (and faster) than no floor leveler.
    – ojait
    Aug 30, 2015 at 18:35

The concrete board and extra mud will be all you need. Lay the concrete board, then fill in where needed.

  • do you mean lay down the backer board and then use thinset as the "mud"? my cabinet guy was over today and he said he would just lay the backer and then use extra thinset to increase height of tiles where the depression exists. What do you think?
    – Dewey
    Aug 30, 2015 at 17:25
  • i think you will end up with cracked tile Dec 28, 2015 at 15:44

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