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Bought a house 2 years ago in which the kitchen was subject to a slow but constant leak at the main water line. So, the subfloor got damaged and in order to repair it I removed big chunks of the subfloor and replaced it with two layers of plywood: ¾ plywood sheet with ½ higher grade plywood on top of it. All the rest of the subfloor seems to be solid and good with the exception of the fact that back then in my stupid rush to dry the subfloor I removed some of the plys of the good plywood as shown below:

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I plan on using thinset mortar and cement board throughout this small kitchen to then install ceramic tiles there. So, I was wondering whether or not simply filling the plywood with missing plys would suffice?

Another issue that is not easily seen on the pic above is that I used a circular saw to cut and trim the cut of the old plywood subfloor bad portion and unfortunately I am not good at making precise cuts without much of a reference and with obstacles along the way, as a result it was left with gaps of close to to ½” in some spots. 2X4 lumber was also attached to the joist as extra brace wherever and whereon the edges of the new plywood would lack proper support.

Any insights on how to approach this repair would be welcome.

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Proper way to install double-layer of subfloor? : don't screw the top layer to the joists. And I think, the screws for the backerboard shouldn't hit the bottom layer.

I can't seam to find any manufacturers coming forth and prescribing what the maximum size void is that you can fill with thinset, under a CBU. This is probably dependent on whose thinset you use.

I'd say: anything over a half inch probably needs special attention (fill and let it set completely before proceeding because it will shrink too much). Anything less, I'd just fill as I layed the board.

Try to insure that any edge of the CBU that's going to be parallel to these gaps, ends up being no where close to them. Avoid it like you're supposed to: the seams of the plywood should not match up with the seams of the CBU.

A few screws into the old subflooring might help get everything level.


This doesn't look so far out-of-wack that you'd need to level it, but here's how:

How to level bathroom plywood subfloor before tiling?

  • Yeah, I am planning on filling any larger void (around 1/2 inch) with piece of plywood (1/2 or 1/4 thick plywood) and then glue and nail them in place whenever the need present itself (there are only two small spots that likely would need that approach). Yeah, I will try to make sure that the edges of the cmu do not intersect the edges of the plywood below and that the cmu are positioned perpendicular to the plywood.. Does 2" screws seems appropriate to attach the first laywer of 3/4 subfloor to the joists, and then 1 1/2 to attache the second layer to the first layer? Thanks! – tk3000 Jul 15 '16 at 3:44
  • @tk3000 - You may want 1-1/4" screws for the top sheet. Personally, I just screw the crap out of everything; the rest of the house can go ahead and expand and contract around my tile jobs. What if it was just a single layer... they would hit the joists anyway. Just make sure your backer board screws are short enough not to hit your second layer or the joists. – Mazura Jul 15 '16 at 4:03

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