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I'm doing a master bathroom remodel in which all electrical and plumbing must be moved.

The old floor had two layers of plywood: 15/32 as the bottom layer, and 19/32 as the top layer. (The numbers I'm giving are those on the APA stamps.) This setup supported ceramic tile for 30 years without any issue.

I plan to leave the first 15/32 layer there and put 3/4 as the new top layer. I've done the math. Structurally speaking, a single layer of 3/4 plywood by itself would be enough for the tile. (I know I'm raising the floor level, and taken that into account relative to the level of the floor outside the bathroom.) If I were in a setup where the bottom layer is needed to provide the structure that the tiles need, then I'd patch the bottom layer. However, since I don't need the bottom layer for structure, can I skip patching the holes left in it by the old plumbing setup? Or am I setting myself up for trouble?

I've given consideration to entirely removing all the subfloor but I'm trying to avoid that, as it would lead to a whole bunch of complications in the basement. (The master suite is on the 1st floor, with the bathroom just above the machine room.)

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You are correct in your thinking. The original layer, even with the holes will still add strength to your top layer. No need to patch the holes.

  • +1 for acknowledging that the first layer is sufficient, even with holes. Also, make sure the top ply runs perpendicular to supports. – Lee Sam Aug 31 at 19:21

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