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I'm trying to fix the floor in my bathroom the tiles that were in here all cracked their grout in the section along where the two plywood pieces meet. I don't feel any vertical play but I was going to add a few more screws. The thinset seems to come off fairly easily with an oscillating tool (and seem to be sparsely applied in the loosest tiles). There was no underlayment just big tiles on top of the plywood.

Anyway I get the idea that I should add an underlayment first like Ditra, but my wife like's the 1" mosaic tiles and Ditra has a 2" minimum. Is there an underlayment that will help decouple the plywood floor but accepts 1" or am I dreaming?

For reference I measured the plywood thickness and it came out to exactly 0.75" with calipers. The tile is 0.3" high and that ends up flush with the wood floor in the other room (nothing fancy just thin oak board). House is from the 60's but it looks like this plywood has been replaced.

This is the floor. enter image description here

And also this is where it meets the shower. I don't know what they used to seal it here, it's not soft like silicon but it isn't grout. enter image description here

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A layer of 1/4 inch cement board would be better, but there is Uncoupling Mat that doesn't mention any limit on the size of tile.

Waterproofing membrane paints, like RedGuard and Aqua Defense also work as a uncoupling membrane.

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