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I'm renovating a cabin and my last inside project is to finish the shower/tub enclosure. I really don't want to be doing this but can't find reliable help locally. I have two big issues; the window in the center of the wall and I have never done tile and don't feel this is the time to start.

I had tried to use a pre-formed tub enclosure but none of reasonable quality could be cut out to deal with the window. I had one that would work but it was so cheap and flimsy I decided not to use it. I stumbled upon a product called Duma Wall that I feel comfortable installing. Even though it is waterproof, it recommends a water proof layer underneath.

The tub lip is off the studs so to get the water-tight layers to hang over the lip of the tub I shimmed the whole enclosure out with 1/4" plywood. Unfortunately it has been very humid since I started this project and the plywood is warping a little. My plan was to cover this with 1/4" HardieBacker and then the Duma Wall. I have a plastic vapor barrier installed at the stud level. I plan to flash the window on the inside the same way I would flash it outside.

Is this a reasonable plan? Should I just tack the plywood down some more and assume the HardieBacker will smooth out any warping in the 1/4" plywood? Or just rip it out and do something else?

I've enclosed pictures of the tub and my plan.

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That’s a heavy duty sandwich. I would probably leave the ply up. I don’t use it just backer board. I would leave it up because of the holes in the barrier would be leak points. In the past I have tiled directly to plywood 3/4 and thicker. using thin set to bond the backer to the ply and then screw them together this will make a very solid base. once all the moisture from the thin set and grout are gone everything will be fine. Make sure to seal your finished job and that will last for 50 years.

I have seen tile on Sheetrock last 30 years and if it would have been sealed more often It could have lasted longer.

Thin plywood flexes with moisture changes I would not be concerned about that at this point. Keep on going it looks like you are going to have a solid base and that is best for tile.

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