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Unread 01-20-2022, 05:22 PM   #4
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 95,317
I would use nothing thinner than nominal 1/2" plywood in your application. While the nominal 3/8ths" might be technically suitable, most of the plywood that thin you'll find is not sufficiently flat and if it is flat it's still difficult to install flat on the existing subfloor.

If you actually want to make a waterproof bond between your Ditra and the face of the tub, you'll really need to use some KerdiBand, fold it up onto the tub face a quarter-inch or so and bond that with some KerdiFix. Then bond it to the Ditra with thinset mortar. Still won't give you a waterproof floor, but at least it would take care of that particular joint if that's your goal.

Entirely up to you.

My opinion; worth price charged.

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