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Unread 03-14-2018, 04:00 PM   #23
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Join Date: Dec 2015
Location: Berkeley, CA
Posts: 1,087
If you have 3/4" plywood on cleats over the whole bathroom floor, wouldn't another layer of 3/4" everywhere be overkill? Unless you want all the build up for some reason, I would think that your second layer could be 1/2" plywood, before considering height matching concerns.

Since you've intentionally set the 3/4" plywood on cleats just above the top of the joists, and since I assume you won't be fastening the second layer of plywood to the joists, I could see using a cementitious patching compound over the joists to fill in between the strips of cleated 3/4" plywood. I suggest getting a second opinion on that idea, though, as I've not seen it discussed here before. Perhaps with the patching compound you'd be happier with the idea of setting the USG foam base directly on your cleated 3/4" plywood.

As to using different build ups under the USG foam base and under the Ditra Heat, I calculated a height difference to make up of 29/32". A single layer of 3/4" plywood is actually 23/32", so you'd be 3/16" low. That's why I suggested two layers of 1/2" plywood (actual 15/32"), which would be 1/32" high. Or a layer of 7/8" plywood (not sure if actual is 7/8" or 27/32"), which would be 1/16" or 1/32" low. Or if you want plywood under your USG foam base, you could use 1/2" plywood under it and two layers of 3/4" plywood elsewhere; then I think the Ditra Heat would be 1/16" higher than the USG foam base.

Cheers, Wayne
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