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Old 03-10-2018, 10:32 AM   #21
wwhitney
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Join Date: Dec 2015
Location: Berkeley, CA
Posts: 1,019
So if you're going to run the USG membrane out on top of the Ditra Heat a few inches, then it seems to me you need to get the top of the Ditra heat flush with the edge of the USG pan. Relative to their substrates, the USG pan will be about 1-1/8" tall (1" pan plus 3/8" notch), while the Ditra Heat will be about 11/32" tall (1/4" plus 1/4" notch).

That makes a 29/32" difference. You could probably use 1/2" plywood under the USG pan and two layers of 3/4" plywood outside of the pan. Or if the cleated plywood between the joists is sufficient support for the USG pan (I don't know), then you could just use two layers of 1/2" plywood outside of the pan. Or perhaps one layer of 7/8" plywood would be close enough, but that's typically a special order item, you may not be able to buy it by the sheet.

Please double check my accounting.

Cheers, Wayne
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