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Unread 05-17-2021, 09:06 PM   #1
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Plywood + Ditra + natural stone question

Hello Everyone

Was looking for advice about installing Ditra over plywood. I have read the manuals about needing a second sheet if your going to install a natural stone, but it only talks about the first sheet being 3/4". So my question is... If my subfloor is 1 1/8" plywood 16" OC, do I still need a second sheet? It's going in my kids bathroom which is a small area. If I do, can I just run 1/4" backer board instead? Thanks for any input

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Unread 05-17-2021, 09:34 PM   #2
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Welcome, Brian.

The reason for the double-layer subfloor for natural stone, regardless the joist spacing, is to eliminate any place where the end gaps in the plywood panels over the joists extend all the way up through the subfloor. The second layer is installed such that it covers all the joints in the first layer.

Neither the ceramic tile industry nor the natural stone tile industry recognizes, nor makes provisions for, single-layer nominal 1 1/8th" plywood. You wanna take a chance that it'll work for your natural stone tiles, that's up to you.

It would make no difference whether you used an uncoupling membrane or a CBU in that application. It's either gonna work or it's not.

My opinion; worth price charged.

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Unread 05-17-2021, 09:56 PM   #3
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Thanks for the quick reply. So do you think I would be fine with 1/4" backer board?
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Unread 05-17-2021, 11:29 PM   #4
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CBU is a compatibility layer, and has no structural benefits on a floor, regardless of the thickness.

Deflection between the joists can cause the ends of the ply to act like a small lever over the joist. That movement can propagate up and into the tile above it. Manmade ceramic tile is pretty homogenous, so there (should be) no veins, weak points, or other defects in the tile, but all bets are off for natural stone. So, while you can do what you want in your house, the tile industry and the stone people all want that second layer of STRUCTURAL material, properly installed, to isolate the natural stone from those movements. Neither cbu nor Ditra can protect from vertical movement, but can offer some help horizontally.

When you install that second layer of ply, you install it so that no panel joints line up. Combine that with the proper spacing and fastener, and you increase the stiffness and isolation between the layers considerably. You might be successful with 1-1/8" ply, but nobody in the industry is going to bless it...
Jim DeBruycker
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