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Unread 03-03-2021, 12:27 PM   #9
Registered User
Join Date: Mar 2021
Location: Portland, OR
Posts: 7
Thanks so much for the advice y'all - I will return the CustomBlend. As much as I would like to use AllSet, I can't find it within 150 miles of here, and shipping it is pretty expensive and slow. VersaBond it is, I'm afraid. Unless the Mapei "Uncoupling Membrane Mortar" is better for the floor?

I do have another matter I would like input on, if you would be so kind:

We will be installing 4" hexagon mosaic marble tile, already purchased and en route. This is NOT going in the shower or tub areas, just the flooring for the broader bathroom. For this I will probably go ahead and use a professional installer, so that we don't damage the marble (feels a bit out of our depth DIY wise!). I followed as much internet guidance as I could find on the subflooring, but I am not sure if it is sufficient and don't want to ruin the marble by putting it on an insufficient subfloor.

What we had was this: 1" tongue and groove on 16 inch joist spacing. The joists are 2x8's, which gave me a deflection of L/480 on the handy deflection calculator here. Obviously, this is insufficient for stone tile.

I went through and screwed down anywhere there was a squeak or movement, and then installed 1/2" exterior grade plywood running perpendicular to the joists. So, with the T&G and the plywood, I am at 1 1/2" total, which exceeds the 1 1/8" minimum for stone, to my mind. On top of that, I am planning to use Ditra instead of CBU as the underlayment for the decoupling benefit, which I have seen can be particularly useful for stone. I read CBU won't technically improve deflection so I figured Ditra was the way to go.

Is this sufficient? I am seeing mixed messages on whether the required 2 layers for stone can be one of T&G and one of plywood. Do I need another layer of plywood (one installer recommended it - the others haven't balked at the current setup)? Should I use CBU for additional rigidity or stick with Ditra for decoupling?

Thanks in advance for any guidance.


Last edited by ZoeJR; 03-03-2021 at 12:46 PM. Reason: Edited to clarify total thickness
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