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Unread 01-28-2015, 11:35 AM   #1
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Thinset between OSB and plywood?

Hey folks, first-time poster here. I've spent several days researching this question to avoid posting a question that's already been answered, but have come up with nothing... so here goes.

Want to tile the master bath in my new house (new construction). Engineered 2x4 trusses, 19" OC, deflection way good for ceramic. 3/4" T&G OSB subfloor with correct axial orientation (perpendicular to truss).

Plan to use Ditra membrane under ceramic tile. Would like to use warm tile heating wire, too. Want to keep all additions (subfloor, mat, wires, thinset, and tile) to an inch.

My plan (OSB up) is: 1/2" plywood, thinset, Ditra, thinset/heating wires, tile. I want plywood instead of CBU because I want added stiffness from the plywood to combat the inherrent springiness of the OSB.

My question pertains to installing the plywood. I understand why adhesive doesn't work and may actually make things worse. And I undertsnad that thinset under CBU is not for adhesion, but for stability. Then why wouldn't we use thinset under the plywood too?? Plywood is less uniformly flat than CBU. Why does everyone just screw it to the OSB?

And intuitively, it seems I could help the thinset stay uncoupled from the OSB by starting with roofing felt, then putting the thinset and plywood down over that?

Am I on the wrong track entirely, and should I just plop the plywood down (with correct orientation and staggered joints of course) and quit my fretting?

Thanks for any help!

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Unread 01-28-2015, 11:51 AM   #2
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Welcome, Scott.

Here is a good article from our Liberry on what I consider the very best way to install that second layer of subflooring.

Layering the structural subflooring you want to take advantage of the friction between the two sheets as well as the inherent stiffness of each layer. You don't want anything between the layers to compromise that.

There is no more "inherent springiness" in OSB than in plywood subflooring, but with your 19.2" joist spacing I'm very much in favor of a second layer of subflooring.

The CBUs have no real structural strength of their own and any gaps between them and the subfloor would be a very flexible area even with the required mechanical fastener schedule. That's why they require a bed of thinset mortar under them.

If you use an electric radiant heating system and Ditra, you want the Ditra on top. But I would suggest you look at the Ditra Heat system and incorporate both in one layer, thus reducing your overall floor height and making the heating installation easier.

My opinion; worth price charged.

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Unread 01-28-2015, 07:04 PM   #3
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I used the Ditra-Heat in my tile install, Scott. It is likely the most expensive way to go in terms of floor-warming but it really is a breeze to install. It is further a breeze to take up and reinstall and take up and reinstall again on accounta you keep messing up where things are supposed to end or not with the wire. Finallly, it works beautifully! Warm tiles...can beat 'em.

Check out the link below my sig for my thread.

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Unread 01-28-2015, 07:12 PM   #4
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FWIW, not all OSBs are created equal. FOr example, Advantec's typically tests out at 10% stiffer than the same thickness plywood.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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Unread 01-29-2015, 08:42 AM   #5
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Thanks, everyone- much appreciated! I love it when things make sense... due to the iffy performance of adhesives I had given up on adhesion in building up the subfloor, and ignored friction. I get now why floating another layer of plywood on thinset wouldn't add as much stiffness as screwing a layer directly over the OSB.

And good to know more about the OSB itself.

I'll check out the integrated Ditra / wire, too. We live in central Minnesota and as far as I'm concerned, warm floors are the only way to keep these bones thawed. First floor is slab on grade with hydronic loops... looking forward to that, too!!
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