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Unread 12-31-2019, 04:56 PM   #1
RickWally
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Advice on Plywood and Ditra

Hi All,

New to this forum. Many good things here!
I am installing new plywood underlayment on top of 1x6 plank subfloor. After the plywood is in I want to put Ditra down and then install porcelain 1x2.
Two quick questions:
1.) Can I use BCX as the plywood that will eventually have thin set put on it to receive the Ditra?
2.) I know I have to use a polymer modified thinset to attach the Ditra to the plywood, so does anybody have experience using flexbond from Home Depot and if that is sufficient stuff?

Thanks in Advance,
Rick
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Unread 12-31-2019, 05:02 PM   #2
jadnashua
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The plywood needs to be at least 1/2" stuff, but that BCx stuff should be okay.

ON the tile size, was that 1' x 2', or 1" x 2"? If it's the smaller size, you CANNOT or at least should not, try to use Ditra underneath. Schluter has a 2"x2" minimum tile size requirement. There needs to be enough 'posts' of thinset in the pockets so that a point load won't let the tile potentially rock,and thus create a failure.

To attach the DItra to the plywood, Schluter calls for at least a mortar meeting A118.11 specs, and you can use a higher .x thinset, but it isn't needed.

A quality (there are some that are mostly sand, thus, not really high quality) A118.1 thinsets is all that is required on top of Ditra, but Schluter does make some modified ones, and others can work.
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Last edited by jadnashua; 01-01-2020 at 10:03 PM. Reason: corrected the ANSI spec...
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Unread 12-31-2019, 05:45 PM   #3
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Yep, the plywood is over 1/2 inch so that should be good. Just wanted to make sure that the BCX is an appropriate substrate upon which to apply a modified thinset with Ditra.
Sorry, the tile is 1’x2’, or 12 inches x 24 inches.
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Unread 01-01-2020, 07:15 AM   #4
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Don't forget to screw all those 1X6's to the joists first, Rick, then be sure the ply is installed with the long edge perpendicular to the joists before you screw it to the 1X6's.
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Unread 01-01-2020, 12:22 PM   #5
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Welcome to the forum, Rick.

Just a quick correction to Jim’s post #2. Schluter requires using a thin-set mortar that is “suitable for the substrate”. With your plywood, you would need a thinset mortar meeting at least ANSI 118.11. And your Flexbond would, indeed, be suitable for adhering the Ditra to the plywood.

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Unread 01-03-2020, 12:27 PM   #6
RickWally
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Thank you for your help, everyone!
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Unread 01-03-2020, 01:36 PM   #7
RickWally
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Expansion Joint Inquiry

Hi Everyone,

Putting down 5/8” plywood over planks in kitchen. Just need to screw the plywood at this point as it is cut.
Ditra is going over the plywood, then porcelain tile to finish.
Left expansion joints at the walls and in the field wherever plywood touches other plywood (1/8-3/16” gaps).
The question is: Won’t the expansion joints just get filled with the thin set, defeating the purpose of leaving space? Do you just fill these gaps with silicone or caulk instead?
Just confused on what to do with the gaps.
Any help would be much appreciated and needed. Thank you.

-Rick
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Unread 01-03-2020, 01:53 PM   #8
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The short answer is: any mortar that gets in there isn’t causing a problem. If the plywood expands and closes that gap, Schluter says the mortar in the gap is crushed/powdered.

If you want to go into longer discussion, say the word and we can talk about the differences between new construction & remodeling as it relates to plywood and moisture content.

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Unread 01-03-2020, 10:19 PM   #9
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Yeah I would be interested in hearing about that!
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Unread 01-03-2020, 10:42 PM   #10
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Basically, you can download the plywood industry's handbook that has a section on underlayment versus subflooring. When the house is being built, prior to it being closed in, there's little control over the moisture content of the materials used. In fact, it's not uncommon for things to get rained on, so there's a need to account for the swelling and shrinkage of the materials, thus the requirement for gaps. Once you get ready for the finished flooring, it is assumed the house is and has been enclosed for awhile, and things are much more stable. If the plywood is rated for underlayment, its moisture content is low and stable unless it was stored improperly. The plywood institute calls for underlayment with no gaps. They don't hurt, except in the case of something like a sheet good like vinyl, where you not only butt things up, but fill any imperfections with a hard filler prior to the final material.

So, a little gap doesn't hurt under tile, and maybe is useful, but in reality, assuming things are stable at that point, any thinset that gets in there does not become an issue in the long term.
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Unread 01-04-2020, 08:08 AM   #11
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Rick, We've merged your two threads on your project. I really helps to have all questions/answers/history for a particular project within one thread so everyone has the big picture.

If you'd like us to rename your thread to something more generic we'd be happy to do so, just let us know.
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Unread 01-11-2020, 10:50 AM   #12
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Thank you so much for The help, everyone! Very much appreciated.
One more question: I was just told by a store that someone from Schluter came out to their store and did seminars and told them that their literature is incorrect and that unmodified is supposed to go on the substrate and the Ditra on top of it and then modified on the Ditra and the tile attaches to it. I feel like this is backwards and if anybody’s heard any sort of information on this.
I thought modified goes on the plywood with Ditra on top and then unmodified on top of the Ditra and the porcelain tile to that. Any help would be appreciated. Thanks
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Unread 01-11-2020, 11:32 AM   #13
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Unmodified under Ditra is Schluter's guidance, unless you use one of their modified mortars. The won't recommend anyone else's modified mortars because they haven't/won't test them.

Everything I've read says to use a modified to stick anything to plywood.

And I wouldn't care at all about using a modified on top of Ditra. Mortar doesn't actually stick to the top of Ditra - it is mechanically held in by the shape of the columns. And of course using a modified under porcelain tiles is recommended.
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Unread 01-11-2020, 11:48 AM   #14
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Welcome, Rick.

You are correct about "a store" giving you incorrect information. The orange vest commonly knows nothing at all about the orange membranes.
Quote:
Originally Posted by Dan
Unmodified under Ditra is Schluter's guidance...
What Schluter actually recommends is a thinset mortar appropriate to the substrate, Dan. For installation over a wood framed floor, that would require a modified mortar meeting ANSI A118.11 as Jim points out in post #2. Over a concrete SOG floor, the unmodified mortar would be appropriate according to Herr Schluter.

My opinion; worth price charged.
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Unread 01-11-2020, 12:18 PM   #15
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Quote:
Originally Posted by CX
Over a concrete SOG floor, the unmodified mortar would be appropriate according to Herr Schluter.
Ah, thanks CX. I thought I'd read, somewhere, that Schluter recommended at least one of their modified mortars to bond Ditra to any substrate.
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