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BridgeBlder559 06-09-2019 08:45 PM

Ditra install
I’m going to do some tile on 2nd story house has 3/4 inch osb with 16 o.c timber strands. I took linoleum of and has 1/4 plywood underneath with staples I got glue off looks really good. My question should I remove 1/4 plywood or just install Ditra over it ..? Thoughts suggestions please. Also in master bathroom also upstairs ripped up existing tile in which they previous owners had installed tile over Ditra but they did not thinset it down . They stapled the hell out of it . I’m excited though easy demo .

cx 06-09-2019 09:33 PM

Welcome, Chris. :)

I trust by "timber strands" you're referring to your floor joists (a geographic location in your User Profile would help here)?

Have you evaluated the joist to determine their suitability for a ceramic tile installation?

The quarter-inch plywood needs to be removed before installing the Ditra. The nominal 3/4" OSB meets Schluter's requirements for subflooring, but you'll need to select a thinset mortar whose manufacturer will accept bonding to that material.

My opinion; worth price charged.

Kman 06-09-2019 11:01 PM


Originally Posted by CX
The nominal 3/4" OSB meets Schluter's requirements for subflooring, but you'll need to select a thinset mortar whose manufacturer will accept bonding to that material.

What if he just staples the hell out of it? :D

cx 06-10-2019 01:27 PM

Well, yes, there's that method, Kevin.

Actually, as I recall, a manufacturer of a competing product argued in court that the fact that his membrane separated from the substrate somewhat catastrophically was simply an indication that it "uncoupled" and that since it was advertised as an uncoupling membrane that did not constitute a product failure. I'm thinking they did not prevail with that argument, but I can't actually testify to that.

But that's at least part of the reason that the tile industry, although there is still no standard for such products, requires that the membrane at least remain bonded to the substrate with a shear bond strength of a minimum of 50psi. Can you achieve that by stapling? I dunno. :D

jadnashua 06-10-2019 03:30 PM

OFten, that 1/4" ply underneath is something like a luan, or similar hardwood. That stuff has an oily substance in it that doesn't agree with cement based mortars. If it's a pine or fir, and you anchored it really well, you've got a chance. That's assuming that it's got C-faces or better...something you can't really tell at this point. A D-face will have voids, something you do NOT want in a tiled substrate.

BridgeBlder559 06-10-2019 07:37 PM

Yes joists are timber strands . Cannot evaluate them cause it is subfloor for second story no access. But house is 10 years old . So I’m going to just rip off the sheeting plywood or luan . And go over the osb . I’m laying 36’x6” tiles . What size not trowel for laying the ditra .?? 3/16 v notch is what I’ve been seeing . What notch trowel for tile laying 1/2x1/2 I’m guessing. That’s what I used on concrete floor does ditra change that ..? And also what about prefilling ditra before tile installment. I’ve read it’s easier. I would think more likely to fail as it is cured already I might not bond as well .?? Thanks a ton for all the info appreciate it very.!

Elkski 06-10-2019 08:04 PM

Yes to your ditra trowel size.
It takes a lot of mud to fill those voids in the ditra. just mix up a batch and start at it if you have any extra when you want to take a break go ahead and fill up the voidz in a far corner you're not working at. Mix another batch and continue. or you can feel all the voids first but the problem is it's going to have a bunch of bumps on it and hard chunks that you need to clean up first.

Have some real good knee pads.

cx 06-10-2019 08:10 PM

OK, so we're talking about TimberStrand engineered wood joists, right?

Engineered wood framing is generally specified to at least meet building code, but one can never be sure. If you're planning to tile over it without first finding out what you've got, I guess we'll just start from there.

If you'll visit the Ditra website and download the Installation Handbook, you'll find manufacturer recommended trowel sizes there.

As for pre-filling and bonding tile to the Ditra, keep in mind that thinset mortar does not adhere to the Ditra membrane, it simply fills the engineered contours of the material, sets up, and provides a mechanical bond. The manufacturer approves of the pre-filling of the "waffles" as an acceptable method of installing tiles, but I believe they'd still prefer that you fill and set at the same time. Many pros here report pre-filling all their installations.

My opinion; worth price charged.

Elkski 06-11-2019 06:10 AM

prefilling does let you put guide lines that have a half a chance of being there when you set the tiles. I really need to put some ditra down in the far side of my bath floor so I can start prefilling with any leftover thinset as I tile the shower...

Kman 06-11-2019 07:41 AM

Prefilling allows you to see your chalk lines, and have a little time during the installation. But that's about it.

You'll also find yourself sweeping the mortar dust every few minutes, since simply walking across it scrapes a fine layer off the top.

That said, when I install Ditra I use whatever mortar I have left over to pre fill. No sense in wasting it.

jadnashua 06-11-2019 11:19 AM

IF you're using chalk lines on Ditra, the magic solution is hair spray. After snapping the line, give it a spritz of hair spray. It will stay in place then. As said, the thinset doesn't stick to it, so it doesn't produce a bond breaker situation. The dovetail holes in the mat are what holds the thinset and therefore the tile in place...it's not a cement bond to the mat (but is to the tile).

The holes in Ditra will be solid if you prefill, but any thinset that is above them tends to crack and flake off if you walk on it, and that should be removed prior to later applying the setting bed and tile. So, in some ways, it's more work, but you will save some money rather than throwing away excess thinset...so, it's a tradeoff of time versus money. Unless you contaminate the prefill, new thinset will bond to the old. You will find it works best, similar to most things, to wipe the surface with a damp sponge prior to spreading the thinset, as what's already there will tend to suck moisture out of the fresh stuff unless you add some first, making it get stiff sooner.

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