Improper coverage on tile / Ditra not filled correctly

Photo 1

I am not writing this to tell you why your tile is cracking or why your grout is cracking – I have other posts that may tell you that. (Click on the pretty little links :D ) If you happen to have Schluter Ditra as your substrate, this post will tell you why either one of the above may be happening.

While Ditra is my preferred membrane for floor tile installation (as well as countertops and tub decks) it absolutely needs to be installed correctly. The two main techniques for this are fairly simple:

  • Make sure the cavities (waffles) are filled correctly
  • Install it over an approved substrate (and with the correct type of thinset mortar)

Improper coverage on tile / Ditra not filled correctly

Photo 2

There is a lot more to ditra than those two items but if either one is incorrect I can nearly guarantee a failure. See photos 1 and 2 there? The tile was cracked and it was a direct result of a) not getting the waffles filled correctly and b) improper coverage on the tile. Now b may be due to not backbuttering the tile, an improperly-sized trowel, letting the thinset skim over or set too long before installing the tile or simply incorrectly mixing the thinset. All three of those things will cause any tile installation to fail – whether you use ditra or not.

Not filling the waffles correctly, though, will cause the tile to not be fully supported and/or not ‘locking’ the tile into the ditra. Because it is not correctly locked into the ditra you will lose the mechanical bonding properties of ditra and you may as well install it directly to particle board at that point (That was sarcasm – don’t do that!). For more specifics about exactly how ditra works you can check out Provaflex vs. Ditra wherein I describe exactly how the mechanical bonding process works – and rant about a particular jackass. But the mechanical thing – that’s what you want to concentrate on. :D

You need to use the flat side of your trowel and spread thinset in every direction over the ditra to ensure that all the little waffles are full. Since the cavities are dovetailed (that means they go down and away from the opening) you need to ‘force’ thinset into the bottom corners of the cavities. Simply running the trowel over the ditra will not do this. Simply running the trowel over the ditra did that (photos 1 and 2).

Improper substrate for Ditra

Photo 3

Installing ditra over an approved substrate is much, much easier. In fact, nearly every bare substrate you find in a modern house would be considered an approved substrate – shiny linoleum is not one of them (Photo 3). While there are thinsets that ‘say’ they will bond to linoleum (and some of them will) apparently the jackass who installed that particular floor was not aware of that. :guedo:

See photo 4? I lifted that up with my pinkie – literally! It was not attached at all. He may have had correct coverage beneath the tile and all the little waffles filled – I have no idea. There was not enough stuck to get enough leverage to tear one off and find out.

Improper substrate for Ditra

Photo 4

Most any plywood (even osb :whistle: ) is an approved substrate for ditra. And  if you use a thinset approved for that substrate, there are no problems at all. Photos 3 and 4 had an unapproved substrate and, apparently, incorrect thinset (and a shitty tile job, but that’s a whole other post). It was nearly guaranteed to fail.

When you buy ditra for your installation every roll comes with a handy little instruction booklet. You can go to Schluter’s Ditra Page on their website and access the instruction booklet (This link is a PDF!). They even have a flash video about the proper installation technique. You can leave a comment below and ask. You can email me. You can send up smoke signals – I’ll answer.

Given the 17 ways to acquire correct ditra installation information above there is absolutely never a reason to do it incorrectly. Ditra, in my opinion, is the best membrane for most floor tile installations. The only time I’ve seen it fail is due to incorrect installation. And that isn’t just the common BS everyone accuses failures on. Me, personally, every one I’ve seen fail is incorrectly installed.

If you use ditra, and if you have an approved substrate, and if you have the correct thinset mortar, and if you fill the waffles correctly, and if you use the proper trowel and get proper coverage it will not fail. Yes, that’s a lot of ifs – when you read it. In practice it really is not that many things to get right. It’s just common sense, mostly.

So here’s one more if: If you have any questions at all about correctly installing ditra and using it for your tile installation please, for the love of all the marble in the Sistine Chapel, ask me below in the comments. I WILL answer you. I’m just super-cool like that 8)

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  • Mitchell Z

    Hey Roger 👋,

    Is it possible or allowed to reuse Ditra? I am removing a tile floor and the previous installer used ditra under the tile and it looks in good condition. Is it possible to clean out the loose old thinset and properly bond the new thinset to it?


  • Henry

    I am a little concerned over thinset. I am applying on bathroom floor with recommended substrate of two layers, 2000 construction home and wood underlayment, my thinset is Acrylpro ANSI A136.1.. DITRA recommends ANSI 118.11. One is a material spec and the other is a construction spec. Can I use the Acrylpro?

    Thank you in advance for taking the time helping me

  • Edmond Despins

    I thank you for providing vcry detailed information on tiling. I have read your posts and find them most instructive.
    I would like to know what you consider the best, second best, and third best modified thinset to lay Ditra on plywood.

  • Mike

    Hello Rodger you are the man. You helped me about 8 years ago on bathroom vanity and it is still holding up perfectly. I’m tiling my kitchen and have pulled up the old tile. There is 2/8-1/2 of mortar left on the floor and 1/2 durarock underneath. There were no cracks in the tile and the durarock is solid. My question is should I sand/scrape the old thin set off down to the durarock, or can I knock down the high spots and use ditra over the old mortar? Thank you for your life saving advice.

  • Tammy

    I plan to put ditra down with All Set and then put a 2×4 mosaic sheet tile down. Do you have a recommendation for the trowel size for the tile install? I am using a 1/4 x 3/16 for the All Set below the ditra. I do plan to use All Set above the ditra as well. Thanks.

  • Elspeth F Mackenzie

    Where Ditra meets Kerdi wall board should :
    a) the Kerdi wall board come right down to the floor i.e. no gap?
    b) the Ditra butt up to the Kerdi or
    c) should the Ditra have 1/4 ” gap between it and the Kerdi or
    d) should the Kerdi sit on top of the Ditra?
    Thank you

  • Trish Bowden

    Is it acceptable to install Ditra Heat Cable on the Ditra Membrane? They now have a special Ditra Heat membrane that has a why to lock the cable in place. My tiling is using the regular Ditra Waffle Membrane and I’m concerned it’s not a good choice.


    Is it acceptable to install Ditra Heat Cable on the Ditra Membrane? They now have a special Ditra Heat membrane that has a why to lock the cable in place. My tiling is using the regular Ditra Waffle Membrane and I’m concerned it’s not a good choice.

    • David Fenderson

      No, you should use the Ditra-Heat membrane if you want a heated floor. Running heater cable with the regular membrane turns the cable into a load-bearing structure – it will fail.

  • David DeMauro

    I am an amateur builder building my retirement log home. The floor/subfloor is 2-6 tongue and groove pine. Bathroom tile is being installed over the pine flooring. The bathrooms will have electric floor heat. I purchased a danfoss lx floor Matt to install under the tile. I do not want the tile/grout to peal or crack when I am done. Should I use an uncoupling membrane under the floor heating Matt? Or what is your recommendation for laying the tile over the pine floor? Please email me with suggestions on how to best proceed

    • Roger

      Hi David,

      You need plywood over that t&g floor. Or, better yet, just install plywood directly to the joists where the tile will be. You can not install a lasting tile installation over t&g flooring.
      When using heating elements with uncoupling mats you want to install the heating under the mat. So whatever the installation instructions are for the heating under the tile, simply substitute ‘uncoupling mat’ into the instructions wherever it says tile.

      Do you really have a 67 shelby? Because that’s friggin’ NICE! :D

  • vy

    First time using this membrane, do i have to lay down morter under the membrane or can i just lay the membrane flat on concrete?

    • Roger

      Hi Vy,

      You need to use thinset mortar to bond the underside of the ditra to the concrete.

  • Angie

    Hello, I’d like to know if i lay my Ditra down, do I need to lay my Tiles soon after or can I wait and walk on it for a couple weeks? Reason is my helpers are not available on the same weekend. But i want to get the job started.

    • Roger

      Hi Angie,

      You CAN wait if you need to. If you do wait I would recommend pre-filling the top of the ditra with thinset (the holes) level and let it cure. That way you aren’t crushing the ditra when you walk on it.

  • Greg Hammock

    As part of a bathroom demo I’m tiling the floor – I’ve removed the old tile down to the 1-inch OSB subfloor which is level and secured. The tile needs to meet up with a wood floor which is approx 3/4 inch above the OSB subfloor. I’ll be putting down mosaic square tile boards and the height of the tile is ~3/16 inch. What do you recommend to raise the subfloor the add’l ~1/2 inch … plywood, backer board? I want to use Ditra for my underlayment which will eat up 1/8 to 3/16 of the gap. Thanks in advance.

    • Roger

      Hi Greg,

      I don’t know what you mean by ‘mosaic square tile boards’. 1 foot square mosaics? (the sheets). Regardless, to raise it 1/2″ for tile I would recommend 1/2″ backer board. It’s the easiest and quickest.

  • Rick

    Hello, I’m new yo using Ditra. I was originally going to use wonderboard but got talked into Ditra. Kind of regretting it now. My problem is this, I laid the Ditra down and now there are waves in spots on the Ditra. I’ll admit the mortar was a tad thick, but I made sure to press hard with a wooden float to squeeze it down & out.
    What can I do now? When I place the tile down to check the placement, some hace a slight wobble. Will the tile mortar make up for the wobble if I make sure to back butter the tiles? The tiles are 18×18.
    Thank you

    • Roger

      Hi Rick,

      Yes, the amount of mortar beneath your tile will compensate for that wobble provided it is less than 1/4″ or so.

  • Leah

    Hi! First time working with ditra and I noticed today after it dried there are a few spots that move up and down when I step on them? What should I do? I’m using a large format slate tile on top of the ditra.

    I used versabond modified mortar, what did I do wrong?

    Thank you for any advice!!

    • Roger

      Hi Leah,

      You didn’t get the ditra fully embedded into the thinset in all areas. You can actually cut out those areas and put more thinset beneath them, then reinstall them. The ditra does not need to be in one uncut sheet to work correctly, it can actually be pieced in and still work the exact same way. Just redo those unbonded areas.

      • Leah

        Thanks for the reply! Great to know I can cut out sections and redo them if needed!

  • Finley

    Hi, we have a kitchen floor we pulled up that revealed flooring leveler that was spread on top of small tiles possibly asbestos. The tiles came up fine. The Plywood that is left is covered in a black mastic glue we believe. We have tried chemicals, scrappers, a heat gun, and searched all over but made little progress. What’s the easiest way to go about getting it off to lay the ditra? Does it matter?

    • David Fenderson

      You should use an asbestos-encapsulating primer. You’re not going to be able to remove that mastic chemically, and any kind of mechanical scraping or grinding is going to release asbestos particles into the air, which is not where you want them to be.

      I recently used PerfectPrimer after removing an asbestos tile floor in my basement – it keeps the mastic sealed and it provides a good surface for thinset to adhere to.

  • Steve Spenceley

    I am installing 12 x 12 light green marble tile over ditra. What are the acceptable non-modified thinsets to use? I have a home depot 45 miles away, also some flooring stores that sell Schuter Set, I think. What would you recommend?

    • Roger

      Hi Steve,

      Get the schluter-set. Any unmodified you normally get from any big-box store isn’t worth the price of the bag that holds it.

  • Evan

    My old tile floor isn’t removable as it was installed in an inch of concrete (70s). Can I put ditra heat above the old tiles then retile?

    • Roger

      Hi Evan,

      That is a mud bed, and yes, it actually is removable, although it is a pain in the ass. Yes, you can provided you scarify or prime the existing tile so the mortar can bond to them correctly.

      With a big enough hammer, and a rotary drill or jackhammer, ANYTHING is removable. :D

  • Joel

    The subfloors in my home appear to be particle board except in kitchen and bathrooms the used plywood. Can Ditra work over top particle board?

    • Roger

      Hi Joel,

      No. It needs to have plywood put over it or, better, be removed and replaced with plywood or osb.

  • Eric Smith

    Hi, had another question. I have a heat vent coming up thru the floor how would you waterproof around it?


    • Roger

      I don’t understand why you want to waterproof around a heater vent – water is going to run into it if it gets above the level of the tile. Beneath the tile you can use silicone. You can also use spray foam, it will keep residual amounts of water out of it, which is fine because, as I said, if it gets above the tile it’s going down into it anyway.

  • Eric Smith

    Hi. I am doing a bathroom reno. Replacing vinyl flooring with tile. I want to use Ditra for waterproofing. Found K3/particle board under the flooring. It has been removed and what is remaining is 3/4 OSB. The K3/particle board was glued and over stapled. When removing the K3/particle board some of the top layer of the OSB came off so the floor is not 100% level. How or when should you level the floor? Use self leveling on top of the Ditra? Also there seems to be some adhesive from the K3/particleboard on the OSB does it need to be removed and if so how?

    Thanks Eric

    • Roger

      Hi Eric,

      If you want to remove the adhesive just soak it, most linoleum and vinyl adhesives are water soluble. If you don’t, you can prime the floor, use self-leveling cement, then install the ditra over that. Schluter does not wand slc over the top of ditra. Not that it won’t work, but…

  • William D Parsons

    Nice presentation regarding Ditra, thanks.

  • John Z.

    I plan to install Ditra Heat in a room with wood floors that were covered with asbestos tiles. Can I attach the Ditra Heat mat directly to the asbestos tiles with quick setting modified mortar, or do I need to install plywood first? Thanks!

    • Roger

      Hi John,

      You need to install plywood first.

  • Mark

    after installing ditra on floor, can you fill all the waffle holes on top, and allow to dry before installing tiles. I feel this would be better than to try to do it in conjunction with laying the tile, given the shorter open time of the mortar

    • Roger

      Hi Mark,

      Yes, you can. I do it all the time. It also makes the installation of tile go quicker.

  • LaNell

    Tiling over pre existing tile over slab.
    Attaching Ditra to tile.

    • Roger

      Hi LaNell,

      You need to either rough up the existing tile or put a primer over it to allow the thinset to bond suitably.