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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  • Ronald Grass

    Should my tile installed over Schulter did a give a little?

    • Roger

      Hi Ronald,

      No, it should not.

      • Ronald Grass

        How long should you let mapai grout sit before actually walking on it. The bag says 3 or 4 hours.

        Rom Grass

  • Simon Hardesty

    Hello mate! I’ve just installed the ditra over ply as a diy job with a friend. We didn’t buy the whole schluter install kit so we have no Kerdi band to go over the joints. Is it absolutely necessary to use the band as the joints have been sealed with tanking and Mapei jointing tape underneath. Or can we save a few bucks and leave the ditra as it is without the kerdiband?

    • Roger

      Hi Simon,

      You don’t need it unless you need the installation to be waterproof. That is the only reason to put kerdi-band on the ditra. The seams don’t even need to be taped, they can simply be butted to one another.

      • Simon Hardesty

        Amazing! Thanks for the peace of mind mate!

  • Gregg Warren

    Question: I want to install ditra over a treated plywood floor to install quarry tile.
    Schluter says to use “unmodified thinset”. Mapi sells a thinset specific for uncoupling membrane but recommends I use their polymer additive on plywood.
    So… Is adding the polymer creating a “modified thinset” which Schlutter does not recommend?
    “The only truly dumb question is the one you don’t ask.”

    • Roger

      Hi Gregg,

      Yes, adding polymer to the unmodified creates a modified thinset. There are two ways you can go:
      1. Use the unmodified as schluter recommends, or
      2. prime the treated plywood with a product like mapei prim-grip, then use modified over it.
      I normally opt for the second option.

  • Rich

    Hi Roger. Getting ready to lay my Ditra to tile a 7×9 bath with 12×24 honed marble. Then started reading on-line comments about beefing up either joists and/or doubling up subfloor due to the weight and fragile nature of the marble. I really don’t want to add height to the subfloor. Joists run lengthwise in the room 12 feet (under shower and thru the remaining 9 ft.). 2×10, 16″ o.c. with 3/4 ply subfloor. Weighed my tiles and they’re 11 lbs each. Do I really need to address these concerns in this small application? Thanks for your reply!

    • Roger

      Hi Rich,

      With the information you’ve given me your floor should have a deflection rating of L/1083. A minimum of L/720 is required for natural stone so you should be just fine going over that with the ditra and marble with no problems.

      • Rich

        Awesome and thanks for the previous response. Another question: I have some small spots of the plywood subfloor that I need to patch/level with an SLU before I lay the Ditra (not the whole surface just small areas). I know I need to use modified thinset over the ply for the Ditra but what about the small leveled areas? Ditra says to use unmodified over SLU but since I will only have small leveled patches what am I to do? Can I get away using all modified? Thank again. Rich.

        • Roger

          Yes, you can use modified over all of it.

  • Jonathan F

    Building my house and taking the time to study and learn *before* things go wrong.
    Installing 700 sq ft floor of porcelain body tile
    using Ditra over a flat new construction (2 years old now) concrete slab.
    Mortar is Bostik Ditra-Set (lucky me for locating a nearby supplier that stocks it!).
    I’ve read the fine Ditra and mortar instructions very carefully, watched Schluter install videos and also perused Sal Diblasi’s Elite-Tile extensive ‘U-Toob’.

    I’m just begining to prep for Ditra install. The house has heat/AC running to keep steady 75 F, 50% RH air.

    I called Bostik tech support but the usage ‘guidance’ is limited to 5-6 quarts water / 50 lbs bag. Open time 20-30 mins and pot life of 2-3 hours. No other information about temp of water / substrate / mixing time or environmental factors is available.

    The info about ‘peanut butter’ creamy for tile and ‘loose but holds a notch’ for Ditra is chanted time and again. The tip about ‘wetting’ the slab with a sponge could be less than spitting or more than Niagara Falls. Nobody can say how many cups of water / sq ft.

    I’m starting out to cover 30 sq ft of slab with 10 lbs mortar mix (150 sq ft / 50 lbs bag rate).
    I experimented with 3-4 minutes of mixing 38 Fl oz water, 10 lbs size test batch (6 qts / 50 lbs rate=max water recommended) and spraying 1/2 cup water before keying in skim coat. Mortar seems completely wetted without dry powder. The mortar began drying in 10 mins and I couldn’t apply even 15 notched sq ft before it became too stiff (coarsely uncreamy).

    Just scraped up the excess, and washed off the surface.
    Good learning experience for me!

    Any ideas about wetting the slab more or mixing (additional) water beyond the labeled amount?
    Thanks for any suggestions!

    • Roger

      Hi Jonathan,

      You’re an engineer, aren’t you? :D

      Yes, you can mix more water than recommended to get your desired consistency for beneath the ditra. It doesn’t take a lot more, go for the consistency of pancake batter for ditra installation.

      As far as wetting the slab, you want SSD – Saturated, Surface Dry. Essentially you are simply wetting it so the concrete will suck in any moisture it will before installing your thinset over it. If you don’t the concrete may suck moisture out of the thinset prematurely, leading to a weakened bond. You want to fill the pores of the concrete with moisture so it doesn’t leech it from the thinset. The surface shouldn’t have water on it. If you can run your palm across it and your hand gets wet, it’s too much water. If, when you spread your thinset, it seems like the thinset is “rolling” across it rather than laying down on the slab as you spread it – it’s too much water.

      • Jonathan F

        Thanks for a very prompt reply. If I use terminology like ‘Dilantant’ and ‘Thixotropic’ does that make me a physical chemist? Consider me well read (even if I did have a bang up time working at Bell Labs back in the ’90s). Remember what Yogi Berra said, “You can observe a lot, just by watching”. Seriously, what we know, we learn from others. Thank you for imparting some of your knowledge to me.
        Also remember this, scientists ‘experiment’ but engineers ‘test’.

        • Roger

          I use thixotropic all the time – the thinset used beneath large porcelain panels is thixotropic, that’s why vibration sets it. :)
          My crack about the engineer is that they normally ALWAYS overthink everything, it’s just their nature.

  • Jim

    Doing bathroom remodel job which involves removing vinyl sheeting from floor. Plywood subflooring has residual tar adhesive. Can Ditra be applied with thin set over the remaining tar?

    • Roger

      Hi Jim,

      Not with thinset only. You’ll need to get as much of that off as possible (I’m sure you already have), then put a primer over it so the thinset will bond. It won’t bond to the tar. Mapei Eco-prim grip would be ideal, but you can also use the primer they have for self-leveling cement in the tile aisle. It’ll be next to the slc’s.
      Cover the existing stuff with the primer, let it cure, then you can install your ditra as usual with thinset.

      • Jim

        Best advice I’ve had so far. Many thanks! 🙏

  • Barbara

    Have you ever heard of this schluder system having uneven tiles after it’s been installed? Thanks Barbara

    • Roger

      Hi Barbara,
      Uneven tiles over schluter is a result of poor tile installation – it has NOTHING to do with the schluter.

  • 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?

  • 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.