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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  • Gregory Cichon

    Hello, I tried to install ditra heat with superflex2 over self-level and for some reason I didn’t get good banding to the membrane when lifting corner. I made thinset loose just so it hold a natch. Any idea why?

    Reply
  • Clint

    First time floor DIY, I would like to install porcelain, stone or thin brick on my 1955 kitchen and hall floor. I plan to use the thinner Ditra uncoupling membrane. Floor joists are 1.75 x 9 , 16 OC, longest support span is about 12 feet. The subfloor is 3/4 x 7 tongue and groove plank, with some knots, set on the diagonal. Shrinkage (I guess) has left about a .25 gap between the planks, but most “tongues” appear to be engaged (I can see some daylight in the basement!). The floor has zero squeaks, and it appears reltatively flat. One online calculator says this structural setup (deflection L/579, or 0.239″) should be good for porcelain, but not for stone (and I presume, thin brick). What do I put down as a subfloor prep to receive the Ditra? (.25 plywood?) Also, what must I add to strengthen for stone installation? Thanks

    Reply
  • Greg

    Hi Roger,

    I’m trying to install Ditra XL onto my concrete slab and having no luck. Per Schluter I’m using a 1/4 x 1/4 trowel and an unmodified mortar (Mapei Uncoupling Membrane Mortar.) Mapei recommends 7.5 qts – 8qts for the 50 lb bag, but after working through those ranges I had no luck in getting the mortar to bond to the Ditra and it was drying out to fast to be workable. Mapei technical support said I should look for a “runny milkshake” consistency; I found a good workable mortar at 11 qts. The mortar bonds well to the floor but no matter how hard I press into the ditra with the wooden trowel I can’t get it to embed. On my latest attempt I was only able to manage about 25% coverage.
    Do you have any suggestions? Thanks!

    Reply
  • Colette Benson

    Do you have to use thin set over concrete to adhere the Ditra to substrate or can you just lay it down, put the thin set and then tile?

    Reply
  • Bengt Jim Staring

    Hello Roger, can the waffles be pre filled during the initial ditra installation, or should that process wait until tile is being installed? We are laying ditra but may not tile for another couple weeks.

    Reply
  • Scott

    What is the procedure for installation ditra over linoleum/vinyl? I have been trying to find a quality answer. They say it’s possible just don’t say the best way to accomplish this. Thanks

    Reply
  • Josh

    Hello Roger, i am laying ditra over an bathroom osb floor , then tile. Should i use wonderboard or just modified thinset the ditra to subfloor then unmodofied the tile?

    Reply
    • Roger

      Hi Josh,

      Just a good modified over the osb then an unmodified over the ditra under the tile.

      Reply
  • Kelly

    I know this is an old thread but wondering why premixed mortar is not recommended? Also wondering if after installing the membrane , should I wait 24 hrs before installing the tile.

    Reply
    • Roger

      Hi Kelly,

      Because premixed ‘mortar’ is not actually mortar at all, it’s mastic with sand in it (to diminish shrinking). It CAN NOT be used over a membrane. The premixed products ALL require air to fully cure. Placing it between a membrane and a large tile, or between a subfloor or slab and the membrane, is just like placing the lid back on the bucket – it will never fully cure. You HAVE to use a powdered mortar that you mix with water.

      Reply
      • Kelly

        Thank you so much for your response. I have all my Ditra pieces cut n was getting ready to start. I’m assuming that I have to wait 24hrs for it to dry before doing the tiles? Sorry I’m new at this , I have only done a bathroom floor before.

        Reply
        • Roger

          Sorry, missed that part. You can tile over it immediately, no need to wait.

          Reply
          • Kelly

            Thank you for your help Roger

            Reply
  • Al

    What is a good way to level a concrete floor prior to installing Ditra? Previously the room had carpet.

    Reply
    • Roger

      Hi Al,

      Self-leveing cement, or SLC is the fastest and easiest. Depending on how out-of-level it is you may be able to do it with thinset beneath the ditra by using a large enough trowel and smoothing the ditra out with your level, once cured it should be level.

      Reply
      • Al

        Thanks. Is SLC something you can use over an entire house? Also would you recut the sawed in control joints after using a leveling cement?

        Reply
        • Roger

          Yes, and yes.

          Reply
  • David

    Hi Roger

    If I prefill dita cells, should I use modified or unmodified mortar to set tiles?

    Reply
    • Roger

      Hi David,

      If you’re concerned about the warranty you need to use unmodified for both. If you aren’t concerned about the warranty you can use either. I normally use modified (don’t tell schluter…). If you use modified give it an extra day to cure before grouting.

      Reply
  • Scott

    Roger, I just finished a recessed (curbless) Kerdiboard shower with Kerdi tray that slopes down from the entrance to a Kerdi-line linear drain at the wall. Next up is a waterproof bathroom floor using Ditra. The question I hope you can help with is about the transition from Ditra to the Kerdi shower tray.

    Are there issues overlapping the Ditra floor membrane a few inches onto the top of the shower tray?

    Kerdi instructions show butting the Ditra right to the edge of the tray and overlapping both with Kerdi-band. With that, it seems to me that the Kerdi band is decoupling the subfloor from the tray, not the Ditra.

    I will run the same 12×24 tiles from the floor, into the shower, The finished tray is about 1/8” lower than the subfloor where they meet at the entrance.

    Reply
    • Roger

      Hey Scott,

      There is no issue at all running it into the pan. For clarification, though, the kerdi-band is placed there in order to waterproof the transition only, it doesn’t uncouple anything.

      Reply
      • Scott

        Thanks for reply Roger!

        I understand your point about waterproofing with the Kerdi-band at the shower entrance transition. I was thinking that with large format tile running over the transition, using ditra to decouple possible movement between the subfloor and the Kerdi tray would be important.

        I will go ahead and overlap the Ditra over the transition joint. If I overlap the Ditra 2 or 3 inches onto Kerdi tray with thinset, would it still be best to also use Kerdi band on top?

        Reply
        • Roger

          I would, just to have a completely waterproof transition. Ditra is waterproof, but the cavities filled with thinset will absorb and hold water. I like to eliminate that where I can.

          Reply
  • Tau

    Hi Roger,

    Ditra install question… nuHeat may has been installed and now requires a skim coat of thinset over it before tiling. I will be installing Ditra, and wonder 1) if the thinset to install the Ditra is sufficient as also the skim coat to protect the heat may; and 2) to avoid nicking a wire with a trowel, is it possible to apply thinset to the back of the Ditra, then lay it over the dry heat may? (of course weight is an issue the consider). Thanks.

    Reply
    • Roger

      Hi Tau,

      Yes, the thinset you use for ditra will be fine. Trowelling the thinset to the back of the ditra is not a good idea, it’ll be a mess! They make plastic trowels you can use over the wires that will not nick them. If you pay attention while you’re doing it the metal trowels work fine – those cables are a LOT tougher than you may think.

      Reply