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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  • SR

    We are getting a few bids from different tile guys and this last one likes glues the ditra down and uses a self leveling material for uneven areas on the subfloor. He uses glue because he says thin set doesn’t stick to the subfloor. Should we find a new tile guy?

    • Roger

      Hi SR,

      Yes. As far as I know there is no glue that is approved for that use. On top of that, any ‘glue’ requires air to cure, if you slam a piece of waterproof plastic over it, it will never cure. It’s like putting the lid back on the bucket.

  • Rebecca Gardner

    I am installing over linoleum. I’m concerned, even though instructions say it’s okay if I use unmodified mortar. The linoleum is old, and hard. I was wondering if it might work to pour a layer of lever over it.

    • Roger

      Hi Rebecca,

      The best thing to do would be to remove the linoleum. But you can do leveler over it, just be sure to use the primer over the lino first.

  • Tory

    I am installing Ditra Heat Duo over our hardwood floors. We need to use self leveler to correct a dip in a portion of the floor. The bathroom is approximately 80sqft, with about 1/3 of that area covered with self leveler and the other 2/3 as bare plywood. My question is, should we go with modified or unmodified thin set? Or should we use both types—modified to cover the areas with plywood and unmodified to cover the areas where self leveler was used? Thanks for your help!

    • Roger

      Hi Tory,

      Use modified over the entire thing.

  • Robert

    can you use flat head screws on ditra mat that is not adhering properly

    • Roger

      Hi Robert,

      You can, but it won’t last.
      If the ditra is not adhering properly there is an underlying issue that may cause problems down the line. If it is not adhering properly JUST on the ends, or it’s rolling up, you can place weight on those areas until it’s cured. You can also mix the thinset a bit thinner so it embeds into the fleece better. If it is just rolling up while the thinset is wet then using weight to hold it down is the proper thing to do.
      The bigger issue is – where is it not bonding properly, and why? If it is JUST the ends, you can solve that. However, you need to ensure it’s JUST the ends, and it’s JUST because it’s rolling up. If there are random areas throughout you will have bigger issues once you install tile over it. Issues screwing the ditra to the floor will not solve.

      • Jim

        Have you set Ditra over Advantech subfloor? Would you suggest Latacrete 253 Gold or Schluter Allset, both Modified?

        • Roger

          Hi Jim,

          Yes I have. I normally use 253.

  • Cindy

    If your thermostat and relay all test fine and you have one room not working what can it be.

    • Roger

      Hi Cindy,

      I need a LOT more information about your installation before I can even venture a guess that would be accurate.
      Are all the rooms on a separate heating wire hooked up to the same thermostat? Is it all a single heating wire? Do you have a relay?
      Do you have specifics about your heating system?

  • Fred Wyss

    I screwed up and used tile mastic to put down 100 sq ft of sutra heat sheets in may bath floor installation instead of thin- set. Don’t ask why please. Installing 24×24 porcelain tile over with correct thin -set. It is install over 3/4 “ tongue and groove OSB with 2×10 joist 16” of doubled every other one. Should I trash the sheet install and start over or can I salvage this mess. The mastic seems to be holding fine and I rolled the whole install with a heavy tile roller. Oh and I used a 1/4” square notch trowel for laying down the mastic.

    • Roger

      Hi Fred,

      I would not trust that. While it may be holding now, water causes mastic to re-emulsify, which will cause it to lose bond. If your floor gets wet beneath the sheets it will no longer be bonded to the substrate.