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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  • rb

    Do you need to fill knots or gaps in plywood before installing Ditra?

  • Harold

    Can you use Masonite siding on floor then use Ditra over it?

  • Fred

    Quick question…is it necessary to tape the joints of ditra if we are not seeking the floor to be 100% water proof?

  • JP

    Hi Roger, All,

    Remodeling my bathroom. Demo’d old ceramic tile floor which had been installed directly on 11/32″ plywood underlayment over 3/4″ plywood subfloor (some of the tiles in the main field were cracked, where foot traffic heaviest.) I pulled up the plywood underlayment. There was probably 3 or 4 spots where the floor squeaked (small bath ~45 sf), I screwed down subfloor into joist and eliminated all the squeaks, though this took a while as eliminating one squeak sometimes seemed to spring another mini one elsewhere. I just had a tile contractor install 6″ x 38″ porcelain wood look tiles on Ditra Mat on the floor about 2 weeks ago, and after about a week my floor which was quiet as could be…is now squeaking and squawking. Most of the squeaks seem to be coming from the corner of the alcove wall (alcove tub/shower) next to the tub apron.

    My question – would improper ditra mat installation cause this? I understand you need to fully fill the waffles etc. Or is it likely that something else caused this, perhaps the weight of the new tiles?

    I’m considering trying to fix the squeaks from below (unfinished basement below), as Ive done this sucesffully in the past (sister blocking to joist up against subfloor), but I wonder if rather than fix the squeaking it might intensify the issue, cause cracking etc?

    Thanks in advance

  • Anna

    Hi! Does the ditra getninstalled flush with the wall? Or is there 1/4” expansion gap used for both the ditra and the tile?

  • Dave

    Hello, how much gap is acceptable between the Ditra Heat mats? I just had some installed recently and some of the mats have a fairly large gap between them (almost an inch). Is this a problem? Will it affect the support of the tiles or cause an issue when passing the heating cable?


  • Dave Austin

    Hi, I have an exterior balcony above an exterior patio. The balcony is ceramic tiled with ditra but must have been installed incorrectly as it leaks to the patio below.
    Can ditra be installed directly over the existing exterior tile, with the proper thinset, and new tile installed over the ditra?