Effects of Improper Ditra Installation

by Roger

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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Roger, great site! I want to install Ditra over a concrete slab (then porcelain tile on top). Slab has old yellow mastic on it.

How much of the mastic do I need to remove?

If I use a chemical dissolver won’t it clog the concrete pores inhibiting the Ditra / thin set adhesion to the slab?




I’ve sanded down my concrete floor pretty good but on the corners and around plumbing I wasn’t able to get it sanded down 100%. There are a few little patches of black but it’s very minor and the floor is still pretty even/flat. Originally, I was told to lay down 12×12 tiles directly onto the concrete but now I’m thinking of using the ditra as a way to cover the little imperfections on the floor. Will it work? Or how well do I have to sand down the concrete?



Hey how are you? I I did everything right, have the right thinset, right trowel, damped the subfloor first, and spread a thin layer of thinset with the flat end of the trowel first before using the notched side of it, and took my wooden trowel and smoothed it out. Yet I still have a good many spots where it didn’t take. Could you please email me as soon as you can I have a few other questions and I gotta get the the ditra and told laid before the weekend so the cabinet guy can come back and finish. I would greatly appreciate it. Thank you in advance.
My email is em_ecton85@yahoo.com,


Not A Tile Guy

I want to put Ditra over our existing bathroom counter(now formica over 5/8 plywood). Can I just put 1/4″ hardwood plywood(glued and screwed)
over the formica as a base for the Ditra?

Please Help!



Hi, I have just spent all day making a very big mess of my bathroom ditra install. I cannot whatever I do get proper coverage under the ditra. I was using a 1/4×1/4 square notch trowel at first. My first attempt I think I had my thinset too thick. I added more water and it worked a little better but still not acceptable. I threw it out and started a new batch of thinset. This time mixed it even thinner. Which ended up being too thin. It wouldn’t hold a notch. I tried installing the ditra and still didn’t get good coverage. So I tore the sheet up and scrapped all my thinset up again.

I’m using ditra heat 2×3′ sheets, installing on top of 3/4″ plywood. I was sold and recommended to use Ardex X5 modified thinset for under and on top of the ditra.

Using a 2×4 as a float I couldn’t get it to cover. I even tried walking on it and working it in the my feet and still won’t work. Tried my 3/16×5/32 v notch trowel as well. Also was dampening the plywood prior to applying thinset.

Any suggestions on what I’m doing wrong?



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