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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Drew

Hi Roger,

Thinking about doing the ditra-heat floor in my master bath. I have a plywood subfloor. do I need to put backer board down before the ditra heat waffle board panels or do the orange panels replace the backer board.

Additionally, I am to use modified thinset to bond the waffle sheets to subfloor?

Thanks.

Reply

Doug

Hi, I just removed all the glue down floor tiles of my kitchen floor down to the plywood. Do I have to sand the glue off the plywood surface to get a good bond for the Ditra with Modified thin set ?

Reply

Dave

I made the mistake of using unmodified thinset between my osb substrate and ditra. I realized my error about 16 hours after doing it, and was able to pull up the ditra mat and scape about 95% of the thinset off before it was fully cured. All that remains is an extremely thin layer in some spots. Question is: Should I remove 100% of the unmodified thinset by sanding it off? Or will the tiny bit that remains be ok? Thoughts?
I will be using a modified thinset under the ditra once I know to what extent I need to clean off the unmodified.
Thank you.

Reply

Andy

Hi there,
In a moment of stupidity (several moments) I installed Ditra Heat cable every two rows and not every three as they tell you to. I haven’t turned on the floor yet because I am not sure what will happen. Any thoughts? Thanks.

Reply

Roger

Hi Andy,

Your floor will get warmer quicker.

That’s it. You’re fine. :D (Rebel…)

Reply

Dale

A friend of mine was telling me of his kerdi installation on a shower floor and told me he used modified thinset. Will this be a disaster as it calls for unmodified?

Reply

Roger

Hi Dale,

No, it will not be a disaster. The thinset will simply take a little while longer to cure – that’s it.

Reply

Sharon

I want to install my Ditra and fill the waffle with thinset, then let it dry overnight, before laying the tile. Most instructions I see say to lay the tile immediately. I understand I CAN do that, but my question is: Is it OK to NOT tile immediately? Will the thinset drying cause the tile not to bond? Thanks!

Reply

Roger

Hi Sharon,

That works just fine. I do it on nearly every job, it bonds fine.

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John Mackey

I am installing porcelain tile on a whirlpool
tub surround. The top of the base is 3/4″ plywood. Can I use schluter membrane with unmodified thinset or do I need to use ditra with modified thinset?

Reply

Roger

Hi John,

You can use either in that application. Modified is better, but both will work.

Reply

John Mackey

Thanks so much for your response Roger. I just picked up the Schluter shower kit and it contained an entire roll of the membrane. I installed kerdi board on the shower walls so I was looking to use the membrane on the tub surround. I’m glad you said it will work. Thanks again.

Reply

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