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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Dear floor elf.
Thank you so much for all of the information on your site.
We are about to lay shluter ditra down. Our substrate is 3/4″ plywood and our chosen tile is 12×24 volakas 3/4″ thick marble tile. We know that we have to use an unmodified mortar ON TOP of the ditra, that is between the marble and the ditra. But shluter recommends ‘latex portland cement’ for BELOW the ditra, between the ditra and the 3/4″ plywood underlayment.

Waaaah? Which product do you recommend. A ‘latex portland cement’ is a ‘modified mortar’ correct?
PLEASEEEEEEEeeeeeee which brand do I use for this?

Thank you



Hi Ava,

Over a wooden substrate you need to use a modified thinset to bond the ditra to the substrate, then an unmodified thinset to bond the tile to the ditra.


Tony Z

I have an old marble floor on a send fl bathroom that has thermal heating under it. The heating still works great but the marble has developed some cracks. I know there is a 3/4 inch ply sub fl and 1/2 inch concrete board on top of that. My guess is there is still enough movement that the marble cracked. It does not sound hollow under the cracks so it appears that the marble is still bonded. Can I tile over this fl to save the thermal heating? If so would I use Ditra or RedGuard under the tile?



Hi Tony,

Yes, you can. I would rough up the marble and apply a proper thickness coating of redgard, then tile directly to that.



hi roger,

i have a question about floor height, on a concrete slab, what would be the best approach to add more height to the tiled area? i will have laminate and the underlayment at about 13/16″ height against the tile floor. Should i use multiple layers of ditra in order to try and get close to level or should i go in a different direction and use a different product?




Hi Anthony,

Ditra XL is twice the thickness of regular ditra. You can do two layers of ditra if needed, but the xl is a better option.


Don Roberts

The Schluter manual says to use unmodified thin set to install tile. The supplier where I purchased by Ditra heated floor roll said that since I was using travertine I could use modified thin set because travertine is so porous there would not be a problem with the thinset drying. What is your opinion?



Hi Don,

Unmodified thinset works fine on travertine. Modified also works fine, but you’ll lose your warranty.



While Ditra can be installed on OSB, I’ve never seen a thinset approved for OSB, do you know of one that is approved in writing for OSB?



Hi Gary,

No. The problem isn’t the thinset, it’s the WILDLY varying types of osb and coatings. A good modified thinset will bond just fine to the UNSEALED side of any osb, but you’ll never get it in writing.




I am planning to use Ditra in my kitchen. I wanted to use it because it is thin and I have a short ceiling. After demoing the kitchen, and tearing up a layer of osb I was left with a 15/32 layer of plywood. I don’t want to add too much height so that the new cabinets will fit, but I am worried the plywood Ina’s too much flex. I am reinforcing all the joints with 2x6s between joists. Will this suffice, or am I asking for trouble. Also with the thin plywood, am I better off just going with backer board instead, would that stiffen it up enough?


Steve In Denver


I’m not a pro (and I don’t even look like an elf), but that 15/32 plywood is not sufficient for tile. I believe the bare minimum is 5/8″ nominal thickness plywood or OSB, but I don’t think you will find many people voting for the bare minimum. If I were a betting man, I’d bet a six pack that Roger is going to tell you that 3/4″ is his minimum recommendation.

So, you probably don’t want to hear this, but rip up your 1/2″ floor and put down 3/4″, followed by the Ditra – that’s probably as thin as you will get. (there are other products thinner than Ditra that serve a similar function (greenskin is one) but I don’t know anything about them. I believe Ditra is 1/8″, so not a lot to save there anyway)

– backer board does not appreciably increase your floor stiffness.
– The plywood must be exterior glue and have all face grades C or higher (no CDX / sheathing allowed – internal voids cause problems)
– The plywood should be tongue and groove or blocking should be added to support the seams.
– Blocking should be added at the perimeter of the new flooring (where there are no tongue/groove joints)
– The single layer of 3/4″ is based on a 16″ joist spacing – I don’t know if it would work / is advisable for 19.2 or wider.
– The single layer of plywood assumes you are using porcelain / ceramic tile. If you were planning on using stone, stop and research it thoroughly before you do anything else.
– Have you evaluated the joist structure and verified that it meets the L/360 deflection requirement for ceramic tile. Determine the size, spacing and unsupported span of the joists – look for a grade stamp if possible. There are span tables available on the googlebrain – look it up and find out if your floor is rigid enough – if not, be prepared to stiffen it by adding (sistering) joists or reducing the unsupported span. (or be prepared for cracked grout and or cracked tiles at some point in the future.)

Just one DIYers two cents.



Steve is correct, but it needs to be a minimum 3/4″ under ditra, not 5/8″. Thanks Steve.



Hi Will,

You need another layer over that. A 3/8″ layer would suffice, but I would REALLY use another 1/2″ layer. Cement board will add zero dimensional stability to your floor, it is simply a proper substrate to bond the tile.


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