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)

{ 2067 Snarky remarks… add one }

Leave a Comment

  • Luke

    Hello Roger!

    If I used a modified thinset between the Ditra and substrate can I use what is left of it to fill the waffles even though the leftover modified will only fill appropriately 10 sq ft of a 40 sq ft area?
    I do plan on using unmodified when laying my tile 24-48 hours later.
    Thanks for your help.

    Reply
    • Roger

      Hi Luke,

      Yes you can, just don’t tell Schluter I said that. :D

      Reply
  • David Crooks

    Howdy- here’s one for the books- I have an entry floor that we will be setting 16×16 limestone inside a diagonally laid wood picket floor, over Warm Board.
    The area measures about 16×20. About half the floor is covered with a thin, 1/4″ wood underlayment to build up the pickets, which the floor contractor glued down to the warm board with high quality wood glue. I want to adhere Ditra to that prior to dropping the 16x16x 5/8″ limestone insto the picket squares. ( There will be a 1/16 gap on each side to be filled after setting with silicone. Do you think unmodified thin-set or maybe Mapai-Granorapid will work over the plywood veneer? I cant tell if the wood underlayment is Luann… I am concerned about that. Thanks.

    Reply
    • Roger

      Hi David,

      You need modified thinset over any type of wooden substrate. If it IS luan you NEED to remove it. The moisture in the thinset will cause it to warp, it doesn’t matter what it’s installed with. This happens between the layers of luan, not the bonding to the substrate. If you can’t tell you should remove it just to be sure.

      Reply
  • Brian

    Can the ditra be glued to a concrete floor? The laundry room floor in the basement has been painted and the thunder will not adhere as well

    Reply
    • Brian

      Thinset not thunder – darn self correct!!!!

      Reply
    • Roger

      Hi Brian,

      You’ll need to remove the paint. Either chemically or mechanically. You need to get down to the concrete, then regular thinset can be used.

      Reply
  • Jack

    While working on my first tile job, I discovered that I have a high spot on the bathroom floor after the ditra and thinset had dried. It’s about 1/8 inch or so.
    Should the ditra be removed and replaced? Or is it okay to tie over a high spot like that?

    Thanks
    Jack

    Reply
    • Roger

      Hi Jack,

      It should be fine. Just be aware of that spot as being the high spot and use less thinset in that spot and a bit more around it.

      Reply
  • Michael Rego

    I am about to install Ditra in my bathroom and the substrate is plywood but I had to level a portion of the floor with leveler. So my question is, do I use non-modified thinset or modified since I have a mix of plywood and concrete substrate?
    Thanks

    Reply
    • Roger

      Hi Michael,

      Use modified over all of it.

      Reply
  • Robert James

    How much will Schluter DITRA help minimize concrete slab flatness?
    I realize that this question may not have a simple answer!
    I am planning on laying long planks – like 48″ – the longer the more critical the flatness is I imagine? Where do I find a tool to check for flatness (once I tear out the old flooring which is mostly engineered hardwood and some tile).

    Thank you very much for your time and advice.
    Bob

    Reply
    • Roger

      Hi Robert,

      It can help minimally, by that I mean about 1/4″ or less. It’s not intended to help with that at all, it is intended to go over a flat, prepared substrate. A very long straight-edge is your best tool. The parameters for large format tile are no more than 1/4″ in ten feet and no more than 1/8″ in any two foot span.

      Reply
  • Bob Mayville

    Hi, I think your tile articles are amongst the best on the subject. I’ve had a CA tile license since 1975, my dad got me into the trade at an early age and I went through 3 years Union apprenticeship and love to read articles by someone who tries to show how to do tile correctly. I was going to do a website of my own but just didn’t get around to it. So many stupid mistakes made by people trying to save money, they buy the tile, get some tools and go through some motions and eventually get it laid and probably looks okay, and never think about the little steps that they didn’t do properly. I like the way you point these things out. Back to to point for writing to you, most people who do their own Ditra don’t know the correct consistently of the thinset ( including some tile contractors ). Ditra does have a great video, but if one didn’t see it, the mixture should be wet enough to pour but thick enough to hold the the notched trowel ridging for several minutes before the ridging starts to slump. Immediately roll the Ditra over freshly laid first mat and then roll it back 12″ to see the coverage. It should be 100%, if not but almost, stand the trowel up more and it should be correct. If not, your trowel may be worn down too much or the notching was too small. I hope I wasn’t too long winded but I probably was. Keep up the good work.

    Reply
    • Roger

      Thanks Bob! And you’re correct, a lot of people don’t know that. I may write something up for it because it’s one of those things I don’t even think about but needs to be said.

      Reply
  • Christine P

    Had a contractor come in from Lowe’s to do a travertine tile job. It ended up being a bad install and because of it, my tiles cracked everywhere. They need to remove the tiles, the hardiebacker, the vinyl sheet floor, and the pressboard for the vinyl. Once that’s done, they plan on using Ditra over the original subfloor. My issue is that no one seems to know if the layer underneath the pressboard is OSB or particleboard. Will Ditra adhere to particleboard?
    To make matters worse, Lowe’s is sending the same guy out to fix the job he screwed up to begin with. But this time, we are going with very strong porcelain.

    Reply
    • Roger

      Hi Christine,

      Absolutely not! On the upside, unless it is a manufactured home (trailer house) then the substrate is normally osb.

      Reply
      • Christine P

        Nope, we have a raised ranch built in 1967. My husband and I lifted some of the vinyl and pressboard ourselves (it’s under the sink so no one will ever see it) and when we put flathead screwdrivers to the subfloor, it feels very soft. Hoping it’s OSB but really we have no idea…

        Reply
  • Jason

    Was wondering if i could use ditra under my shower pan to flush up with bathroom floor? Floor trusses where built a 1 1/2 ” lower in shower area to eliminate curb. The bathroom floor is ditra so i got to make up the 1/4″. I seen no reason it wouldnt work as long as i use pm on wood floor.

    Reply
    • Roger

      Hi Jason,

      Yes, you can. You need to fill it first and let it cure before putting the shower pan over it. You already knew that – but the one time I don’t specify that someone’s gonna set the pan over unfilled ditra and a dog is gonna burst into flames somewhere… :D

      Reply
  • David DeMartini

    We have purchased uncoupling mortar for this job as well as modified thinset. Which one goes on the bottom and which one fills the waffles and sets the tile? Or have we made a mistake somewhere along the way. The job is approximately 400 square feet and we can’t afford to do it twice. Thank you for any help!!
    David

    Reply
    • Roger

      Hi David,

      If you are going over a concrete substrate you use unmodified thinset for both over and under the ditra. If you are going over a wooden substrate you will use modified thinset beneath the ditra and unmodified thinset between the ditra and tile.

      Reply
      • Matt

        I am curious why unmodified on top. I had a tile installer (claimed to have 30 yrs of experience) say to use modified under Ditra and on top. Wasn’t sure what to make of it… I was assuming unmodified on the top because it helps create a cohesive unit with the Ditra.

        Reply
        • Roger

          Hi Matt,

          Because modified thinsets are modified mainly to help the mix retain water, which assists in the hydration process allowing a stronger bond. Since ditra is a non-pervious membrane water will not dissipate quickly, it helps the thinset retain water throughout the hydration process – modified is not needed. Also, most polymers in modified thinsets require air to cure, that’s a difficult thing to obtain beneath a tile in the ditra cavities.

          That said – I use modified all the time. :D

          Reply
  • Kathy

    I pulled up old linoleum….some of the paper backing is still stuck to the floor in some places….it is over plywood…..will the ditra adhere to that ok?

    Reply
    • Roger

      Hi Kathy,

      Ditra doesn’t adhere to anything, it depends on the thinset you use. It is ALWAYS best to get as much off as you can. An orbital sander normally makes quick work of that leftover paper.

      Reply