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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  • Brandon

    I have standard wood subfloors but had to use a floor leveler where the house settled a bit. I want to put ditra down as my membrane. Do I have to use 2 different thinsets? One for the leveler and one for the wood substrate?

  • Chris

    Hi Roger,
    I’m having my kitchen redone and my contractor seems to have vanished. So I’ve been pulling up the layers of old floor myself as the new cabinets should be ready soon. The last layer is sheet vinyl glued to 1/4 inch subfloor over the 3/4 inch plywood. I was going to pull up that last layer of 1/4 inch and replace with new (so that the tile will be at the same profile as the hardwood it’ll meet up with) but the vinyl comes off really easy. The glue substrate left on the floor is smooth. Can I put the ditra on that or should I rip it out and install new? Or maybe just run a sander to remove the old glue?


  • Mark Rimer

    Hi Roger
    I am in a 1950s home in pacific north west with many layers of flooring on top of each other through much of the house. I ripped up the tile in my bathroom to find a 3/8 inch layer of plywood which is on top of lino which I presume is on top of something else. The hallway is a good inch above the bathroom floor.
    There is a bit of old thunder that is left where I removed the tiles. Do I need to sand it down to pure clean wood to lay down the ditra membrane? Do I need to remove the existing plywood and lay down 3/4 inch plywood first? Would it be ok to lay down 3/4 inch plywood over the existing plywood and the put in the ditra?
    I’m planning on putting in the heat floors and 1 x 2’ porcelain tiles on top.
    Please help! First time tiler!

  • Megan

    We need help! We applied ditra heat with non notified thinset on a raised foundation after self leveling because our floor is not level despite this, we realized that we are still not
    Level and need almost an inch to put the floors down. I know ditra has a 1/2 inch in order in order to get proper heat. could be self level over the ditra and still
    Get enough heat or should
    We self level and apply new ditra heat membranes on top of old ones I really don’t want to remove the other ones

  • Megan

    We need help! We applied ditra heat with non notified thinset on a raised foundation after self leveling because our floor is not level despite this, we realized that we are still not
    Level and need almost an inch to put the floors down. I know ditra has a 1/2 inch in order in order to get proper heat. could be self level over the ditra and still
    Get enough heat or should
    We self level and apply new ditra heat membranes on top of old ones I really don’t want to remove the other ones

  • Tim


    What a fantastic site. A guy that calls a spade a spade. We need to work together, hell, we could solve all of the worlds problems!

    I’m writing from the great white north of the border. Canada eh!

    I’m a contractor who’s done this stuff for years but have found myself stumped! I’ve got a 30 yr old kitchen floor with a 3/4″ ply substrate matching up to 3/4″ hardwood. I want to use Ditra XL instead of another layer of ply to get me to the required substrate thickness prior to laying 24/24’s in hopes of keeping the transition to the hardwood to a minimum. Schluter Canada is telling me that I cannot do this as it does not get to me the required 1 1/4″ prep thickness. Thoughts?


  • William

    I am wanting to install real “sliced” old brick for a new kitchen floor. We had hoped to install the 1/2 inch thick, 4×8 inch pavers onto a DITRA membrane which would be adhered to 1” thick Owens Corning Foamular 150 insulation board sitting on top of the newly poured slab floor.
    Does this sound reasonable or is there a better way?
    Thank you for any advice.

  • Ron

    I don’t know how old this article is as it doesn’t have a date on it.

    I am ready to tile my floor . I have ditra and schluter all set mortar. I am putting it over 2 x 10 floor joists, 3/4″ plywood flooring and a layer of self-leveling (about 3/8″). In the shower I am using the schluter pre-formed floor which is where my question comes in. Also, large format tile….12 x 24.

    The formed floor has kerdi membrane on it. Can ditra be put over that kerdi in the shower? Not sure why I want to do that (maybe a security thing….i dunno) but I am interested to know.



  • Bryan

    Hi Roger,
    Please help! I just recently tiled our bathroom floor and afterwards realized I got my mortars reversed for the ditra. I used Mapei uncoupling membrane mortar (unmodified) under the ditra over plywood floors and Mapei ceramic tile mortar (modified) on top of the ditra. I realized this before I grouted everything. The tiles I am using are small 2″ round tiles on mesh backing. Am I going to be OK or do I need to redo everything??

  • Ron Marek

    Hello Roger,
    I am preparing to tile a 4’ x 8’ bathroom floor with 12” x 24” tiles. This is a very old house, 75-80 years + old. The floor joist are 16” OC. Next are full 1-1/4” plank board followed by hard wood flooring on top. The HW flooring has no finish left on it and seems very flat and tightly in place. What is your recommendation for proper underlayment prior to tile?
    Thank you Ron

  • Jeff Spooner

    I am laying 12 x 12 porcelain tile over Ditra using Schluter All Set mortar. I have quit for the day and have a fair amount of thin set on the Ditra at the perimeter of the work area. I wiped up as much as I could with a damp sponge but there is still mortar in the squares. Is this a huge concern? What will the consequences be if any?
    Thanks, Jeff

  • Katie

    Hi, I’m 9 years late to this post, but I’m having issues with my grout cracking between tiles over the ditra mat. I’ve already replaced one tile near the entrance of the bathroom and am trying to avoid replacing the entire floor of tiles. I think my tile man may have not fully filled in the ditra mat studs with mortar. Is there an easier way to check and repair this other than tearing up the entire floor?

  • Kristina Vernyi

    Hi Roger! I had a tiler put thin set over sites flooring already wired. Some places the thin set is on and in some isn’t. Happens to be that he did not level the floor before doing all of that. He no longer works for me. The new guy said he can’t put self-leveler over Ditra heated flooring. Is he correct? Is everything lost? Do I have to rip it all out and then level? If he’s wrong, what self-leveling substance can you put on top of the membrane and the wiring? (I’ll need an actual brand please). Thank you so much.

  • Sara

    Hi Roger. I am going to tile a kitchen floor. Is it necessary to use the keri-band on the seams and floor/wall transitions?

    Thank You

  • Joel

    Hi Roger! I have some vinyl sheet flooring, on top of 1/2″ particle board, on top 1/2″ plywood. I was hoping I could just install ditra right on top of the vinyl layer. Thoughts?

    • Duncan

      My contractor installed ditra membrane in the bathroom, he used the recommended mortar and then screwed down the membrane. Does this make sense and does it void the warranty. Is there an advantage to screwing the membrane down?

  • Khou X


    I just bought an apartment that is partially renovated. The seller used Ditra over a cement substrate. He never got around to installing the natural stone tiles, which he left for us to do. The space is about 400 sq ft. It appears that he started to fill the Ditra cavities with thin set and never finished, so about 1/4 of the entire Ditra underlayment has dovetail cavities filled with thin set. It all appears to be months old now, and the rest of the Ditra underlayment was installed nicely but has been walked on so much it’s all dirty. My questions are:

    1. Should we take it all out and install fresh Ditra, or do I try to clean the dirt off and use what he had put down?

    2. Can I simply put fresh thinset over the old thinset that fills those dovetail cavities, or should it be removed (in which case I will simply replace it instead of doing all that work)?

    Thank you!

    • Roger

      Hi Khou,

      You can vacuum all the dirt and dust out of the cavities and off the cured thinset and use it, provided the cavities are not completely crushed down from being walked upon. Unless the walked on portion is flattened down you should be fine to use it.

  • Kurt

    Hi Roger,

    Thank you for the informative site. I am installing a bathroom on a loft that has 2×6 t&g decking supported 39″ o/c. I want to insulate under my finished floor. My plan is to place dense 1″ styrofoam sheets over the decking with 3/4″ ply screwed through the styro into the decking and Ditra over the ply. What do you suggest?

    • Roger

      Hi Kurt,
      I don’t see any issues with your plan. I would ensure that you only screw into the decking, not the joists. Screwing into the joists can cause differential expansion issues. While not really common, it does happen. Provided you make an effort to avoid the joists you should be fine.

      • Kurt

        Thanks Roger,
        A second question involves creating a monolithic thinset floor over the Ditra. I want to fill the Ditra with a ‘brown coat’ of thinset and trowel it smooth after which I would finish with a product called SkimStone that will be sealed with polyurethane or epoxy. The idea is to use the Ditra as a waterproof membrane for the bathroom…How thick of a ‘brown coat’ would be required over the Ditra?

        • Roger

          That will be the finished surface?

          • Kurt

            Yes, the thinset with about 1/4″ of the MileStone / SkimStone finish on top…no tiles.

            • Roger

              Only fill the ditra cavities with thinset. The skimstone finish will bond to the cured thinset in the cavities. Thinset does not bond to ditra, it stays put using a mechanical bond (the cavities are dovetailed back and holds the thinset in place once cured). If you use a grout float to fill the ditra with thinset it will leave a clean finish with the top of the ditra clean, which is what you want. Does that make sense?

              • Kurt

                Yes, thank you. The thinset on top of the Ditra might be too thin and crack with use. But, I am wondering if the Kerdi might be a better product for this application since I am not sure I need the decoupling of the Ditra…

                • Roger

                  The Ditra is the correct choice for a thin, monolithic lamination like you’re talking about. Any thinset on top of the ditra is simply excess, it does nothing for an installation. The ENTIRE bonding process of the ditra is the mechanical dovetail fastening into the cavities. What you are essentially doing is installing your skimstone onto a bed with thousands of little ‘feet’. It would be best for your application. It would be more apt to crack if installed over Kerdi because there would be zero compensation for movement.

  • Josh Jackson

    hello! I am installing ditra before tile on a bathroom floor. I took off old linoleum. I am wondering the extent to which I need to remove the paper and adhesive that is stuck to the concrete slab. I am chiseling it all off I just didn’t know how much residue would be a problem. Your expertise is greatly appreciated.

    • Roger

      Hi Josh,
      You want to get as much as you possibly can off of the slab. Try to soak it with water and let it sit for ten minutes or so, it should scrape right off after you do that. The issue with leaving it on there is that the thinset will bond to the paper, but the bond to the concrete is reliant on how well the paper is bonded to it, not how well the ditra is bonded to the paper.

  • Sue

    Schluter all-set is not available in our area. Is there a substitute I can use? Menards has a modified latex thin set, will it work?

    • Roger

      Hi Sue,

      Will it work for what? According to schluter a latex modified should only be used when bonding ditra to a wooden substrate, everything else requires non-modified.
      That said, a modified WILL work, but you’ll lose your warranty and you normally need to allow more cure time.

  • SR

    We are getting a few bids from different tile guys and this last one likes glues the ditra down and uses a self leveling material for uneven areas on the subfloor. He uses glue because he says thin set doesn’t stick to the subfloor. Should we find a new tile guy?

    • Roger

      Hi SR,

      Yes. As far as I know there is no glue that is approved for that use. On top of that, any ‘glue’ requires air to cure, if you slam a piece of waterproof plastic over it, it will never cure. It’s like putting the lid back on the bucket.

  • Rebecca Gardner

    I am installing over linoleum. I’m concerned, even though instructions say it’s okay if I use unmodified mortar. The linoleum is old, and hard. I was wondering if it might work to pour a layer of lever over it.

    • Roger

      Hi Rebecca,

      The best thing to do would be to remove the linoleum. But you can do leveler over it, just be sure to use the primer over the lino first.

  • Tory

    I am installing Ditra Heat Duo over our hardwood floors. We need to use self leveler to correct a dip in a portion of the floor. The bathroom is approximately 80sqft, with about 1/3 of that area covered with self leveler and the other 2/3 as bare plywood. My question is, should we go with modified or unmodified thin set? Or should we use both types—modified to cover the areas with plywood and unmodified to cover the areas where self leveler was used? Thanks for your help!

    • Roger

      Hi Tory,

      Use modified over the entire thing.

  • Robert

    can you use flat head screws on ditra mat that is not adhering properly

    • Roger

      Hi Robert,

      You can, but it won’t last.
      If the ditra is not adhering properly there is an underlying issue that may cause problems down the line. If it is not adhering properly JUST on the ends, or it’s rolling up, you can place weight on those areas until it’s cured. You can also mix the thinset a bit thinner so it embeds into the fleece better. If it is just rolling up while the thinset is wet then using weight to hold it down is the proper thing to do.
      The bigger issue is – where is it not bonding properly, and why? If it is JUST the ends, you can solve that. However, you need to ensure it’s JUST the ends, and it’s JUST because it’s rolling up. If there are random areas throughout you will have bigger issues once you install tile over it. Issues screwing the ditra to the floor will not solve.

      • Jim

        Have you set Ditra over Advantech subfloor? Would you suggest Latacrete 253 Gold or Schluter Allset, both Modified?

        • Roger

          Hi Jim,

          Yes I have. I normally use 253.

          • Sue

            Where can I find that? Our options for stores is limited in Wyoming

            • Roger

              Hi Sue,

              Menards in Cheyenne (you mentioned menards, I assume that’s where you are) has TEC full-set plus 380. That is an equivalent non-modified.

  • Cindy

    If your thermostat and relay all test fine and you have one room not working what can it be.

    • Roger

      Hi Cindy,

      I need a LOT more information about your installation before I can even venture a guess that would be accurate.
      Are all the rooms on a separate heating wire hooked up to the same thermostat? Is it all a single heating wire? Do you have a relay?
      Do you have specifics about your heating system?

  • Fred Wyss

    I screwed up and used tile mastic to put down 100 sq ft of sutra heat sheets in may bath floor installation instead of thin- set. Don’t ask why please. Installing 24×24 porcelain tile over with correct thin -set. It is install over 3/4 “ tongue and groove OSB with 2×10 joist 16” of doubled every other one. Should I trash the sheet install and start over or can I salvage this mess. The mastic seems to be holding fine and I rolled the whole install with a heavy tile roller. Oh and I used a 1/4” square notch trowel for laying down the mastic.

    • Roger

      Hi Fred,

      I would not trust that. While it may be holding now, water causes mastic to re-emulsify, which will cause it to lose bond. If your floor gets wet beneath the sheets it will no longer be bonded to the substrate.