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 
) 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)
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.
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).
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. 
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.
Photo 4
Most any plywood (even osb 
) 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 
Ditra….can you prep fill the membrane (filling in all the waffles full of thinset) a day before the tile install-looking to making the marks on an intricate, precise layout standout and not get lost in all the gaps etc. Thanks!
http://www.bunyantilemarble.com
Hay guuuurl… (are you a girl?
),
Yes you can.
Hi Roger,
I just had an asbestos abatement company remove they vinyl flooring in my bathroom. They had to do some serious scraping to get the glue off the 3/4″ plywood and it’s quite torn up in many areas. Can I still use Ditra on this project and if so, how? Would you recommend Hardi Board instead?
Thanks!
Hi Denise,
I would actually probably use cement board over that. There is really no way to know if the thinset will bond to your existing floor. You can also go over it with a new layer of 3/8 or 1/2″ plywood, then ditra.
Roger,
Our upstairs master bathroom has a 23/32″ T&G Oriented Strand Board sub-floor on 16″ floor joist. Is it okay to put the Ditra membrane down on this sub-floor and then lay large ceramic tile?
Hi Dwight,
It depends on your unsupported span of the joists. How far apart are the supports for the joists?
Hi, I’m installing the detra over particle board in our bathroom, I want to make sure that I can use the polymer ‘ porcelain ‘ tile mortar underneath and then use an unmodified on top for my porcelain ceramic tile. Thanks
Hi Btown,
Yes, modified between wood and ditra and unmodified between ditra and tile. But you can not set it over particle board, it needs to be plywood.
Hey Rog,
I am a carpenter by trade, ( don’t hate me!) I am installing tile in my house, long story, but I’m gonna do it. I’m hearing, reading conflicting info
regarding the installation of Ditra over plywood. The Schluter videos indicate
that the use of modified thinset is proper, and when I called the Schluter headquarters they told me to use unmodified because some people forget
and end up using the modified under their tile as well which is never recommended.
I just wondered if you’d be willing to chime in on this. Don’t worry, I agree to
hold you harmless etc. for anything including burning dogs.(actually,I have cats, but hurry with your answer,they’re starting to smoke…)
Thanks for all the e- books and help, – Phil
Hi Phil,
You NEED to use modified thinset between plywood and ditra. If you ‘forget’ to not use that with tile then it is my opinion that those people have no business setting tile.
Hi Rog,
Too late for me,the die is cast for
Thanks for the reply.Alas, in my wood butchering stupidity, I got my wires crossed, your info is correct, my confusion was with types
of thinset going under Schluter’s premade shower pan.They have
said to use modified under the pan if you are using ditra next to it
on the rest of the floor. I catch your drift about “those people have no business setting tile” hahaha!
my ham-handed wood butchering self!
Have a great Thanksgiving! Big Phil
Has anyone ever used SLC to fill the waffles in Ditra, then installed the tile after the SLC has set? Is this a bad lazy man idea or a why didn’t I think of that moment? Thank You.
Hi Glenn,
I’m sure there are a lot of people who’ve done it, but you should not. It essentially negates the purpose of the ditra (what I was told at Schluter).
Floor Elf…..i have plans sub floor screwed down in bathroom with 1/4 inch concrete board taped, thin setted and screwed down. I purchased ditra heat mat and heating cable. Can I install the ditra mat over the cement board then tile over that?
Hi Peter,
Yes you can.
Hello, I’ll be installing Ditra over Advantech. The Advantech has the dust and mud from the drywalling process and paint splatter and dust from the spray priming process all over it. It will be very hard to fully clean the Advantech. That being said how clean does the Advantech need to be to install Ditra over it? Any insight would be greatly appreciated!
Hi Erik,
It needs to be as clean as you can get it. Make sure you vacuum it as well to get as much of the dust off as possible.
Hi Roger,
Great site! Quick question. Can Ditra be installed on a painted subfloor? I’ve got 5/8 plywood down on 16″ OC joists, but it’s painted. I wasn’t sure if that would prevent the Ditra mat from bonding. If I can’t install it on the painted subfloor, do you recommend putting down 1/4 plywood on top, or 1/4 hardiebacker before laying the Ditra?
Any suggestions?
Kevin
Hi Kevin,
No, the paint will affect the bond of the thinset. Ply or backer will both work over it.
Hello Roger,
Great knowledge. Thank you! I am tiling a kitchen and bathroom.the existing floor is linoleum, original from 1996. I have used ditra over concrete in a basement with 12×24 slate. 3 years in looking perfect. I read on the as bluster site that I can lay ditra directly over the lino with proper thinset. I now see that the Lino pulls up very easily, I was advised to nail it down?? Other alternative is to pull it up and go direct on the existing subfloor. Is the very thin layer on blue an issue?? Which this set for old plywood!
Thanks so much!
Jason
Hi Jason,
The lino should be removed. You may get the ditra to stick to the lino, but the glue holding it down is water-soluble, so if water gets under there it’ll come right up. You need modified thinset over plywood.
Sorry for the delay, you got lost in the shuffle somehow.
Have a Ditra installation where one part of the floor in bath is heating up properly but another is not. The area that is not heating up apparently needed to be built up with more thin set (uneven Floor) — could that cause it too not be as warm as the other part? If so can it be corrected without tearing up entire floor?
Hi Mark,
It will cause it to heat up more slowly. The only other reason would be a separate heating wire (more than one wire under your installation) and that one isn’t heating properly. If you only have one and you want it to heat up uniformly, unfortunately that tile will need to be removed and the floor built up under the heating wire.
Hello from the UK, great site..
Our kitchen floor is 60sqm – down so far is: beam & block construction, 100mm of celotex over it, piped UFH, 65mm of liquid screed on top of which we are due to lay 20mm thick Limestone Floor Tiles. Our builder is questioning whether we need Ditra Matting…What do you think?
Thanks!!
Hi Gemma,
You don’t NEED it, but it’s always a good idea.
Hi Roger.
I’m about to do our dinning room and kitchen using ditra for the first time. Is it ok to use a 12″x24″ tile on this. I’m using a polymer modified mortar to adhere the ditra to my 3/4″ substrate and a non-modified for the tile. Thanks. And great site by the way.
Hi Don,
Yes.
I’ll be installing natural slate (12″x12″) tile in mudroom over 3/4″ OSB subfloor (crawl space, std joists, circa 1990). The rest of the first floor will be 3/4″ hardwood and I’d like to get as close to a level surface between the two surfaces as possible. Do I need to glue and screw another layer of OSB on subfloor for tile or will Ditra on existing OSB be enough (assuming I get the IF’s correct). Thx!
Also, what about a layer of backer board, is that necessary or does Ditra replace that too?
PS – changing name to Also Not a Tile Guy
The backer is not necessary when using ditra.
Hi Matt,
With slate one layer of ditra is normally enough to flush up with hardwood.
Hi, I bought a very thin glass accent tile and the 1/8″ Ditra. The Ditra will be applied under the accent tile to make it flush with the wall tile. The strips of the accent tile are about 3″ x 12.5 inches. When I told my tile man to use unmodified thinset between the glass tile and the Ditra, he would not listen. He said it is ok to use modified thinset on both sides of Ditra ( below and above Ditra ) . I am confused. Please help.
Hi confused,
It is fine to use modified with an accent tile both above and below ditra. He’s correct.
Thank you for the quick reply! Today, I realize he used Ditra on the floor and used modified above and below it. The floor was done now and I decide not to worry about it anymore. Won’t do any good. The thinset used above Ditra is TEC full flex. He said he has been in the tiling business for 30 years and used Ditra since it ever existed. I could not believe you have time to respond my question and keep doing the beautiful work you posted on Facebook as well. It is such fun to visit your web site and Facebook. I admire your work. Thank you!
We have to install 1×1″ stone mosaic over Ditra Heat, over dbl plywood subfloor. We also plan to run a band of Kerdi Band around the perimeter of the room for waterproofing (shower is zero threshold).
My understanding Schluter warrants only 2×2″ and larger material, but I’ve heard 1×1 can be installed successfully. Over Ditra Heat would you recommend settin the Ditra Heat mat, then running the heating wires, then thinsetting in the Kerdi Band and prefilling, and after a cpl days to cure, then setting tile on top of the cured thinset fill? And if so, would you prefill with modified or unmodified? Per Schluter instructions, we’ll be anchoring the Ditra Heat with modified.
Thanks for the most current thinking.
Hi Mark,
I wouldn’t do that at all. But since I’m not gonna stop you from here
– I would fill it all with modified first, let that cure, then install the mosaics.
Roger, what part of this plan would you not do at all? Thanks for elaborating. –Mark
Putting 1×1 mosaics over ditra heat.
What are the problems you anticipate? We could go SLU over the heat then either set tiles direct or use an uncoupling membrane. The Ditra Heat seems like a natural for this application, though the tile size is small. Appreciate if you can elaborate. Thank you, Mark
Mark,
I won’t try to speak for Roger, but the problem with smaller tiles, as I understand it, is that they may not be supported well enough by the grid of columns the Ditra provides. You are risking cracking tile/grout by going with a smaller tile, I think.
I think encasing bare heating wire in SLC is workable, but most SLC products require lath to be installed over wood framed floors. You would want to find a suitable plastic lath to avoid nicking/cutting the heating wires. Mapei makes something (Mapelath, I think) for that purpose. Rumor has it that some SLC manufacturers permit heat mats (not just the bare wire) to be used in place of lath – something you’d want to verify for the specific SLC product if you choose to go that route. You then set the tile directly to the SLC – I think.
I went with a different option, which is Ardex Liquid Backerboard. I’m not sure how they manage it, but they don’t require lath to be installed over wood flooring. Maybe that would be an option for you?
I’m pretty far above my pay grade, so don’t take anything I said as solid advice…more as ideas to consider and research. The manufacturers of various products usually have very good customer support via phoned and email….use them.
Hi. Great post, more info on here than anywhere I’ve found. I’m building a shower on a slab house. Shower measures 4’6″ wide 6′ long, concrete was sloped 1/4 per foot when I poured it. So ditra is going on the floor and walls I guess. I was told to put drywall on the walls for backing. Is this right? Just seems crazy that drywall will hold up tile on walls and ceiling.
Hi Guy,
Yes, if you’re using kerdi drywall is fine. And yes, it will hold up tile just fine, sheer strength is what you’re concerned with, and it has more than enough.
Do you have any advice about applying self-levelling compound over ditra-heat before finishing with thin-set mortar and tile? Installer needs to compensate for thickness of adjacent mortar bed poured for curbless shower (I, and probably the installer, think it would have been better to build up plywood floor thickness before laying ditra-heat…)
Hi Nick,
Just make sure you mix it all up one right after the other so you’re pouring wet slc into wet slc, and be sure to block off anything you don’t want leveled.
Can I lay Ditra on exterior pressure treated plywood? We installed the plywood over a poured cement floor to control the dampness and prevent bugs. The plywood is installed indoors and clean secured. Would it be better to remove the plywood and level the cement then apply the Ditra?this would be a lot of extra work.
Larry
Hi Larry,
No, you can’t. But you can install 1/4″ or 3/8″ ply over the pt, then install the ditra over that.
AFTER MY DITRA HEAT WAS INSTALLED MY ESTIMATE OF LENGTH WAS CORRECT, AND NOT THE INSTALLER’S. (HAD A KNEE REPLACED RECENTLY SO COULDN’T DO IT MYSELF)
WHAT KIND OF OPTION DO I HAVE, CAN I RUN SOME TILE UP A WALL TO TAKE THE ADDITIONAL FOOTAGE, OR CUT SHEET ROCK ALONG A STUD AND INSTALL CABLE IN THE DITRA IN THAT WITH OR WITHOUT TILE COVERING IT OR AM I JUST SCREWED.
THANKS FOR ANY SUGGESTIONS. (ALREADY FIRED THE INSTALLER BUT IT WAS TOO LATE)
Hi Ken,
It depends on how much extra you have. You can alternate the number of nubs between the wires in portions of it (1 closer, 1 with three between) or weave it back and forth to eat up some length as well. You do not want it uncovered anywhere, nor inside a wall, although you can run tile up the wall provided you have backer behind the ditra heat mat and/or wire on the wall.
Will a modified thinset work to install ditra over sheet vinyl? Website says use unmodified thin set over ditra for ceramic tile. Why?
Hi Rich,
Only a thinset approved for use over vinyl, provided it’s not cushioned vinyl. The plastic waffles will cause the latex to hold water for an undetermined amount of time, leading to inconsistent cure.
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?
Thanks!
Hi Rick,
You need to remove as much as you can. It is best to just scrape or sand it off. Most dissolvers, however, won’t affect the pores of the concrete, nor the bond of the thinset.
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?
Hi Kal,
Yes, it’ll work just fine.
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 Not a tile guy (if that IS your real name…),
Yes, you can.
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?
Thanks!
Hi Nick,
Use a larger trowel, a 5/16″ u-notch, and make your thinset just a bit thicker than you did the second time when it wouldn’t hold a notch.