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 
Great site! I have a beautiful waterproof tile shower thanks to you! I have a small bathroom (about 20sqft) on my 2nd floor I plan on tiling with 18×18 ceramic tile using ditra. can i use modified thin set on both sides of the ditra or am I asking for problems?
Thanks again
Hi Keith,
Yes you can, just don’t tell schluter I said that.
Hi Roger,
First off I wanted to say thanks for this website! It has been extremely informative to me, and I feel more confident in my first DIY bathroom tile job.
Situation: I noticed a water spot on my ceiling in the kitchen, which is directly below my kids’ bathtub. After going to take a look, it was easy to tell they had been splashing a lot of water out and it had soaked thru the floor in the corners. I went ahead and ripped up the Vinyl tiles and the 1/4″ plywood underlayment. I am not entirely sure if I need to replace the section of plywood subfloor around the bathtub, but I probably will just incase. (It does not seem weak or squishy, its just discolored like it can’t absorb anymore water hence the water spots..)
So, after hours of research and finally finding your website I have decided to do a ditra underlayment and then do ceramic tiles. My issue is that the 1/4″ plywood underlayment that I ripped up was held in by 9 billion staples. I have tried to pull these staples and they are not coming out.. I have hammered down quite a few and they seem flat and level with the rest of the subfloor. Is it okay for me to just hammer down the 9 billion staples and then do the ditra underlayment?
Thanks in advance,
-Josh
Hi Josh,
YES, just hammer them down. They’re a pain in the ass.
Laid Ditra and tile in a master bath 14 years ago. Now replacing that tile. We pulled the tiles. The existing Ditra has some orange showing…other spots (most of the surface area) that are still covered with thinset. Can I use a product over the existing Ditra to even things out? Or do I need to remove all of the existing Ditra and start from scratch?
P.S. There are a few gouges in the Ditra in spots where the guys used some tools to remove the tiles.
Thanks!
Hi Patrick,
You can use slc over it, or a very large trowel (large enough to make up for that difference).
Is there a thinset that is approved by the manufacturer to attach Ditra to OSB (23/32″ T&G Sturd-I-Floor)? I was looking at a data sheet for TEC 338 and it stated that while Schluter approves installation of Ditra over OSB, the TEC 338 is not warranted for that application.
Hi Richard,
It has nothing to do with the schluter, the liability and recommendation will rely on whether or not a particular thinset is approved for use over OSB. I don’t know of any that specifically state that. However, I have never had any problems with that with ANY good modified thinset.
Hi, I purchased 150 Sq ft of ditra for a entryway tilemail job on my home. I have already laid 3/8th Osb over my 3/4″ subfloor. The tile area is 140 Sq ft. My question is must ditra be laid in the same direction. I’m worried that with the odd shape of room, that cuts to lay the ditra will mean I don’t have enough, so I am curious if I can lay the pieces down in any direction to cover floor?
Hi Jeff,
It can go down in any direction you need it to, makes no difference at all.
Great website, lots of useful info! I will be installing Ditra-Heat in my bathroom and am wondering if it makes sense to use Ditra mats only for the heated part of the bathroom (about 50%) and cover the rest of the bathroom with cement boards of the same thickness (1/4″). Using cement boards is cheaper but adds weight to the floor. Can you tell any other cons and pros? Thanks in advance.
Hi Sergei,
No reason not to do that, if that’s what you want to do. It’ll work just fine.
Thanks, Roger. Appreciate the prompt response.
Hi Roger,
First time using Ditra membrane. I saw somewhere that a 3/16 x 1/4 trowel square notched trowel is acceptable for applying Ditra. I was an idiot and thought I had that already, and installed last night, over OSB, using a 1/4 x 3/8 trowel.
I noticed as I was progressing that the Ditra was deforming a bit underfoot (or underknee) . . . which I thought was weird since I knew you can install tile right away. I realize now, it was because I had too much thinset under there.
So, I got a piece of plywood (~2×3′) to distribute my body weight and stomped up and down on it all over to flatten as much as possible. . . and think it looks okay now. (I think adherence is good). I can let it cure for as long as it takes. . . but is there any other consequence to having too much mortar between wood and ditra layer?
Hi Matt,
Nope, it’ll be fine provided you let it cure before crawling all over it.
I will be installing this product in a small half bath upstairs. joists on 16″ oc, with 3/4″ osb tongue & groove subflooring. I was told, and have researched, to use modified polymer thinset (Dietra set) to stick dietra to wood. was told that once done I could immediately start laying tile? was told to use regular unmodified thinset to fill squares on dietra, and that once done use grooved trowel to apply more unmodified thinnest to stick tile. is all this correct? or should I let some of the thin set dry between ?
Hi Dean,
It’s mostly correct, except ditra-set IS NOT a modified thinset, it is an unmodified thinset. You can use it to bond the tile to the ditra, but you NEED modified thinset to bond ditra to plywood. You can tile immediately, you do not need to wait.
thanks for the quick response. I will indeed make a trip to the store…again, lol…to get modified thinnest. thanks for the heads up
Great info here!
Planning a curbless shower with a straight slope to the back with a linear drain.
subflooring is 3/4″ plywood and can easily remove a 36″x48″ shower floor, trim the “over-sized 2″x12″ floor joist to get my 1″ slope, then replace the shower floor.
Any reason I could not just continue the ditra down the shower floor slope and use the 5” kerdi tape to “seal” the “joint” along the sloped shower floor and level floor?
similar to the photo below?
Any preference to slope direction being shower has two “wall sides”?
THANKS!
Hi Gary,
You want to cover the ditra with kerdi. The issue is that the dovetails will retain water and it will never drain. It’s waterproof, but you don’t really want standing water in your shower substrate. Slope direction doesn’t really make a difference. Whichever way you want is just fine.
Hmmm interesting point about the subsurface water not being able to drain, never thought about that, I was more concerned about water seeping thru ditra joints even with the 5″ tape. I like your thought and all the effort and time you spend on this blog. Excellent content!
Gary
I am considering installing 18″ travertine tile or perhaps and 18″ with 8″ pattern on a balcony space to match my pool deck below. The substrate on the balcony looks like what the pool folk call Sundeck. It is rolled on coating over a think cement layer. Is a proper ditra installation over this substrate enough to properly support an 18 inch travertine installation? Should I be considering special flexible sets as well?
Hi Chad,
That should be sufficient, but I don’t know what ‘sundeck’ is. You need to ensure that thinset will bond to it properly, if not you’ll need to go over it with some sort of coating to allow thinset to bond. Products like redgard and hydroban work well for that.
I installed Ditra heat over plywood using Tec 3n1 modified thinset which was mixed to a loose consistency I checked for good coverage and it looked ok. After 24 hours it doesn’t look like the ditra is very well bonded when I peeled back a small piece by a register. I won’t be tiling for another week. How can I tell for sure if it is adequately bonded to the plywood? And what would happen if the ditra is not bonded adequately to the plywood? Would the tiles set over it come loose or break? Thanks this is my first time using ditra.
Hi Grant,
Give it a couple of days to set, 3n1 is a HIGHLY modified thinset and takes a bit to fully cure beneath ditra. It’s great stuff, though. After about a week you won’t be able to peel it up without destroying it.
Hi there
Can left over modified thinset (used to adhere ditra to plywood) be used to fill the waffles of ditra? Of course I would let it dry then use unmodified mortar for my porcelain tile.
Thanks
Bob
Hi Bob,
Yes.
I am thinking about installing floor tile over tile. The current tile is in good shape, I just don’t like it. What advice would you have for this?
Hi Dan,
Scarify the existing tile and use a mortar approved for use of tile over tile installation. A product like greenskin over your existing tile will eliminate the need to scarify.
Hi Roger,
After 8-10 DIY tile jobs over mortar board, we tiled our master bath with ditra 10 years ago and are very happy with it. Now getting ready to tile another bathroom with 1″circles and with transition height a concern we planned to install ditra heat. Old floor was 12×12 tile on top of 1/4″ hardiebacker on top of structural plank 16″ OC. No problems with it, just ugly. We have added 3/8 ply to the structural plank and it is very solid. Plan was to proceed with ditra heat then we read that minimum tile size is 2*2. Have read we can put thinset on top of ditra and allow it to set for 24 hours prior to setting the mosaic tile. Do you agree or have another suggestion?
Thanks!
Noel
Hi Noel,
Doesn’t matter if you put the thinset in there first or not – same result as doing it when you set it. I would go with either backerboard or another layer of ply and a product like greenskin. If it were hex tile or something I’d say go for it, but 1″ tiles are really small and could definitely cause issues.
Hi, we just installed ditra heat in our kitchen and it looks like one small edge in the middle of our kitchen didn’t adhere. What do you recommend we do? Should we rip up just that section and reapply? Second question is how long do we need to let the thinset dry? Normally I would wait 24 hours, but ditra says you can tile right over it after a few hours. What gives? Thanks!!
Hi Natalie,
If you can’t get a trowel under that edge to put more thinset under there, then yes, take up that sheet and reinstall it. You can tile over the ditra immediately.
I am using Redguard and not Ditra. I’ve noticed that some of the edges have not adhered to the thinset firmly. So, do I need to try to get under there and use more mortar or would the problem be solved when I go over the seams/edges with the Redguard seam tape and mortar? Thanks for your help.
Hi Kerry,
I have absolutely no idea what you’re talking about. Redgard and ditra are two completely different products, what are you using it for? And do you mean the edges of the redgard are not adhered? Help me out here.
Hi Roger,
Not much unlike Dave above I bought modified and unmodified mortar for my Ditra installation but when installing my buddy was helping and I failed to mention we needed to use the modified under the Ditra and he grabbed the unmodified when we laid it down. I didn’t notice this until it was completely dry. Do I really need to tear up all the Ditra and remove all the unmodified mortar?
Hi James,
If it is over wood then yes, you should. Or you can take your chances. A failure is not guaranteed, but using the correct (modified) mortar over a wooden substrate greatly eliminates the chances of a failure.
it is over wood 3′ x 8′ why does it make a difference? and what are the possible bad issues?
Because wood expands and contracts – A LOT. Unmodified thinset does not have polymers in it, polymers allow a bit of flexibility in the cement crystals once it’s cured. Unmodified thinset is more brittle than modified, which means that expansion and contraction may cause the crystals to unbond from the wood pores by breaking off at the surface.
Hi. I would to install the schluter ditra Heat system in our kitchen / morning room which is one big open space 36×16′ and is a partially concrete (24×16) and partially wood subfloor (12×16). We are planning on putting down ceramic tile for the finished floor. From what I have read, I should use a modified thinset down over the
Subfloor, and non-modified thinset down over the concrete, then put down the ditra mat, run heating elements and pour the thinset. Is this correct? Are there any potential problems considering that there are two different subfloor types?
Thanks,
Jason
Hi Jason,
You should not be ‘pouring’ thinset anywhere.
That is the technically correct way to install the ditra. However, I would likely just use modified over both substrates. There are potential problems with an installation like that – the wood and concrete expand and contract at different rates, so you NEED to separate them with a soft joint.
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.
Hi Drew,
No, the ditra replaces the backer. Yes, use modified to install the ditra to the subfloor (wood).
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 ?
Hi Doug,
Yes you do. Any type of cured glue is difficult to bond correctly to. You can also prime over the top of it if you can’t get it all off.
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.
Hi Dave,
A little bit is just fine, it won’t hurt anything.
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.
Hi Andy,
Your floor will get warmer quicker.
That’s it. You’re fine.
(Rebel…)
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?
Hi Dale,
No, it will not be a disaster. The thinset will simply take a little while longer to cure – that’s it.
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!
Hi Sharon,
That works just fine. I do it on nearly every job, it bonds fine.
THANK YOU! I have been looking for this answer in the various forums for DAYS until I finally found this one. If you do this on nearly every job, why is it such an obscure question?? But thanks again! Back to work…
Hi Tim,
Because nobody wants to piss of schluter.
I don’t care…
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?
Hi John,
You can use either in that application. Modified is better, but both will work.
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.