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 
I had to make several cuts to the ditra in order to cover my bathroom floor without buying another roll. I know you can cover in several pieces without any problems with ditra function (found that out from reading one of your previous posts thanks!). There are some small gaps about one eighth inch at the seams of same of the smaller pieces. Is that going to be a problem? If so, remedy?
Hi Kelly,
Nope, no problem at all.
So just morter and fill the seam gap and morter ceramic to ditra as normal? Thanks so much for help and quick reply
Correct.
We are new at the diy thing and decided to tile our small bathroom floor. We installed the membrane, and now the area near the door is 1/4″ to 1/2″ uneven from the rest of the area. How do we level this out before tiling the floors?
Hi Julie,
What membrane? What type of substrate do you have beneath it? Was the substrate below the membrane uneven or is if from the membrane installation? Help me help you.
We used the ditra uncoupling membrane and the floor became uneven from the installation process.. it was stable and I damaged plywood underneath
Undamage*
You can simply cut out the ditra where it is uneven, scrape down or build up as needed, and reinstall a new piece of ditra. The ditra can go in in pieces, it doesn’t need to be a continuous sheet, it can be patched together with zero difference in the performance of the membrane.
Hello,
Quick question on ditra install in a basement. I have a concrete slab floor that structurally is in good shape. The downside is that there is drylok installed and some of it is flaking off. Is it possible to install ditra over the drylok after cleaning up all the loose areas? Planning on tiling about 300sf in this area. Thanks in advance!
Hi Judd,
You can, but I really wouldn’t trust it unless you scarify the surface of the concrete. If that’s flaking off now you really don’t want more flaking off with the ditra and tile bonded to it.
Floor Elf, I have a cracked slab on a patio. Cracked at the control joint and across it. Everything has settled and I’m going to repair the crack with Emecole 555. The cracked piece is a little lower. I want to get the slope back under control and use a pedestal system with 24×24 pavers on top. I’m also considering tiling. Just not sure what the best approach would be. I was thinking of using Ditra and build up the thinset on the low the part to accommodate the slope. How thick can the thinset be under the ditra? Would tiling be okay if I used two separate sheets of ditra on either side of the cracked control joint? In that case I would build up a thicker mortar bed at the low spot and then slope as I move away from there. I was considering Hydroban as well. Lastly, I have pooling as the patio slopes. Low spots. How would you recommend filing in low spots? Thanks!!! Love the site. Helped me get through my 16×16 slate tile job.
Hi Ben,
The ONLY way that you can tile that with one side lower than the other is to treat it as two separate installations with a soft joint between them. You don’t want to build up thinset any thicker than about 1/4″, at the most. But you can double up the ditra, put two layers on the low side.
Great. Thanks! How about if I put the fancy artificial grass over it? Can the thinset be any thicker? I was thinking about putting regular Ditra on the raised side and the XL on the lowered side if that was the case. Just want some solid advice before I go either way.
The regular ditra and xl works as well. So does the artificial grass. Most thinset are only stable up to about 1/4″ or so, so you don’t want to go thicker than that. The important part is the soft joint between the two – no matter how you level them up.
Thanks Roger! Have you ever heard of anyone using something like this in thinset? http://www.forta-ferro.com/applications/slab-on-grade/
No I haven’t, but I don’t see why it wouldn’t work. As long as YOU are the one doing it.
You may want to look into some medium-bed mortars (now called LFT mortars). Some of them can actually go up to 1/2″ or so, they probably use something similar in the mix.
Russell is it possible and will it work to install the shower membrane on Advantech flooring with modified thin set instead of the ditra??
Hi Chris,
No it is not possible. Well, it is, but it won’t last – so no. Why would you want to do that? And you don’t install the membrane to anything with ditra. Not sure what you have going on there, care to clarify for me?
i have extra membrane left over and was wondering if i could use it on the floor instead of using the Ditra on the floor.
thank you for your response.
Hi. Using a linear drain across the opening of a curbless/frameless shower with door.
Thinking of using Kerdi membrane in the shower and ditra outside the shower. But I can’t find anyone who talks about joining the two membranes with Kerdi band. Is it ok to do that?
Hi Michael,
Absolutely it is.
Hello can you lay 20 by 20 tile over ditra with plywood subfloor space joist 19.2
Hi Uri,
It depends on the size of the joists and the unsupported span.
I’m about to tackle tiling a 45 sq ft area of my 2nd bathroom. The substrate is 3/4″ plywood over an encapsulated crawl space. I’m planning on using Ditra with ceramic tiles. I’ve read your posts and the Ditra installation material on the web. I’m not getting a clear directive on priming the plywood before installing the Ditra. Some sites say yes and some say no. Would it be better to use 1/4″ durarock ?
Hi Joe,
No need to prime. Use modified thinset between the ditra and plywood, and non-modified thinset between the ditra and tile.
I recently had a contractor install the Ditra XL in my master bathroom, and I’ll be installing the tile. Can I do that in two phases? Meaning one day I fill all the waffles with unmodified mortar, let it set up, and then on another day install the floor tile? If so, should I used the same mortar or should I use a modified mortar?
Hi Tony,
Yes, you can do that. Use the same non-modified mortar.
I used unmodified thinset over a plywood substrate to apply the ditra. I just learned that I should use modified thinset for plywood. Is this true and should I remove it if it is ☹️
Hi Adam,
Yes it is true, and unfortunately yes, you should remove it and install it with modified. Wood simply expands and contracts way too much for unmodified to last long term.
I am wondering if there is any thermal break benefits of using the Ditra HeatTB over concrete slab but without the heat cables? Just thinking of a small guestroom where I cannot put the cable heat wires but concerned with the coldness of the tile floor. Yeah throw rugs are in the plan…
p.s. I’ve used Ditra on the cement slab in the kitchen and LR with 24″ tile and it has greatly reduced the dampness I used to loath. Planning to use the heat TB in the downstairs bath with the cables of course.
thanks.
Hi Lesley,
I don’t see why that wouldn’t work just fine.
My son will be setting tile over Ditra in the bath and laundry room. Come to find out “Mr. Know-it-All” didn’t read the directions and used unmodified thin-set over plywood to secure the Ditra. Is that going to be a problem that we need to tear it all up and start over? We will be using un-modified thin set to set the tile.
Hi Carolee,
Unfortunately that’s the one mistake with ditra that will absolutely not last. You NEED modified thinset between plywood and ditra. The plywood simply expands and contracts too much for unmodified to bond long-term.
Hi, I’m installing ditra heat in a kitchen. This will take a number of weekends. What are your thoughts on installing ditra then wire and then filling ditra with ditra set then tile in sections all while still be able to use the kitchen?
Thanks, Russell
Hi Russell,
I wouldn’t do it if I’m using the space – like your kitchen. You can do it if you cover everything up while you’re using it, but I still wouldn’t recommend it.
Hi. I think I can work out a schedule. Another question is, I’m installing the ditra over 3/4″ advantech with modified thinset(hpx) but I have 2 small patches of floor leveler (about 10sqft each). Can I use the same modified thinset on these? If so should I prime again?
Thanks, Russell
Yes, use the same modified mortar over those patches. No need to reprime.
Hi Roger,
I’m curious what your thoughts are on using Ditra on a very wet (in spots) concrete slab. My concrete slab foundation has a few spots that get a lot of condensation/moisture transmission – particularly when it’s cold outside. There are no under slab leaks or other issues. I’m worried there will be a adhesion problem but others have told me it won’t be a problem as long as the slab is completely dry before installing the Ditra. What are your thoughts?
Thanks,
Aubrey
Hi Aubrey,
As soon as it is cured and bonded no amount of water is going to cause thinset to let go of the bond. You should be fine provided it is installed with a good quality mortar.
Hi Roger!
I recently put down Ditra TB heat mat on concrete and noticed some areas where I didn’t get a good bond. I cut them out and noted that the thin set can up pretty easy from the concrete. Should I be concerned with the rest even if there is good coverage elsewhere and good contact with thinset and mat? Other than the soon to be patched areas, it feels rock solid. Just nervous and don’t want to run in to issues down the road. After the patches and wire is in, the plan is to come back with unmodified thinset and 10×36 porcelain tiles and marble Thanks!
Hi Jason,
It should be fine. It’s been my experience that thinset that is not fully embedded (like under the areas you took up) will come off fairly easy. Never had any problems making patches like that.
Hi Roger,
I have a 5′ x 10′ bathroom with original (100 year old) 1 1/4″ pine T&G flooring, it is flat and sturdy. I have stripped the surface down to the wood. Is that an acceptable substrate for Ditra-Heat-TB, or should I remove the T&G and install 3/4″ plywood? I’m trying to keep the transition minimal to other floors. Installing 7×7 ceramic tiles over the Ditra/Heat cables.
Thanks for your advice, I want to do it right.
Kevin
Hi Kevin,
You can go directly over that with the TB.
Hi Roger – You gave me some sage advice about 5 years ago. I’m the poor bastard that had already nailed down lauan plywood before asking if it was suitable. Ripping that up was fun, but the tile held! Well, I think it held – we’re in a new (to us) house now and I’ve gutted a bathroom. What is the total thinckness of thinset – Ditra – thinset – 3/8 tile? I’ve scraped and sanded the plywood subfloor and it is in pretty good shape, but I’d like to put 3/8 plywood down to bridge some spots I had to repair. But the floor butts up to hardwood in the doorway, and the clearance under the door is about an inch. Thanks!
J.T.
Hi JT,
Normally 3/4″ – 1″, depending on the size of trowel you use to install the tile.
Kerdi Install Question.
Our contractor installed Kerdi for tile in our shower. After it set I was looking at it and noticed 1) a decent size air bubble and 2) in the one corner the surface is not flat. You can run your hand from
the top to the bottom and feel numerous ridges. Not like this anywhere else or the other corner. I assume these two issues need to be corrected before they start putting tile down?? I’m going to take to them when they start up work tomorrow but would love some insight to this from someone else too.
Hi Jamie,
I assume your issue is already resolved, but small air bubbles aren’t an issue, and ridges normally aren’t either unless a trowel is used which is not large enough and causes the ridges to interfere with the tile’s ‘flatness’.
We will be tiling a very large area in the basement. It is a good finished concrete floor. My daughter insists that we must use the 2 inch Ditra on the seams and around the outer walls. The installation videos I have seen didn’t use it. What is the deal?
Hi, we would like to put tile down and plan on using Ditra XL, but the 3/8 in plywood is not perfectly level. How level does it have to be and can we use the thinset to fit in the small valleys? There are 2 that are about 1 square foot and about 1/4 in low. PS..we have only installed a small tile entry way so we are newbies…
Hi April,
It doesn’t need to be level, it needs to be flat. You can compensate a little bit under the ditra with thinset, but you need to be sure not to exceed the maximum thickness of your thinset, most products are at about 1/4″. You should be fine provided they are the size you stated.
Hi,
I have 2″ octagon tiles with 1″ squares on the corners. Ditra says no to install less than 2″ however I’m curious if any has installed something similar with no negative affects. I really like using Ditra and the low clearance is something I need.
Thanks
Hi Scott,
Someone probably has done it with no negative affects, I have no idea. I haven’t. Do you want to take that chance? Try something that is approved for that installation – google ‘greenskin’.
Can you install two layers of Ditra? I thought I should go with the regular 1/8″ Ditra on a hearth area. I wanted to lay tiles on the plywood to meet the surrounding 3/4″ hardwood. I thought the Ditra XL product would be too thick because I am using a large tile, 24″X16″ that is a little over 1/4″ thick.
But after installing the 1/8″ Ditra, I see that I will have to use about 1/4″ of mortar to make the top of the tile level with the hardwood flooring. That was too much for me to work with and I just couldn’t get the tile to sit right.
I see now that I should have used the Ditra XL first as I have done in the past in other areas of the house and never had a problem lining up the tile and hardwood floor.
So now can I install another layer of Ditra over top of the first layer to take up the space and allow me to use only a small layer of mortar? If so, which type of mortar do I use, the modified or unmodified product?
Thanks for your help
Jake Cole
Ottawa, Canada
Hi Jake,
Yes you can. Unmodified mortar.
If you are installing a large 24″ x 16″ tile, you should be using a 1/2″ notched trowel for good contact and for the ability to level your tiles with each other. I can’t imagine using 1/4″ mortar under a LFT like that.
A 1/2″ trowel leaves you 1/4″ of mortar.
Roger,
I have a 1 1/4″ plywood subfloor that I am going to lay tile over in our bathroom. There were some damaged pieces which I cut out and replaced but left an 1/8″ inch gap between the pieces which I filled with silicone to prevent the thinset from filling the expansion joints.
I will use VersaBond modified thinset over which I will attach Schluter Ditra uncoupling membrane. I like to take my time since I am not a pro. We are going to install this and then complete the tile floor subsequently. I assume there’s no issue in waiting. Correct? If it takes a few days we can walk over this after it cures for the appropriate amount of time (16-24 hours), too. Right? We then plan to lay the floor tiles (12X12 porcelain tiles) using Mapei’s Uncoupling Membrane (non-modified) as our thinset and let it cure before grouting with sanded grout at 3/16″ gap. I will take care to ensure that the waffles are completely filled.
Have I described an appropriate method based on my underlayment and materials?
Thanks for your feedback.
Hi Sean,
All of your information is absolutely correct.
I have a printed cement floor in downstairs bathroom, would like to lay tile in this room, can you guide me thru the correct steps to do the job right. Muchly appreciated. Thank so much Pat
Hi Pat,
The paint needs to be removed, the easiest way is a scarifying wheel on a grinder. Then a proper membrane such as ditra, then tile.
Hi,
I have 1.25″ thick tongue and groove subflooring on my second story. It is very sturdy, but naturally has high and low spots. Would you lay plywood or OSB over the T&G (1/4″ ok since subfloor is sturdy enough?) or could you lay the ditra over the pine T&G subflooring?
Thanks
Hi Matt,
You NEED to put plywood over that. 1/4″ is too thin to be stable enough, you need at least 3/8″.