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 
Hi, I have an issue with a few tiles not being level and cracking. Can I place Ditra only under those tiles?
I just finished installing the backerboard in a 300ft2 area to be tiled. No tile has been laid yet. Can I install the Ditra heatwire without the orange mat, and then tile? If so, how?
Hi,
We are building a curb less walk in shower and planning to install Ditra heat flooring in the bathroom portion only. (Not in shower) In order to create a properly sloped shower and floor we are putting down dry pac base throughout. My question is… Do we install Ditra heat onto plywood and then dry pac over top? or Dry pac first, and Ditra heat over top of Dry pac? We will be using Mapei Ultra flex 2.
Thanks
Dino
Hi I have 1’x1′ mosaic sheets that I plan on putting on my bathroom floor the sizes are 2″x2″ 1″x2″ and 1″x1″ I saw that they only recommend tiles bigger than 2″x2″. I called Shluter and they said the only thing they recommend with using tiles that small is to skim the Ditra with thunder to fill the waffles let dry, then apply thinset and tile. Any thoughts on this?
Sorry typo. thunder = thinset
Hi Chris,
You can do that, but you’ll lose your warranty. I’ve done it without issues, but if there is an issue they’ll just tell you that 2×2 is the limit and you shouldn’t have installed over it. If you want your warranty I would call back and get that in writing.
Thank you for the reply
I am finishing a tile job for a friend who had the previous contractor leave in the middle. My problem is the ditra heat membrane was installed on an unlevel floor. What suggestions do you have to fix this problem, home owner does not want to rip up the already installed membrane. I have read that you are not recommend to use self leveler over the membrane. Any knowledge will be helpful. Thank you.
Hi Jacob,
If it is not level then you need to remove it, level the floor, then install new stuff. Your floor does not have to be level to install tile, it just has to be flat. You can use slc over the membrane, you’ll just lose the warranty. No big deal to me, but it’s up to you.
Thank you
Help! We had Ditra installed by our local flooring company. They say they use Ditra all the time. Supposedly the installer of the Ditra was experienced. My builder then installed the tile. 18” in kitchen, 12” in bathroom. Within 3 months grout cracked EVERYWHERE. Fine lines mostly along the tile edge the entire length of the room. The contractor is here today cutting out the grout, but after reading many forums, I’m worried the same thing will happen. About 6 tiles in the bathroom are loose/ popping up. I don’t think tiles in kitchen are loose but grout is cracked everywhere. Floor is 3/4” T&G Advantech with I joists 16” o.c. What should we know/ check before re- grouting?
Hi Julie,
What type (modified/unmodified) and brand of mortar did they use below the ditra and between the ditra and tile? How large are your rooms, and are there expansion joints through your tile if required? Is there space around the perimeter between the tile and base or walls? (That last one would be my guess, and it would be no).
We want to install a 12″x12″ marble tile sheet of smaller tiles in top of a ditra heat floor. Our tile guys says the marble it to small and the sheet will wave on top of the dirta floor. He says he can put down heavy thinset but then it would just goop up between the tiles and be a mess. He thinks we can’t do it and we need to find larger tiles. What are your thoughts?
Hi Abby,
Ditra heat has a minimum tile size of 2″x2″. If you want the smaller tiles you’ll need to use the wire with slc over it or another substrate, or you’ll need to find a larger tile for the ditra heat.
One additional question; Are expansion joints handled differently than cracks in there slab?
The TCNA Handbook for Ceramic, Glass, and Stone Tile Installation recommends that control joints in concrete carry through the tile, what does this mean?
Yes. The uncoupling membrane can be put over the cracks and it will compensate for any additional cracking. Control joints are just like expansion joints, they need to carry up through the tile. Grout line or a cut line in the tile directly over it filled with color-matching silicone rather than grout. An expansion joint is put in the concrete as the concrete is setting up. A control joint serves the same purpose, but it’s cut into the concrete with a saw after the concrete cures.
I am using Redguard Uncoupling Membrane. A tile installer advised I use the membrane on only one side of the expansion joint. What do you think?
Also, should I put an expansion joint in my tile over the expansion joint in the concrete?
Thanks for your input!
Hi Matthew,
You need it on both sides of the expansion joint. Yes, your expansion joint needs to carry up through the tile installation. Try to put a grout line directly over it and fill it with color-matching silicone rather than grout.
Hi. Appreciate all the wisdom you offer here. I installed a kerdi board shower and have the schluter shower pan in place. I ordered a plastic kerdi drain but was sent a metal one. We were stuck installing it due to living in a remote community where it would take 3 weeks to get a replacement. Anyways the plumber installed it before the base because we had no access. It is a little high. My question is whether we can use ditra on top of the kerdi shower pan in order the raise the base up high enough to properly meet the drain and not compromise the slope. It is a round drain. Considered dry pack mortar but there is none to be had in this community either. Will the ditra work? The tile we plan to use in the pan is mosaic on a mat. Thank you!
Hi Ros,
You can use kerdi if you put ditra over it. Or you can make deck mud (dry pack mortar) by following these instructions: How to make deck mud.
Thanks! So…the kerdi fabric down on the pan and the ditra over it followed by mortar and tile?
Hi Ros,
No, the ditra down, then the kerdi fabric, then mortar and tile.
Thanks!!
Hello,
My OSB subfloor has a thin layer of glue residue on it, left over from the old linoleum. There is also paint overspray at some of the edges. Will the modified thin-set bond to this, or should I sand it down?
Thanks
Hi Adam,
Splash some water on it. If it beads up then you’ll need to sand it, if it soaks in within 30 seconds or so then thinset will bond to it just fine.
How thick is Ditra? I am worried the added thickness will cause a problem transitioning to my laminate flooring in adjoining rooms.
Hi Monica,
Ditra is 1/4″ thick. There are a ton of transitions which can go from a higher profile flooring like the tile to a lower like the laminate. Google Schluter tile trim. DO NOT compromise your tile substrate in order to try and get flush with a lower floor.
My OSB subfloor is not level, slopes in one direction. The house is pier-and-beam. My contractor wanted to use LevelQuick after installing Ditra. I told him that LevelQuick modified mortar and will take several days, if not weeks to set. I am concerned that leveling the floor before installing Ditra will cause the leveling compund to seprate from the OSB below the Ditra. Suggestions?
Hi Siljan,
The levelquick has to go down under the ditra, directly over the osb. If the levelquick primer is used it will not seperate. And it will not take several days or weeks to set if you use modified, it may take two days, normally it’s set just fine in one.
I laid my Ditra for my bathroom. Unfortunately I have a small section that doesn’t seem to have adhered to the sub floor, and moves slightly up and down, should i cut this section out? and re-lay it with more modified thinset, or will it solidify when I lay the tile and thinnest on top? Thanks for having a great site.
Hi Nick,
You need to cut it out and install it again. It needs to be properly bonded to the substrate.
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.
Hi Luke,
Yes you can, just don’t tell Schluter I said that.
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.
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.
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
Thinset not thunder – darn self correct!!!!
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.
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
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.
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
Hi Michael,
Use modified over all of it.
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
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.
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.
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.
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.
Hi Christine,
Absolutely not! On the upside, unless it is a manufactured home (trailer house) then the substrate is normally osb.
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…
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.
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…
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
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.
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.
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.
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?
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.