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 Elf,
I have used Ditra a few times and have a question as I am going to be doing our bathroom shortly. I am using 7×24″ porcelain tiles. We have a newer house with manufactured joists at 19.5″ centers, topped with 3/4″ OSB. I am trying to meet my 3/4″ walnut hardwood(in adjacent hallway) evenly. Currently there is what looks to be a 5mm underlayment stapled over the OSB (a load of 18ga staples from the looks of it) from the previous vinyl floor. Can I scuff up the surface and use modified thin set to adhere the Ditra on the existing stapled underlayment? The level of tile would be near exact the 3/4″ floor height if this is possible.
Hi Rick,
Unfortunately no, that luan (the thin ply) needs to be removed. It is not stable enough to be under a tile installation. You can use ditra xl to achieve the height you need. To remove the luan you can pry it up and pound any staples down that don’t come up with it – you don’t need to remove all of them.
Dang! I will rerun my regular Ditra and order some XL. It looks like the OSB has some drywall/primer oversray on it… How would you prep that? Wire brush? Sander?
On a side note: On a past tile job in our foyer I received some bad advice to put the Ditra over that quarter-inch underlayment (which was also there). I have about 70 sq feet done in this manner…guess I just leave it until tiles start coming up…
Thank you much for your advice on how to do it the right way!
A regular palm sander normally takes care of the overspray fairly quickly.
Replacing a failed shower floor and have done the demo down to the plywood subfloor. I’d like to prevent another failure due to floor movement, so plan on putting another 1/2″ inch of plywood over existing plywood. Then I’d like to use ditra, but not sure about:
1) should I pre-slope the floor before putting down the ditra?
2)I’m thinking of using one of those plastic sloping kits, your opinion? I’m assuming I can’t pre-slope if I do use one.
3) how do I ditra the curb, do I use the same technique as with rubber membrane, or use kerdi for the curb?
Thanks a million.
Hi Ed,
It sounds like you are confusing ditra with kerdi. Kerdi is for use in showers to waterproof substrates – like shower floors. Ditra is an uncoupling membrane used for flat floor installations. You want to install a sloped mud deck and cover it with kerdi, not ditra. Curbs are normally build with 2×4’s and backer over them, with kerdi installed right over it. The tile is then bonded directly to the kerdi. The sloping kits work just fine.
hey elf,
i’m laying tile on a plywood substrate that previously had linoleum on it.
i’ve removed the linoleum but am having a horrible time getting rid of the glue. have scraped, used solvent, used electric sander to get down to plywood and am looking at probably another full day of work to get the area down to clean plywood.
my question:
how important is that?
can i lay the thinset over that ply substrate if it still has some residual glue on it. i am using Ditra.
or must i remove every friggin’ milimeter of old adhesive?
thanks
Hey Brian,
If you splash water on it and the ply soaks it in then you can install ditra over it. If the water doesn’t soak in right away you need to remove glue until it does.
Hi Roger,
I am tiling a small bathroom 45 sq ft. sectioned into two with a door between the double sinks section and shower/toilet. Two questions: Would you allow for an expansion joint in the threshold of this small room and would you use sill seal band. I don’t plan to tile the wall though it might look cool. Probably just wooden base board. Thanks.
Paul
Hi Paul,
With a floor that size you normally only need perimeter joints.
Is Ditra a good fit when installing 1/2″ Travertine onto WarmBoard subfloor?
Hi Mike,
Yes, ditra or slc.
Elf,
Putting either ditra or Homelux GatorGold down in bathroom, over osb, then 12″ porcelain tile.
I understand Latex Modified thinset between osb and decoupling membrane.
Ditra says unmodifed thinset on top. GatorGold says to use modified on top because it was used below. Both products are very similar. I have heard that most porcelain tiles recommend modified thinset. What would you recommend?
I am concerned about the modified thinset taking for ever to dry, but also concerned about the bond strength of unmodified thinset to porcelain tile.
Thanks,
John
Hi John,
I would likely use modified. That said, unmodified bonds to porcelain just fine if you backbutter the tile to ensure it’s embedded into the back of the tile.
How long will I have to wait for the modified thinset to cure/dry before walking on and grouting. I assume 24 hours for unmodified.
You can grout and have light traffic on it in 24 hours. 48 would be better just to be safe. Yes, 24 for unmodified.
Hi Roger,
Is HD Versabond okay between plywood and Ditra or would you recommend another product from HD or Lowes. I found the Custom Uncoupling membrane mortar for on top of the Ditra.
Thanks!
Hi Trent,
Versabond is fine, I use it a lot.
Elf:
Can I pre-fill the Ditra, let it cure over night, and lay tile the next day?
yes.
yes.
I deleted the third one.
And the answer is still yes.
Elf:
I know I’ve already posed this question, but I can’t remember in which forum I posted it. Anyway, is it OK to pre-fill all the cavities in the Ditra, wait a day, and then come back and start laying tile?
yes.
I am using ditra over plywood and going to lay 12x 24 porcelain tiles over that. However, after putting the ditra down I noticed one area by my threshold that seems to not adhered to the thinset (when i step on it, its loose up and down). How do I handle this… continue and tile or cut that section out and remove thinset and lay new ditra??? please advise
Hi April,
Just cut out the kerdi and replace it. You can do that with ditra without problems.
Ok to put ditra under a schluster shower tray? I am assuming this would allow uncoupling between tray and foundation slab?
Hi Glenn,
No, the tray gets bonded directly to your substrate.
Yes sir. I plan to use modified thinset, then Ditra, then thinset, then ceramic tile.
Thank you for the advice on the gaps and voids. I should have inspected all the sheets before I let Lowe’s load them. The top sheet looked great but the rest have up to 1.5 inch voids in the top layer where there were knots in the wood scattered across the sheets.
Thanks again for the advice and all of the great info you post on your website!
Hi Floor Elf,
I have laid a layer of Exposurd 1 11/32 plywood on top of my 3/4 OSB sub floor according to Ditra/Tile Council. But I would appreciate your advice on what to do with the 1/8 inch gaps between the plywood and the occasional voids in the tolayer of the plywood. Thanks!
Hi Trent,
Fill the 1/8″ gaps with caulk or silicone so they don’t get thinset in them. The voids can be filled with thinset as you install your membrane or backer over the ply. You ARE installing a membrane or backer over the ply, right?
Hi Elf,
I am having tile installed on Ditra over plywood and used the Kerdi-band for waterproofing the seams. The kerdi-band makes the Ditra surface unlevel. Is this just something I need to add extra thinset to account for? Is there a “proper technique” for leveling tile over Ditra? Or do you just grab a four ft level and adjust as you go?
Thanks in advance,
Hi Michael,
Unless it makes your floor more than 1/8″ or more out of flat then the thinset used to set the tile will compensate for the unevenness. I normally just use a four and two foot level as I’m setting the floor. Everyone has their own technique. This is one of mine: setting a flat floor
I installed the backerboard, warmwire heating wire and the ditra as explained in radiant floor heating parts 1 and 2. I laid down about 10 ea. 12×12 tiles and after about 12 hours of cure time, took two back up because not flat. These straddled a ditra seam which came up on one side. Pulled by hand some more and was able to pull up about 6 inches back and then got hard to do anymore. Went to an empty ditra 40 inch wide piece and was able to pull whole sheet up by hand. Thinset coverage on Ditra matched the wire path and the trowel notches I flattened out but did not seem to get into the membrane. I Used Laticrete 317 with 1/4 x 1/4 trowel (to insure got thin set notches above the wire). I plan to take the ditra and tile off and do over. I would like to keep the thinset/wire layer I have down already. What is best way to flatten out the thinset that was used to embed the wires so that I can put the Ditra on like any other substrate?
Hey Ron,
The easiest and most consistent would be to skim-coat the existing to fill the notches flush, let it cure, then install your ditra.
Hey, Elf.
We are getting ready to install the ditra this weekend and are planning to install the tile a day or two later due to our work schedule…is that okay or will installing the ditra first and the tile later make for a tiling nightmare?
Thanks!
Hi Krista,
No problem with that at all.
Hey elf. About to do a bathroom remodel and researching schluter products. I plan to use a shower pan & wall kit and also ditra on the floor.
Couple questions.
How would you transition from the curb to the floor ditra? Any concerns with filling in the ditra while mudding In a kerdi seam strip where the curb and ditra meet. I would think you would fill in your ditra in some areas while sealing the kerdi seam to the ditra…I’m concerned that I will fill in some of the ditra squares prior to me laying the tile and then those holes would be filled with solidified mortar.
And, if I wanted a floating glass shower door..how would one install this..I can’t imagine the curb would be used for a bottom mount. Should I plan for a wall stud mount only? (Before talking to glass guy..wanted you input on how this is handled usually)
Thanks
Hi Scott,
The holes in the ditra can be filled with cured thinset, it’s not a problem. I often pre-fill my ditra the day before I tile it. I normally have frameless glass installed with clips into the wall stud, nothing on the curb.
Ok…here are my questions. You’ve probably answered these a million times, so I apologize in advance for requesting that you answer them one more time.
1.) I have 16″ OC floor joists with 5/8″ osb subfloor. Do I need to install osb on top running perpendicular to the seams? If so, what’s the minimum thickness.
2.) I think I need to use an un-modified thin set (portland cement) between the Ditra and sub floor. Is this true? And, what do I use between the Ditra and natural stone tile (travertine)?
3.) Do I need to back butter the tile during installation?
4.) I am using planks and sets of stone tile. Do you recommend a tile leveling system?
Nick
Hi Nick,
1. Yes, you need another layer of plywood (not osb) running perpendicular to the first. It needs to be a 1/2″ minimum.
2. Over wood you want modified thinset between the ditra and subfloor. You want unmodified between the ditra and stone.
3. Yes. Always. Especially with natural stone.
4. It is always a better result using one, but they do not help with MAKING your floor flat, they simply fine tune a properly set floor installation. Many people think they will force the floor to be flat but all they do is hold them in position as the thinset cures. With planks they do assist greatly in pulling the planks together (flat) and holding them there. You just need to understand the limitations of those systems.
Thanks Roger.
Hi Elf,
I’m installing marble tile counter tops, as for the substrate I’m using 2 layers of OSB 23/32″ T&G. I’m wondering if it would be incorrect to use bakerboard with Ditra? Also, using Ditra/Kerdi band would it be ok to use a modified thin vs unmodified thin-set with Marble ?
Thank.
Thank you!
Hi Dustin,
Nothing at all wrong with using backerboard with ditra. Make sure you get thinset beneath the backer, though, to ensure everything is rock solid. You’ll lose your warranty from schluter if you use modified thinset. (I do it all the time – don’t tell anyone…)
Tell anyone what! Thanks for the quick reply and advice.
– Dustin
Hey elf, I am installing 12×24 porcelain tiles over osb using standard ditra and an approved thinset to go between the osb and the thibset and also an aproved thinset to go between the ditra and the tiles. My question is, how flat does the osb need to be. E.g. should I sand it to be perfectly smooth? Some of the joints are slightly raised also there is some residue from drywall mud.
Hi Court,
As long as it’s within about 1/16″ over six feet and everything is scraped off the surface it’ll be fine.
Hello Elf, We need some help
1. We are tiling our kitchen with porcelain tiles that are 6×24 inches. Is this Schluter-Ditra product appropriate for this type of tile?
2. Our sub-floor is plywood (one layer about 1/2 inch) that has a tar paper like covering on it. We can remove the paper but the tar like residue is still “staining” the wood. We can’t simply scrape this tar like substance off easily but it is thin and fairly smooth more like a stain and is not sticky. Can we simply lay your product over the tar stained wood? Should be replace all the wood? I am not sure what to do here. Also do we need a this sub floor?
3. At Lowe’s we also saw this something from the Schlulter-Ditra called Keba, 5m waterproofing tape. Do we need to use this and if yes then how do we use this?
4. In the on-line video from the Schluter-Ditra website we saw a Schiene edging product being used. We are not going to be tiling up the wall at all. We will have cabinets against the wall in some areas and wood trim in others. Do we still need to use this product?
Any advice is greatly appriciated.
THANK You !!! Nancy
Hi Nancy,
1. Yes.
2. The minimum substrate requirements beneath tile is a double layer of plywood totaling 1 1/8″ in height. You can just place another layer of 5/8″ or 3/4″ plywood over it and screw it down, then install the ditra. (Ditra is not my product, I’m just a contractor who uses it)
3. You can use keba if you want your floor to be waterproof. No real need otherwise, the seams of the ditra simply butt against one another. It won’t be waterproof but your kitchen floor isn’t a swimming pool.
4. You do not need to use any of the edging products, they are simply (expensive) optional add-ons.
Thank you for your help!!! Our other option is Hardy Back Board ( I think that is what it is called). WOuld we want to go that way instead? Your thoughts?
Oh one more thing – There was tar paper ( we think) on the oak and plywood subfloor. We can’t seem to get the black feltish type stuff off the subfloor. Do we need to remove it? It is not sticky at all. It is kind of dry feeling. Again, thank you!!
Yes, you can use hardi but it goes in place of the ditra, you still need the double layer of plywood beneath it. If you are installing the additional layer of plywood you can go right over the cutback (the black stuff).
Hey Roger, back again. Tonight I was trying to install some ditra over slc (levelquik rs) over concrete on a small 5×5 floor. Slc is 3 days old. I was trying out Customs Uncoupling Mat Mortar. When I went to key in the thinset everything seemed fine, but when I tried to notch it (3-5 min later) it seemed REALLY dry, like the slc pulled all of the water out of the thinset. Anyhow, the coverage seemed pretty poor, so I pulled it back up and cleaned everything off. Hoping to re-use the ditra. One other thing of note, when I was Mixing the thinset it seemed really wet, so I added more mortar to the bucket a few times while Mixing, before letting it slake.
So, what went wrong? Did I over mix the thinset? Is it normal for the slc to take all the moisture? Do I need to notch the thinset in an area as soon as I key it rather than keying in an area and then notching it all? The thinset in he bucket was still good to go and it even seemed a little wet to me, but the stuff on the floor came up almost like deck mud.
Any help is appreciated before I eff’ it up again
!
B
Hey Brooks,
It may be the mixing – adding more mortar to the bucket. SLC will pull some of the water out of it, but not enough to affect it that much. Add what you had initially, the stuff that seemed wet, and use it like that.
Elf,
I am planning on installing 18″ travertine in my second floor bathroom. I was reading the ditra handbook and had some questions. You are required to put two layers of osb or plywood? I’m thinking I need to add an 11\32″ of what they call underlayment(plywood or osb) in the PDF? Over this what trowel do I use for the modified thinset under the ditra? When installing the travertine I use unmodified thinset? I use the flat side of the trowel to mash the thinset into the dirta and then back butter the travertine with what size trowel? Would this be how you would go about the install? I am worried about the height of the travertine when finished as the doorway leads to carpet. Any ideas on that front? Thank you in advance for your help!
Travis
Hi Travis,
You have the basic order correct, but once you use the flat side of the trowel to fill the ditra cavities you can use the other side to comb thinset onto it as you would a flat substrate. Yes, you do need an additional layer of ply or osb. The total minimum height of the two layers should be 1 1/8″. I usually use a 1/4″ x 1/4″ trowel to install ditra, that works if your substrate is flat. If it is not flat use a larger trowel. The correct size is the one that creates full coverage, with both your ditra and travertine. This explains it better: Using the proper trowel
The height of your threshold is something that needs to be dealt with in nearly every installation. With carpet you can actually ramp the pad and carpet up a bit, set your tack strip higher and tuck it to the tile edge or, better yet, an edge trim like schluter. Never compromise anything on a tile installation due to the threshold height, they can be dealt with, tile failure is much more difficult to deal with.
Hi Elf – I’m doing a tile install (5″ x 24″) in a large kitchen/dining room. It had linoleum (or vinyl) flooring, with two layers of 1/2″ plywood as a subfloor, on joists 16″ o.c. Over a weekend, thinking he was helping things along, the homeowner peeled up the decorative layer of the linoleum, leaving the gray paper and glue. Can I lay Ditra on top of this, or do I now need to sand it down to the plywood? Hope I’m explaining the situation clearly enough. Thanks for the help.
Hi Bill,
The paper needs to be removed or another layer of plywood put over the top of it. The problem is not that thinset won’t bond to it, the problem is that glue used for linoleum is water soluble. If water gets beneath the thinset, which it will at some point, the glue may release and everything above it is useless. It may be bonded together, but it’ll be floating.
Hi Elf,
I know you are busy this time of year and it may be impossible to answer this before I am in way too deep BUT my question has to do with the thickness of the thinset over the Ditra. I’m laying 16 x 16 Saltillo tiles that are 3/4 inch thick and often cupped. I’ve been very careful to fill the grid cups, but am I supposed to lay out a half inch or more thinset on top of that? I have the 1/2 inch trowel and I have backbuttered because of the cupping, but lawsy…that’s a lotta thinset. I think I cheated today…first day of actually laying them in. I may have as little as 1/4 in on top of the grid and a little less on the back of each tile. Am I in trouble? Thank you!
Hi Beth,
You can do however much or little you want provided you get full contact and you have a minimum of 3/32″ layer of thinset between the tile and substrate. I would use the 1/2″ trowel but skim the backs of the tile with the flat side to get them flat. That should leave you about 1/4″ of thinset between the tile and ditra, which is perfectly fine.
thank you so much, Elf. I have done it now and have my fingers crossed. You are a great help to many of us. Happy New Year!