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 
Oh, and I am a ditz! I used 1/2-inch plywood over the subfloor, not 3/8. And yes, my 56-year-old back is feeling lovely after all this work, as is my hubby’s back. There, I’m done.
Hi Roger,
After consulting Jay from Schluter, he basically agreed with your advice. We ripped off all the flooring down to the 3/4-inch subfloor – I thought it was 5/8 until I could see it. We laid 3/8-inch plywood in the pattern described in Schluter’s manual and then followed their instructions with the aid of your advice, i.e., I made sure the waffles were filled, and it did require many swipes with the flat side of the trowel to accomplish this important aspect of the installation. Basically I became the ‘Barney Fife’ of this project by making sure everything was ‘by-the-book’. I hope this works! Thank you for your help! This blog is invaluable for the do-it-yourselfers out there, like me.
Do I have to use a modified thinset when setting ditra on plywood, or is a unmodified thinset ok?
Hi Matt,
It needs to be modified thinset.
Hi Roger,
Thanks for taking your time and answering so many questions. I have 3/4″ planks going diagonally over joists 16 on center. I am attempting to match the height of hardwood flooring and was wondering if I could put 3/8″ plywood over the planks and then Ditra and than the 1/4″ tile? I think that should make the transition close? This is my first time tiling so maybe I am completely off or am missing something. I have also read about Ardex x5 that a few places have used. What is your opinion on using Ardex?
Thanks for your help.
Hey Andy,
You NEED at least 1/2″ of ply over planks, that is the bare minimum, 3/4″ is better. Never used ardex, it’s not available around here. Guys I’ve talked to who use it absolutely love it.
Preparing to install 12×12 marble floor in a bathroom addition. Planning on laying Ditra, using Custom (Ditra Specific) un modified Uncoupling Mat Mortar over 1/2″ hardie backer, 1/4 x 3/8s trowel. Anything wrong with that? or any potential problems with that set up. Also acrylic shower base and tub. I”m handy but schluter unexperienced.
Thank you for this site, and any input. really been a huge help
Hi Matthew,
No problem at all, you’re right on track.
Roger,
Thanks for all of your posts. Can I ask a few quick questions of you on installation in a beach house? I will be putting new 5/8 in board down ove some older 3/4 plywood. First question is OSB or Plywood? I planned to put down Hardie Backer on top of the 5/8 for extra support. Should I screw it to the plywood or put a thin set in between the 5/8 and the Hardie? Next I planned to use Dietra. What type of thinset should I use between the Hardie and the Dietra. Finally, what thinset would you use between the Dietra and the tile.
By the way, the tile is 3/8 ” thick 6″ x 3 feet tile that looks like planks of wood. I am particularly wanting to get your feedback as this is a first floor (of 3) condo at the beach and I would think there is some shifting, movement/ expansion over time. You can send me an email if you like at the address above.
Thanks so much for your response. I really appreciate it!!
Hi Ed,
1. Plywood.
2. Both. The hardi needs to be thinsetted down, then screwed. The thinset is not to bond it, it’s there to fill voids.
3. Unmodified between hardi and ditra.
4. Unmodified between ditra and tile.
Thanks so much Roger for your response. I can’t tell you how much I appreciate it! One last question – I promise!
I want to be super conservative towards cracking tile (as much as possible) given that the tile is 6″x3ft wood looking tile planks, and they are in a stilted beach house which assemble deals with some degree of shifting.
If given a choice between the two seneraios below, which would you choose for the use described:
Scenario 1 (the scenario mentioned above)
Existing 3/4 plywood
Plus new 5/8 plywood on top
Plus unmodified thinset
Plus 1/4 Hardie
Plus unmodified thinset
Plus Dietra
Plus unmodified thinset
Plus 6″x3ft tile
Or
Scenario 2 (no Hardin and thicker plywood)
Existing 3/4 plywood
Plus Vapor Barrier
Plus new 3/4 plywood on top
Plus unmodified thinset
Plus Dietra
Plus unmodified thinset
Plus 6″x3ft tile
Thanks you so much for your responses! I really appreciate it!
Ed
Niether.
Go existing 3/4″ plywood, additional layer of 5/8″ ply, modified thinset, ditra, unmodified thinset, then tile. If you want even more protection from cracking use an additional 3/4″ ply rather than 5/8″.
Hi Roger,
I just removed all the tiles on my floor. It was set with mesh and a million of tacks. The floor is made out of 5/8 OSB. Do I have to remove all the tacks before I set my Ditra. Thank you in advance for your answer.
Hi Carl,
What do you mean by ‘tacks’? If they are narrow headed staples or small nails then you don’t need to remove them first, you can simply pound them down flush into the existing substrate and go over it with ditra. If it’s something else, I don’t know, you’ll have to let me know what you have.
Roger,
re: Can I, place this Schluter Ditra System on top of the “Hardie Backer Board” with same Flexbond??
thanks for your time. my floor is new. mistake ?? the story will be untold.
it was 1/2″ osb with 1/2″ plywood” with linoleum ,,Now the linoleum and 1/2″ ply wood removed, Original OSB sanded smooth reset nails & new screws. Then new 1/2″ underlay plywood screwed to old floor joists and osb.
The plan was … for new tile with with heated floor,
I have New 3/4″ Maple hardwood floor to transition to.
So, I proceeded with 1/4″ inch Hardie Backer board set to new 1″ underlay with “Flexbond” Crack Prevention Mortar & screwed to floor, my original was to put floor heat (1/4″)and more Flexbond to cover, then tile to complete.
Now upon learning of Schluter Ditra and the floor heat system and application, (this adding 1/4″ approx) Can I, place this Schluter Ditra System on top of the “Hardie Backer Board” with same Flexbond.. Changing mortar/thinset is not a problem. I will do as you suggest. This is in my kitchen, so not a generally wet system (- of course a dishwasher leak ) is there also a concern with mold regarding hardie backer board & Schluter Ditra if I can add the SD heat system??
Hey Rocky,
Yes, you can add the schluter heat system over that. You won’t have any problems at all. You can use the same mortar also.
We are remodeling the bathroom and don’t want to mess with the height of the toilet flange if possible. We only have 1″ total between the bare joists (16″ OC) and the bottom of the toilet flange. Is it okay to just use 5/8″ Plywood, then Ditra, then our 12×24 3/8″ thick tile (oops, that is still going to be greater than 1 inch thick I guess)? In any case, is there a reason whey we need 2 layers of Plywood? Also, any comments on using Laticrete’s Strata mat instead of the Ditra? I was originally going to use the Stratamat, but everyone keeps talking about the Ditra. Thanks for all of your help!!!
Hi Frigid,
You need two layers of plywood to support your tile. No, it is not okay to use only one layer of 5/8″. You don’t need to move the flange at all, just cut around it and use a thicker wax ring. It can sit below the tile and wood. I prefer strata-mat, kerdi is just more readily available since you can buy it at home depot.
Hi Roger,
Thanks for your time in posting all this stuff, it’s very informative!
Can I use Ultraflex LFT by MAPEI to stick Ditra to the plywood, or should I use another product?
Here is why I ask:
We’ll soon be starting a tiling project in our bathroom. When we bought the tiles (12″ x 24″), the tile store also sold us bags of Ultraflex LFT. We thought we’d be using a substrate of Hardiebacker cement board, but changed our mind to use Ditra instead (1/8″ thick instead of 1/4″). (I’m not even sure they asked us what would be the tile substrate…)
Since our sub-floor is plywood, Schluter recommends using a modified thin-set to stick the Ditra to it, and to use unmodified thin-set to stick the tiles on the Ditra. The Ultraflex LFT will not be suitable to put the tiles on the Ditra so we bought some PremiumPlus Standard Thin-Set Mortar by Custom Building Products (should I use something else? it’s not “MasterBlend”, what you seem to dislike in your unmodified-thinsets-a-users-guide post… (Masterblend does not seem to exist anymore, when searching their website)).
Since I’m not 100% the tile store will give us a refund for the MAPEI product, I’d rather use it if it’s good instead buying something else.
Thanks a lot!
Hi Alex,
The lft will work GREAT! Serious overkill, but what the hell.
PremiumPlus? You Canadians and your funny products. (It’s only available in Canada) It should work just fine, although I’ve never personally used it. Guys who have seem to like it. Maybe my post caused them to drop the masterblend? Yeah, probably not… 
Hi Roger,
Great, thanks a lot!
Hi Roger….. Can I use Ditra over a floor that has more than just the subfloor layer…..We tore out the granite-set-in-mud floor in our kitchen and noticed that there are 3 layers of plywood beneath it. The subfloor is 5/8-inch plywood, then comes the 1/4-inch floor with vinyl on top of it, and finally there is a 3/16-inch plywood layer on top. I am concerned that these layers may shift, even though there is already screws through all layers to stabilize it. If this is the case….should we rip out the top 2 plywood layers and lay the Ditra directly over the 5/8-inch subfloor (the subfloor is over a truss system – not sure if that matters). I am looking to start the floor tomorrow, so a timely response would be appreciated. Thank you!
Hi Mary,
Sorry for the delay, been super busy with the day job and all.
I’m sure you’ve already begun, but the ideal solution is to remove it down to the 5/8, install an additional 1/2″ layer of ply and go over that. You can go over the existing with the ditra, but I really wouldn’t. Your tile installation will depend on every layer beneath it. When it was set on mud the tile didn’t depend on that as much, with ditra you still need a proper substrate.
Hi Roger,
Do you have to mesh tape the backer board seams prior to Ditra?
Hi Tom,
Nope, ditra takes care of that with the mesh.
I have removed linoleum from the plywood but in doing so created numerous small
divots in the plywood sub-floor can the Ditra go over these or does the divots need to be filled?
Hi Viginio,
They can be filled as you install the ditra. Provided it is actually the plywood subfloor and not the thin luan, or 1/8 or 1/4″ ply used as the substrate for the lino.
Hi Roger,
I’ve been reading your site for some time now, as I attempt to research information regarding my first time tile install. I get the feeling you really know what you’re talking about, so I feel confident in soliciting your advice.
So, here’s the situation….. I want to tile my roughly 50sq. foot upstairs bathroom. (My townhome was built in 1972). It is covered in non-cushioned sheet vinyl, at least 22 years old, completely (and firmly), adhered to 1/4″ plywood. I tried to pull up the vinyl, so I am certain it’s completely adhered to plywood. It isn’t going anywhere. Under that is another layer of un-cushioned sheet vinyl (original, with asbestos I’m sure), adhered to 1/2″ of plywood. Under that is more plywood, exactly how thick I am not certain. I can visualize this from lifting up the floor register.
I have cut away any curling vinyl from the perimeter of the room (very minimal) and leveled with modified thinset. There is a 1/8″ to 1/4″ expansion gap on all walls as well as next to the bathtub.
The tile I have purchased is 12″x24″, 1/4″ thick ceramic.
Now, here is where I need your expertise…. I have limitations due to a “bad” back and I want to use Ditra. From all the endless hours on YouTube and DIY forums, AND calling the Ditra company itself, I THINK I’m OK to use this product, but I really want your opinion as well.
I also need to know what size trowel to use for laying the tile ….1/2″ x 1/2″ square notch? Would it be better to go with Ditra XL? I sure hope I can use this product….my back wouldn’t be able to lay backerboard. I’d hire some guys to do this…but my budget won’t allow it. Besides, I know I can do this if it isn’t too hard physically.
Thanks so much,
Deb
Hi Debra,
Yes, you absolutely can use ditra. The only issue would be the adhesion of the thinset used to install the ditra to the top layer of vinyl you’re going over. It will work, provided you remove the very top layer of vinyl (meaning the shiny part). HOWEVER (!) be very aware that vinyl adhesive is water soluble! IF water gets beneath your tile it may emulsify that glue and you will have nothing bonded to the actual floor. I also do not like the 1/4″ plywood beneath a tile installation – it tends to compress over time in traffic areas.
Now that I’ve scared the shit out of you
your floor SHOULD be fine removing the shiny part of the vinyl and installing your ditra. Ideally I would remove the top layer of vinyl and the 1/4 luan (ply beneath it) and install 1/2″ ply over the old vinyl, then ditra and tile. What you want to do should work, but I would not warranty an installation like that. I did ONCE – and I paid for it.
Hey there, I am currently at a jobsite which the company I work for did some tile worker. The grout is cracking in I would say higher traffic areas. Not being here for the installation. This is what my coworker said was done in here. The original subfloor was 3/4″ ship lathe with a layer of half inch plywood on top. They removed the layer of plywood and screwed down another layer of 3/4″ plywood (unsure if it was PL’d to the ship lathe..) then they proceeded to lay the Ditra down with modified thinset. Overtop of the ditra they used a 1/4″x3/8″x1/4″ notch trowel unmodified thinset (sceptical on that claim) and set 12″x24″ 3/8th thick porcelain floor tile in with what I believe is called QEP floor levelling clips. Now the grout is cracking in certain areas. Not entire floors. So I removed the grout and was succesful in removing 4 tiles so far and they came up somewhat easily. Back sides are clean as a whistle and you can physically see the backbutter is still on top of the notches with tiny hairline cracks running through the mortarbed. I asked why they never used a 1/2″ trowel and I guess it was to maintain the height of the floor throughout to match up with the new hardwood going in. I checked the waffles. They were packed properlyand I cut and removed a small square of ditra which also appears to properly have been set/mortar bonded to underside fleece. In your infinite wisdom would you have any thoughts on this. Like I said. Im thinking its either movement in floor joists/subfloor or improper mortar mix/trowel size… want to address this properly. And no damn well my superiors would say to just reset and regrout…they dont have to look the clients in the face. The next time we come back to fix this shit…
Hi Ryan,
Trowel was not big enough. The lash system is the worst of all of them in my opinion. The feet that go under the tile are almost 1/4″ thick by themselves, you absolutely need 1/2″ trowel with it.
Sorry in the below comment ↓↓↓ I forgot to mention the grout spacing was 1/8th and the tiles are on a 1/3 stagger. I believe floor joists are 16 o/c 2×10’s
Just uploaded a photo to show you what I meant be the back butter was still on the ridges and tiles were clean
Looking at the photo confirms the trowel size not being proper, as well as the thinset NOT being burned into the back of the tile. They may have put a layer of thinset on the tile back, but they damn sure didn’t burn it in or backbutter properly. It looks as if no force was applied at all to the tile when backbuttering it, that is your main reason. The trowel notches apparently contacted the back of the tile just fine, although they don’t seem to be collapsed properly.
Hello!
It seems that luxury vinyl planks are the way for me to cover an ugly ceramic tile kitchen floor affordably. We replaced some rotten subflooring ( under the washer’s former position a while back and filled in the huge ( 2+ sq ft ) expanse of missing tile with concrete ( or a concrete-like substance, the name of which I can’t recall ) so as to reconfigure the workspace with stock cabinets and counters. It’s functional and sound but unsightly.
I’m about to rent the house out and would like to spiff it up a bit by putting the same flooring in the kitchen then step down about 3/4″ off the tile and continue onto the living room’s oak flooring ( I’m not interested in refinishing beautiful hardwood floors for indifferent renters. ) with the same planks, then down the hall in the high traffic areas. I’ll treat the renters to carpeted bedrooms.
I gather that I should level the holes, cracks, missing tiles, etc. with thinset then proceed to install the vinyl planks?
You are a gem! I’ve enjoyed reading your blog, which I just stumbled upon!
Sandy
Hi Sandy,
Concrete floor patch would be a better option than thinset. Thinset can cure with a sandy finish and the patch does not. The LVT being a floating application would cause it to grind on the gritty thinset surface, the patch would be more durable in that application.
Roger,
I have to remove a laudry room floor (linoleum) due to a water leak. The plywood subfloor is rotten in spots. I plan to use Ditra, but I want to know if I can use 1/2 hardibacker for the subfloor instead of plywood? I don’t want another leak to equal a new floor. The new floor will be tile.
Thanks
Karen
Hi Karen,
You need to use plywood. Hardi adds zero structural stability, plywood does. That’s why it’s there. If you use ditra you can make the floor waterproof by using kerdi-band on the seams.
I have a 10 year old home that i pulled some pergo laminate up. There is sheet vinlyn underneath. Its not linoleum. What thinset would bond the ditra? The instructions say use modified thinset. Or should I just put down bakcerboard? First time installing tile, using porcelean.
Hi John,
If you want to go over the vinyl you need to put something over it before your ditra, 1/2″ plywood is fine. Unless you have luan (1/8″ or 1/4″ plywood substrate) beneath your vinyl, in that case it needs to be removed. In that case backer would be the easier choice.
I’m installing travertine over ditra.after filling the squares completely What is the appropriate towel size?
Proper trowel size
Hi Roger,
Great site, I learned a lot about tiling and still learning more about it. I’m a new homeowner and starting to do the kitchen floor tiles. I leveled the floor by using a 19/32 plywood on top of the slanted planks. I layed down the Ditra-Heat on top of the plywood using Custom’s Flexbond Crack Prevention Mortar. After reading some of your articles I’m afraid that I might have used the wrong mortar to bond the ditra-heat to the plywood. Please let me know if the Flexbond is good enough that I won’t have to redo anything.
Thanks.
Hi Ferd (?),
The flexbond is just fine for that. You don’t have to redo anything.
Roger,
Your site is amazing! I recently bought a house that I am in the process of renovating. I have become quite the DIYer, but I am a complete newbie to tiling. I am about to do my kitchen floors in 12×24 tiles and have a question about the best installation method. The kitchen has vinyl floors (4 layers of various vintage shades to be exact), and I’m sure at least one of those layers is asbestos. I know the best course of action would be to tear everything up, but that is just not an option for a host of reasons, including prohibitive cost. The floors are not exactly level, but are flat. My question is how best to go about prepping the floor for tiling. My initial thought was to do 1/2 cement board over the vinyl followed by Ditra. Is that overkill? Or would other methods be better? Any advice would be most appreciated!
Thank you for your great advice on this site!
Kait
Hi Kait,
I think you’ll be just fine using 1/2 plywood over all the vinyl, screw it down REALLY well, then use ditra over the top.
I just installed ditra for the first time and I am concerned I may have made an error. I checked to ensure the ditra was fully covered by peeling back a corner as suggested. However, my thinset might have been too thick. Once it is cured, is there a way of checking proper bonding before I tile? There are also a couple of small humps. I don’t think they are tall enough to cause lippage though.
If I have mixed the thinset too thick, what do I do now?
Hi David,
If you had full coverage when you peeled it back then it wasn’t too thick. If it is too thick it will not bond to the fleece and there will be no thinset on it when you peel it back. Those little lumps are easily taken care of with the thickness of the thinset beneath the tile as you set it.
Sr: I want to put 13”x13” ceramic tiles on my old concrete floor in the basement. I have blast shot the entire floor and patched the cracks and voids. However, the floor is uneven and some of the “valleys” are 3/8” deep. The entire floor is 600 sq.ft.
Should I put a self-leveling compound on the entire floor or just screed the self-leveling or other compound ONLY where is needed?
Kind regards
Doroteo
Hi Doroteo,
You can do either. Both will work just fine.
Trying to find a definitive answer on whether we need to use Kerdi band with our ditra on our kitchen floor
Hi Lori,
Only if you want your floor to be completely waterproof. It isn’t required.
Thank you. I planned to put down 1/4 hardibacker 500 on top of the 3/4 osb subfloor not plywood. unless there was a good reason you mentioned 3/8 plywood instead of OSB…
Plywood within 3 galaxies of water gives me the heebie jeebies.
That’s what I used in the other bathroom (non ditra install) and it has held up great for14 years. (you mentioned plywood).