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 
Heya! First thanks for the great write up! Its giving me confidence to tackle this job. I have 3/4 plywood floors and I just tore up the linoleum so I can lay the tile over the Ditra. My question is whats the best thinset to use if I still can see traces of the lino glue and very thin parts of the lino backing on the plywood? Thanks! Chris
Hi Chris,
Any good modified thinset will be fine. I prefer Laticrete 253 or 254 or Mapei ultraflex 2.
Thanks for the quick reply Roger! So double checking, no need to worry about the traces of lino and glue?
They are both considered ‘bond breakers’, which means that thinset will not adhere to them fully and it could cause problems being a ‘floating’ area. Depending on how large those areas are it may cause problems. The best thing to do would be to get it all off of there. The second best thing would be to go over those areas with a primer like mapei primer T or even an elastomeric membrane like redgard. Those will bond to the existing surfaces and give a properly prepped surface to bond thinset.
Hi Roger, in following Chris’s question, I’m curious to know what issues may arise if there are a few “floating” sections?
I’m asking bc when I was laying my tile on top of the ditra (the ditra being laid the previous day) there were 2 tiles that I had to pry up after having just laid them bc I needed more thinset under them to avoid lippage. When prying them up, the ditra came up along with the tile (I guess d/t the suction the tile had adhered to the ditra with). The thinset underneath was adhered to the floor, it was just the actual ditra that was suctioned to the tile. Just wondering what issues may arise on down the road? Thank you!
Hi Cassie,
If the thinset was still wet beneath the ditra (I assume it was) then reinstalling the tile likely rebonded it and you probably won’t have any issues. If it did not rebond then the tile may eventually begin moving when walked upon (it does not have full support beneath it) which will lead to grout cracking and powdering out of the joint. If you see that – that would be why.
I’m having an issue in that it’s been over 12 hours since I laid my tile and they’re still not bonding to the ditra. I can still slide them around. This is my first time using ditra. I’ve always tiles on hardiboard in the past. Any advice as to what’s going wrong with this job? Thanks.
Hi Paul,
I would need a lot more information to even hazard a guess as to the problem. What particular mortar did you use to set the tile over the ditra? This comment is two days old so it may be a moot point now. If you used a modified thinset over the ditra it will require extended curing time – and yes, it will still move after only twelve hours.
Stop pushing your tile.
Can Ditra membrane be used to build up a floor.? Meaning, can it be stacked to gain another 1/4” in height? Or should all levelling and build up be done with K-15 by Arden prior to Ditra being laid for the uncoupling membrane and heat application?
Thank you in advance!
Hi Rich,
You can do it either way, but yes, ditra can be stacked.
thanks for the post you wrote about the possible fails of installing ditra.
is it a good idea to pre fill the ditra with schluter modified thinset then come back the next day and lay my porcelain tiles.
Thanks Cliff
Why would you want to? Just do to all at once. It’s do easy.
Hi Cliff (and acstorm…),
You can do it either way. The reason to do it is it actually goes MUCH faster the day you set the tile if it is prefilled. Seriously, about twice as fast. It doesn’t seem like it would, but not having to flat-trowel your substrate in four directions (to get the waffles filled), then having to spread the thinset properly for the tile cuts the time down to about half.
I am tiling a kitchen, hall, bathroom and laundry room. Subfloor looks like real plywood, not osb. the previous tile was installed directly onto the plywood (no backer board) and eventually tiled over (tile directly on tile directly on plywood). I don’t see any failures and the mortar stuck to both the tiles and the plywood. I am told that I should not use the Ditra on a floor that big. Do you think I will have any issues laying a floor like this?
Thank you for your help.
Hi Andrew,
I don’t understand your question. I do not think you’ll have any issues using ditra on a large floor – I’ve used it in a 9000 square foot showroom without issues. I have no idea who told you that but it’s absolutely incorrect. Is that what you mean? The tile on tile on plywood CAN last, but it’s not common. Sounds like you had someone who both did it correctly and got a little lucky.
Hello,
I have trusses between my 1st and 2nd floors. I’m looking to recess the floor in a second-floor bathroom. I know how to build a box and recess for joists, but how do I proceed with a truss? They are 24 OC.
Hi Glyn,
I’ve never done it with trusses. The best I can think of is cross-members on the bottom of the trusses (between the centers supported by the lower), then build the boxes in between to raise it level with the cross-members supporting them.
I am getting ready to lay 18” square ceramic tile in my bathroom. My plan is to use ditra. My current bathroom floor consists of: original 3/4” plywood subfloor (floor joists 16” oc) with a layer of linoleum and then 1/4” plywood floor covering which is stapled down with the longest staples I’ve ever seen. I pulled up the linoleum that had been put on top of that. Can I go ahead and install Ditra on this 1/4” plywood with modified thinset for large tiles? (Then I’d use unmodified uncoupling membrane for large tiles between the Ditra and ceramic tile. Is this next step correct?)
Hi Cassie,
Yes you can.
Hi!
After installing the decoupling mat the previous day, we returned to find large air bubbles in some of the sections of mat. We’re not sure what happened. We installed RedGard over 1/8” hardboard (which was over two subfloors and heavily screwed down) and used modified thinset between the hardboard and the RedGard. Any ideas on what happened and how to fix?
Thank you!
Hi Tom,
I actually have no idea what you mean by ‘used modified thinset between the hardboard and the redgard’??? If you mean you used it between the mat and the redgard (which it sounds like is installed over the hardiboard – for what reason I’m unsure) then the issue is either the thinset was not thin (runny) enough to embed properly in the mesh, it was allowed to flash over (dry skim on top) before you were able to install the mat over it, or you did not embed it properly into the thinset to allow it to bond through the mesh on the bottom of the mat. I’m also assuming ditra. Can you clarify a few things for me?
Can I install Ditra on a painted concrete floor, or do I need to remove the paint?
Hi Seth,
Unfortunately you need to remove the paint first. The thinset cannot properly bond through the paint.
Thanks for the quick response.
I kinda figured that would be the case. Oh well.
When installing DITRA how can you best ensure proper contact over concrete? Also if one seam between two DITRA mats has a gap of like 1″ is that ok if going over seams with Kerdi Band anyways?
Hi Al,
After you embed it pull up a corner and take a look at the back of the ditra. You should have complete saturation of the webbing on the back with no bare spots. Yes, a gap is fine, although 1″ is HUGE. I would only bond the kerdi-band to the ditra, NOT filling the gap between with thinset as that will bond the kerdi-band to the floor, and subsequently the tile, defeating the purpose of the ditra in that particular area.
Thanks for the reply.
Maybe I clean out the gap then and put in a 1 cell run down the middle?
Yes, you can absolutely do that.
Hi Roger,
I recently installed ditra uncoupling mat for the first time and in the process of installing I must have missed about a 18 in long section right at the seam about 3 inches in on the fabric side and the only way I found it was after I filled to honeycombs and was laying the tile I had to pull up a tile and the Matt pulled away from the floor I removed a few tile and tried to get some thinset under the mat but now I’m worried I should have stopped and removed the tile and pulled up the whole sheet of ditra any thoughts?
Hi Ed,
You do need to remove any tile that is over that area and you can cut out the ditra that is loose. You don’t have to pull up the entire sheet, you can just cut a piece out that covers all the area not bonded, put thinset beneath it and put it back in there. Ditra can be pieced in, it doesn’t need to be the full sheet, it won’t affect anything negatively by piecing it in like that.
I have a plywood shower enclosure. l will use Ditra on the bathroom floor and a Kerdi shower floor. Do I need to cover the plywood walls with Kerdi board or can I use Ditra on the walls also with Kerdi strips to waterproof?
Thank you,
Malcolm
Hi Malcolm,
You need to cover the plywood with something. Plywood has way too much expansion and contraction to have tile installed directly over it (putting kerdi on it is just like tiling directly over it). You can cover it with kerdi and it will waterproof it, but you’ll likely have movement issues on down the road leading to an installation failure, beginning with cracking grout. Ideally removing the plywood and replacing it with kerdi-board would be best, but even putting drywall over the plywood then covering it with kerdi would be better than just the plywood.
Hi.This is my first time installing porcelain tile in a bath. My plan was to remove the linoleum so I didn’t have to nail it every 4 inches before using the thin set and applying the Ditra. But after removing the vanity, I looked under the linoleum and it appears to have a lot of adhesive under it and on the wood subfloor. What do you reccomend? removing the linoleum and adhesive or preparing the linoleum for the Ditra over it?
Hi Mike,
I always recommend removing it. I also know that’s a huge pain in the ass. The easiest way is to put a layer of thinset and 1/4″ backerboard over it, that eliminates the need to nail every four inches since you’ll be screwing through the lino and substrate into the base substrate. I NEVER recommend going directly to the linoleum for two reasons: You don’t know what is beneath all that and linoleum adhesive is water soluble – once water gets beneath your floor it releases. I learned that last one the hard, expensive way.
Thank you Roger, I think I am going to use the Snap stone floating porcelain tile system. What is your opinion about that tile?
Hi Mike,
I have no personal experience with it but the guys I’ve spoken to who have installed it seem to like it. I haven’t heard of any issues with it.
Thanks Roger.
Hello! Tile contractor waited two weeks after laying ditra to tile. At the time, there was heavy foot traffic and painting happening. Heavy drywall dust as new home build. Thoughts? Shouldn’t the ditra have been laid just prior to tile install?
Hi Tricia,
Yes, ideally it should be installed right before the tile installation. However, if the ditra was filled with thinset (the little waffle holes) right after he installed it then it should be just fine. If he just installed the ditra and left it exposed (bare) then it really should be replaced as the foot traffic will crush the waffles and may lead to an improper bond, which will compromise your installation.
NOPE….it was absolutely filthy, there is lippage EVERYWHERE, grout comes out with floor washing. Contractor told me today that “if the tile was not going to adhere, it would have been happening by now.” Not sure he gets it…
Installer has layed mortar over too big an area over ditra and trowled it off to return next day and restart with new mortar to lay tiles
Is this OK.
Hi Steve,
Yes, that’s fine provided he removed what he installed.
Hi there, I realize this is an old discussion but your advice seems to be rock solid and we’ll respected. So I hope you don’t mind another question.
I’m looking to install a few15X25 porcelain tiles at my front door. I’ll be removing some hardwood flooring from that area. Here is what I’ve come up with as a plan, but I’d appreciate a second opinion because I don’t want a DIY disaster.
My subfloor is 3/4 inch plywood.
Here is what I am thinking happens next:
-Ultraflex 2 spread with 1/4 inch square notched trowel.
-1/2 in Durock board embedded in that, then screwed.
-Ultraflex 2 applied again, then orange Ditra decoupling membrane.
-Then more Ultraflex 2 applied with a 1/2 inch square notched trowel.
-Then the Porcelain tiles.
Am I way off base?
I found the shelf of mortars at the hardware store confusing. Should I be buying a different type of mortar for the tiles? I didn’t even see anything called unmodified.
Thank you.
Hi Sherri,
Your plan sounds just fine. For an installation that size you won’t have any issues with the ultraflex 2.
That’s a relief. It’s done and seems to be holding up fine.
My teenager was jumping on it just to be a pain in the ass, but it stood up to his pounding.
I’ve since learned that I probably didn’t need both the Ditra and the Durock, but it’s a small area so it was no big extra expense.
Thanks for answering, I appreciate it.
Can you use ready to use Mapei premium mortar to set tile over ditra If so what happens
Hi Anthony,
NO! Absolutely not. It is essentially mastic with sand in it. What happens is that your tile installation will fail. The reason it will do that is because the ready to use “mortar” will never fully cure.
Hi Roger,
I don’t know how old this post is, but have a question – redoing one of our baths, pulled up sheet vinyl to find 1/2″ particleboard over 3/4″ plywood. Planning to install Ditra then 10mm porcelain tile. We removed the particleboard because all info I read was that shouldn’t come anywhere near Ditra. Wondering if we need to add any additional underlayment. We didn’t think so but one friend thought we should put down a layer of Hardie backer. Just thinking with that, Ditra, thinset, then tile would make the floor SO much higher than the floor outside the bath and that it’s probably not needed, but wanted to get another opinion.
Hi Marby,
You can go directly to that plywood with the ditra.
Hi!
We live in a modular home that sits on a crawl space. At the present time, we have vinyl flooring in our baths and laminate flooring in our laundry room. A DIY er told us that because we are on a crawlspace there will be to much give in the foor for ceramic tile. Can this ditra system be used in this type of home?
Do most professionals know how to use this?
Hi Patricia,
Yes it can be used, but it will not compensate for the lack of deflection ratio in your substrate. Your floor will need added bracing to meet the minimum deflection ratio (‘stiffness’) before you can set tile, ditra will not make up for that.
Hi Roger,
I’ve run into an issue with my Tiler’s installation of Ditra 25. It is being laid externally on a concrete slab with Keraquick (grey) underneath and the same on top with the latex additive.
I’ve specifically asked for this as we will be laying large format (1.2m x 1.2m x 6.5mm) porcelain tiles. They are very high spec (and expensive) Italian tiles and we have laid these internally with the same process but different tilers 2 years ago and so far all is great!
The issue lies in the laying of the Ditra into the concrete. Yesterday the Tiler’s mixed up the adhesive very soupy and in a rush poured it into the floor, spread it evenly and then rolled out the mat. They won’t agree to that but I watched this happen all very close to quitting time. I believe they were rushing to beat traffic.
It was obvious to me that the fleece backing had not adhered to the adhesive as I could see light patches through (about 75%) and darker patches where the matt was pressed into the adhesive. I could even loft the mat on the lighter areas and if I pushed down I could see the fleece make contact with the adhesive and darken. After I did this tonseceral of the lighter areas it was immediately evident that it was then stuck down as I could no longer lift the matting.
Needless to say when the tilers arrived this morning I went through this with them and they were not happy and said, “that is how it is installed” along with their reasoning behind it and a bunch of other filler that made no sense.
I simply lifted the Ditra up and showed them the back of it and as they say, “the proof is in the pudding!” large sections of it was still white.
They reluctantly proceeded to take it all up and afternoon I explain the installation instructions to them they sort of followed suit only to find that the results were equally similar.
The thing I noticed them not do was float the matting properly. They used a trowel and pressed down but it can’t imagine that a trowel has the same evenly dissipated pressure as a float would. Thin metal bends and the only area properly pushing down on the mat would be the handle.
I know that is a long story for what is a simple question but I do think it needs to be explained first.
If the fleece side of the mat is not completely adhered to the adhesive will this create a problem down the road?
-Tye
Hi Tye,
Yes, it will create issues. It needs to be 100% bonded to the substrate.
Thanks for that.
I was pretty sure that was the case but it’s hard standing up to bullying builders at times.
Best,
Tye
I am installing Ditra heat over my bathroom floor and schluter foam shower pan over OSB. I had to patch parts of the floor with 23/32 osb and the result is some uneven seams (not necessary unlevel) where the new osb meets the new osb boards. I think the old osb is 3/4” so literally a fraction of an inch off. Since this is a fraction of an inch would All Set mortar fill the voids enough to ensure an even and secure bond between the osb and Ditra heat? I would rather not cover the entire floor w/ a second layer of plywood/osb and most patching compounds are not recommended for osb. Would a belt sander do the trick? What would you recommend?
A representative from LevelQuick suggested using RedGard on the osb shower floor between the shower curbs and walls, then metal lathe, and then LevelQuick before installing Schluter shower pan to ensure the shower is completely level. What would you recommend?
Hi Rex,
Yes, the mortar can fill that void when you install the mats. No need to use the levelquick for your shower pan unless your floor is WAY out of level, you can normally level the pan up with just the mortar beneath it as well.
Hi,
With 3/4″ plank sub-floor, and going to install Ditra for Tiling. Joists at 16 oc.
I see the wood screw instructions for installing plywood underlayment to subfloor. Should I glue as well? I see different opinions on line. If the membrane provides de-coupling then why not make the sub-floor and plywood as stiff as possible with screws and glue?
Thoughts?
Thanks
Hi Tim,
No glue. The glue will actually leave voids between the beads of glue – you are creating empty space under your tile when you glue the ply. Just use screws.
Hi. I am installing tile over vinyl. After days of research and not finding a real “tile over vinyl” solutions, I got a reply email from MAPEI saying that I can use their MAPEI Uncoupling Membrane Thinset mixed with polymer additive to lay tile directly on vinyl (with light sanding on vinyl). I can not imagine thinset adhering to vinyl. Would it really work? will it last? I am willing to give it a try.
Hi Maz,
Will it work? Yes. Will it last? No, not if water gets on the floor. I learned that lesson the expensive way (as I do with most). Vinyl glue is water-soluble. When water hits it the bond will release. Since your tile is bonded to the vinyl – the tile releases as well. Toilet leaked and ruined the entire floor. The thing is Mapei is correct – it bonded ridiculously well to the vinyl – every piece of tile was bonded to ditra – the ditra was bonded to vinyl. The vinyl released from the floor. Best option without removing the vinyl is to go over the vinyl with 1/4″ backerboard.
Thanks. But I am not using Ditra .. this is direct tile to vinyl installation using MAPEI. I can’t use backerboard because firstly the floor is concrete so can’t screw backers to it, also combines with the tile, we are talking .75″ to 1″ rise in floor. what to do, what to do!!!
You need to remove the vinyl.
Hi – I’m looking to use Ditra in my bathroom floor (25 sqft).
The subfloor is 3/4″ planks (5″3/8″ wide) with 1/4″ gap between planks. Joists are 16″ o.c.
My interpretation of ditra instructions is to put 1/2″ plywood ontop of planks. Is that correct?
What type of plywood should I use?
What length/type of screws should I use and space at 6″ along joist with 1/8″ gap between plywood sheets?
Do I need to use that level-up product ontop of plywood prior to installing Ditra? (the floor seems pretty level to me but maybe I don’t understand the need).
Thanks for your help!
Hi Tim,
You want to use a minimum of AB grade ply, I would also opt for 3/4″, but 1/2″ will work. Use screws that are 1/2″-3/4″ longer than the plywood thickness. You do not want the screws to penetrate the joists – only the plank. No, you do not need to use a leveler if your floor is fairly flat.
Hi,
I just installed Ditra in our kitchen; we’re still in the 24hr curing window for the thinset, but I can already see that the Ditra mat isn’t flat. I think the thinset was mixed correctly, we used a 1/4”notch trowel to apply in a linear direction…I’m not sure where these raised bumps would have come from. Should we pull it up and redo the thinset, or is it ok to even out the flooring with thinset when we tile? Thanks
Hi Amy,
It depends on how ‘unflat’ it is. If it’s significant – over about 1/4″, then you really shouldn’t try to compensate for that with thinset. Pulling it up, however, normally causes more issues than initially existed.