Photo 1
I am not writing this to tell you why your tile is cracking or why your grout is cracking – I have other posts that may tell you that. (Click on the pretty little links 
) If you happen to have Schluter Ditra as your substrate, this post will tell you why either one of the above may be happening.
While Ditra is my preferred membrane for floor tile installation (as well as countertops and tub decks) it absolutely needs to be installed correctly. The two main techniques for this are fairly simple:
- Make sure the cavities (waffles) are filled correctly
- Install it over an approved substrate (and with the correct type of thinset mortar)
Photo 2
There is a lot more to ditra than those two items but if either one is incorrect I can nearly guarantee a failure. See photos 1 and 2 there? The tile was cracked and it was a direct result of a) not getting the waffles filled correctly and b) improper coverage on the tile. Now b may be due to not backbuttering the tile, an improperly-sized trowel, letting the thinset skim over or set too long before installing the tile or simply incorrectly mixing the thinset. All three of those things will cause any tile installation to fail – whether you use ditra or not.
Not filling the waffles correctly, though, will cause the tile to not be fully supported and/or not ‘locking’ the tile into the ditra. Because it is not correctly locked into the ditra you will lose the mechanical bonding properties of ditra and you may as well install it directly to particle board at that point (That was sarcasm – don’t do that!). For more specifics about exactly how ditra works you can check out Provaflex vs. Ditra wherein I describe exactly how the mechanical bonding process works – and rant about a particular jackass. But the mechanical thing – that’s what you want to concentrate on.
You need to use the flat side of your trowel and spread thinset in every direction over the ditra to ensure that all the little waffles are full. Since the cavities are dovetailed (that means they go down and away from the opening) you need to ‘force’ thinset into the bottom corners of the cavities. Simply running the trowel over the ditra will not do this. Simply running the trowel over the ditra did that (photos 1 and 2).
Photo 3
Installing ditra over an approved substrate is much, much easier. In fact, nearly every bare substrate you find in a modern house would be considered an approved substrate – shiny linoleum is not one of them (Photo 3). While there are thinsets that ‘say’ they will bond to linoleum (and some of them will) apparently the jackass who installed that particular floor was not aware of that. 
See photo 4? I lifted that up with my pinkie – literally! It was not attached at all. He may have had correct coverage beneath the tile and all the little waffles filled – I have no idea. There was not enough stuck to get enough leverage to tear one off and find out.
Photo 4
Most any plywood (even osb 
) is an approved substrate for ditra. And if you use a thinset approved for that substrate, there are no problems at all. Photos 3 and 4 had an unapproved substrate and, apparently, incorrect thinset (and a shitty tile job, but that’s a whole other post). It was nearly guaranteed to fail.
When you buy ditra for your installation every roll comes with a handy little instruction booklet. You can go to Schluter’s Ditra Page on their website and access the instruction booklet (This link is a PDF!). They even have a flash video about the proper installation technique. You can leave a comment below and ask. You can email me. You can send up smoke signals – I’ll answer.
Given the 17 ways to acquire correct ditra installation information above there is absolutely never a reason to do it incorrectly. Ditra, in my opinion, is the best membrane for most floor tile installations. The only time I’ve seen it fail is due to incorrect installation. And that isn’t just the common BS everyone accuses failures on. Me, personally, every one I’ve seen fail is incorrectly installed.
If you use ditra, and if you have an approved substrate, and if you have the correct thinset mortar, and if you fill the waffles correctly, and if you use the proper trowel and get proper coverage it will not fail. Yes, that’s a lot of ifs – when you read it. In practice it really is not that many things to get right. It’s just common sense, mostly.
So here’s one more if: If you have any questions at all about correctly installing ditra and using it for your tile installation please, for the love of all the marble in the Sistine Chapel, ask me below in the comments. I WILL answer you. I’m just super-cool like that 
I have a kitchen I am going to put ditra and tile in that has a leveling compound over it. However in the edges the compound is missing and there is vinyl squares under that then the wood sub floor. Do I need to pull up all the level compound and the vinyl in this area or can I just patch the areas that don’t have level with more level compound?
Hi Finley,
You can just patch it.
Hi – Been interviewing different tile installers for a new home I am building in the Northeast and the feedback on how the job should be done from each vary. I put down 1 1/8 Advantect that is screwed and glued to floor trusses that are 12 to 16 inches oc. The reason for most 12 oc is I was given 12 extra trusses by the truss company so I used them. The installers seemed amazed I used 1 1/8 inch thick Advantech as many don’t realize it comes this thick. Basically I want to have Ditra over the Advantech sub-floor and I find many pushing other brands I never heard of. Today I was told by one installer he was concerned the thinset would get between the Advantech boards where the TG merge and something should be used to seal these areas. Even suggested maybe we put down another 1/2 inch – from what I’ve read, the modified thinset between the Ditra and Advantech is all that is needed, and I am not sure if these guys know what they are talking about. We bought and plan on using gray, 24×24 porcelain tile from Italy installed on top of the ditra with un-modified thinset to give an industrial look – the entire first floor is this tile, about 1400-1500 sf job. I was hoping for your thoughts on anything I should consider doing besides the Ditra and thinset. I was told with the Ditra I don’t need expansion joints as it becomes all one piece and one guy said I need one every 24 feet. One guy said they put down the 2×8 foot galvanized mesh metal sheets and put the thinset on top of that to create a solid hard floor that never has problems. He doesn’t like the Ditra and this method is best. This guy complained it was hard to hold a mark on Ditra and this metal mesh allows for a hard floor they mark and cover the lines with hairspray. He says because it is like one sheet of thinset, they can do the job in stages and one can walk on it in the mean time. I read this is called a Jersey job that has a lot of problems. I am concerned with my floor trusses this is not the way to go. I haven’t gotten a quote yet from this guy and I figure I knew a lot so maybe he knew he could no BS me.
Hi James,
The guy talking about the jersey mud job? Lose his number. Really. You DO need expansion joints. Every 16-20 feet or every 8-12 in a room with direct sunlight. A sharpie writes on ditra just fine. A laser writes on it even better.
The thing with expansion joints is that it is NOT about the entire installation being monolithic. It is about the expansion and contraction of each individual tile relative to the one next to it. Although the floor may MOVE as one monolithic structure (ditra enables this), each tile is separate and individual from the one next to it.
If you butt two tiles up to one another you can slide them both across a table by pushing on one against the other – they’ll move together in the same direction (monolithic). If, however, you butt them together and nail a board tightly against opposite sides of each, so they are essentially locked together jammed between the two boards, then grab a blow torch and begin heating them, they will begin to ‘tent’ – rise up at the point they meet. This is due to expansion of the individual tiles against one another. They essentially outgrow the area into which they are confined.
If you had an expansion joint between those tile rather than having them butted, they will not tent, the silicone in the joint would shrink to compensate.
Get rid of any of your guys who don’t understand that concept.
I really should do a video on this, that would be awesome because I could play with fire!
My husband and I recently had work done by a contractor. Their subcontractor did a lousy job installing a fairly large area of tile. We got our own subcontractor to fix it and accidentally discovered the Laticrete StrataMat was installed upside down. We communicated directly with Laticrete who advised us to remove and reinstall. This involves a hall a pantry with floor to ceiling shelves and kitchen with a new custom island with waterfall counter. The contractor told us he was told by Laticret it won’t fail. The contractor refuses to reimburse us for the expense it will cost us to repair his mistake. Instead of pursuing this with a court or by lawyers we agreed to not pay the balance we owe him and absorb the extra cost our selves. What do you think about a membrane being installed upside down. This seems like a no brainer to me-there is a reason it has 2 sides!
Hi Sharlene,
What do I think? I think your contractor is an absolute moron.
But you knew that, already.
It MAY last. Maybe. I don’t know. The biggest issue is the underside and how well it bonded to the substrate. There is a reason the mesh is on the underside. I would take it out, have it replaced, something. It is completely wrong and is likely not to, at the very least, work as intended. At the worst – completely fail. My opinion is that it will come down to the bond to the substrate, but even there you don’t have a solid, continuous bond. It’s ‘pillars’ of bonds. I wouldn’t trust it. Unfortunately the courts may be your only recourse here.
I need to know if I should fill in the waffles, float flat, then let dry before tiling or fill the waffles at the time of tiling?. There seems to be an equal distribution online between the two camps and they each defend it like a religion.
Hi Mark,
You can do either, they each work equally well. I normally prefill mine, but just because it makes the installation go faster for me the next day, not because it works better than not filling them. The other advantage for prefilling for me is the ability to stomp, roll, drag and crawl over installed ditra without worrying about any issues. If I have my rolling cart loaded with four boxes of tile I can’t roll that over ditra that is not filled without compromising it. If I fill it I can drive a forklift over it if need be while I’m installing.
I would like to tile over a basement floor.
I saw in a schluter company video that their shower product has ditra over kerdi shower trays, which are polystyrene.
My question is:
Can I put polystyrene or other panel insulation, then the “correct” thinset, then ditra, etc?
Meaning will ditra allow me to tile over insulation without a hard surface to adhere to?
Hi Anny,
No, you can’t. The shower pans are a particular density and you use kerdi over it, not ditra. Wall polystyrene is a much less dense foam.
What is the best way to level a concrete substrate before applying ditra?
Hi Melanie,
Self-leveling cement.
Roger,
Second question: I installed Ditra yesterday, and when I looked at it this morning, I noticed about 8 or 9 “pancake” lumps, about 1/16 – 1/8 inch in height, maybe about 2-4 inches in diameter. I’m laying large format tile, with 1/2-inch trowel. Should I cut out those pancakes and painstakingly insert new Ditra, or will the thin-set, with large trowel, allow me to compensate. [BTW – mistake was going in a forward direction with the installation, with a large sheet which hid the pancakes – poor troweling on my part…never had a problem wokring backwards with everything in full sight.]
Thanks
Lance
It will be fine. Anything under 1/4″ will not affect your tile installation. If it does then your trowel is too small.
Roger,
Thank you for the answer on the 1/16th to 1/8th inch “pancakes” in the Ditra; I did go ahead with tile work and left them “as is”, using 1/2-inch trowel. The tile has been down for over a month, with lots of traffic, and as you said, it didn’t make a difference – no issues at all. Love your blog – great advice!
Hi Roger,
Is it better to fill in the waffles with one application of thin-set, let it dry, then apply the notched layer, or is it better to do both at the same time. I’ve seen mixed recommendations. The last application for me went fine, as I did them both in ne application, but more recently, I saw a professional recommend two separate applications. Thank you !
Lance
Hi Lance,
Either works just fine. I do both. Pre-filling it makes the installation itself go faster for me, that’s why I do it. It doesn’t have anything to do with one way working better than the other.
Hi, I just tried putting down the Ditra, and I think I put too much thin-set (how am I supposed to know??!) it was very stressful. So since I think I had too much thinset and The Ditra looked bubbly and uneven I used my rubber float to push out as much thinset as possible. I think I pushed too hard and damaged the squares of the Ditra….is that possible?? Some are dented. I weigh 100lbs so I’m pretty small but strong, do you think it will be ok??
Hi Isabel,
Unless you crushed it to the point that it is a single-layer sheet now and the waffles have completely disappeared – you’re fine.
Hello I am going to be installing prodeso mat and tile in upcoming weeks. I had done a small bathroom remodel and used 3/4 GP dryply for flooring old osb had rotted. It has some sort of wax on it for waterproofing during new builds. Will the thinset adhere to this dryply. I call GP and they said it probably wont and sanding the dryply first may help.
Hi Don,
Splash some water on it. If the drops are absorbed fairly quickly (within 30 seconds or so) then it’s fine. If it just beads up and sits there then you need to sand or prime it for the thinset to bond.
Hi Roger,
My husband and I have recently installed Ditra on top of level plywood. We spent yesterday dry fitting the tile to install soon (12″ x 24″). However, I am concerned after spending hours dry fitting the tile over the installed Ditra, I noticed that there are lumps UNDER the Ditra. We went through great pains to install it correctly: modified mortar, 5/32″ grooved trowel, mortar sticking evenly to the Ditra, grout float over the Ditra to get it all even. I measured the spot that seems to be the worst hill–>valley and it is 6/32″, so a bit over 1/8″. My understanding is that the next layer of mortar should be about 1/8″ which would hopefully even it all out? I guess I’m worried that the mortar layer under the tile won’t be enough to even it out because I broke a tile by leaning on it when we were dry fitting….that how we discovered the bump. I would love to know what you think we should do. I’d hate to put the tile down and then have it crack, that would be heartbreaking….tiling is such hard work!
Hi Sabrina,
No need. If 1/8″ makes your tile installation an issue then you need to use a much larger trowel, you aren’t using enough mortar under it. There should be a minimum of 3/16″ of mortar beneath a finished tile with 12×24. If 1/8″ throws it off, use more. You should be using a MINIMUM trowel size of 3/8×3/8. 1/2 x 1/2 is my minimum size. 1/8″ height difference in the substrate doesn’t affect that at all.
On an unrelated side note: in the 13 years I’ve had this site this is the first time I have EVER answered two consecutive questions identically – because it was the same issue.
Is Daniel Foley your husband?
Hi Roger,
I laid down the Ditra and I thought it was pretty level when going through it with a level but now that I have cut and dry fit the tiles (12″ X 24″) I am noticing more than 1/8 of an inch give in a couple areas. Am I able to pour SLC or a Self Leveling Underlayment over the Ditra Membrane to get it more level and then install the tile over that? Schluter says no but it seems in some of your comments below that it may be possible. Is there a best SLC or SLU to utilize? Or would it make more sense to rip up the Ditra, pour the SLC and then buy some new Ditra and reinstall?
Thanks and really appreciate the content for DIY!
Hi Daniel,
No need. If 1/8″ makes your tile installation an issue then you need to use a much larger trowel, you aren’t using enough mortar under it. There should be a minimum of 3/16″ of mortar beneath a finished tile with 12×24. If 1/8″ throws it off, use more. You should be using a MINIMUM trowel size of 3/8×3/8. 1/2 x 1/2 is my minimum size. 1/8″ height difference in the substrate doesn’t affect that at all.
I have a 14x 14 screened porch with a concrete base. I’m using 24” square tiles. The floor is relatively level but there is an area that has a slight dip in it. Is it possible to use a floor leveler under ditra? Thanks!
Hi Peggy,
Yes, you can use floor leveler beneath ditra.
I used Ditra for first time and unfortunately had a hard time putting it down. I think my trowel is too big and too much thinset. It left me with some bumps on the first sheet. I realized the problem later and started using a roller to get the excess out but I’m wondering if I should take the first sheet out. I’m putting down 24 x 24 tiles so maybe I can get away with it with sufficient thinset and those levelers, but wondering what you would do in this case. Thank you kindly!
I had the same problem! It was so stressful. I pushed out so much thin-set I think I damaged the little squares. I just payed one sheet and let it for now for my dad to inspect because I’m afraid I ruined it. I hope yours turned out ok!
I took it out completely and put it down with the right trowel and it was perfect. Because the mortar had not dried, I was able to get it out fairly easily. I also power washed the Ditra and it became like new. Just make sure to let it dry well if you do that.
Hi Kosta,
You can leave it. They shouldn’t be big enough to significantly effect the outcome of the finished tile.
Hey Roger, I’m putting .59″ porcelain tiles in my kitchen. There is a 1×8 plank subfloor in generally solid shape perpendicular to joists spaced 24″ apart. I will patch with plywood where it’s needed and reinforce the rest with ring shanks. I’d like to keep the overall thickness as close to 3/4″ as possible, so what I was thinking after that was 3/8″ plywood and Ditra. After tiles, that would put me at about an inch. Does this seem like a good plan or would you suggest any alternatives landing at a similar thickness?
Hi Daniel,
You need a minimum of 1/2″ ply over that. I understand the height issue, but you can’t compromise an entire installation to arrive at a desired height along a limited transition space like a doorway. There are a LOT of transition strips to ease any height difference you may end up with. I mean like hundreds of different ones. I would find one of those and ensure that the floor lasts.
Before putting down ditra do I need to scrape the latex paint overspray off the concrete subfloor?
Hi Peter,
Yes, or the thinset may not bond.
Can I reuse Ditra if it pulls up ok ? I used the wrong thinset and would like to redo it but it’s already cured
Hi Christopher,
Yes.
Liked you analysis/diagnosis of failed installations.
About to tile a concrete floor in the basement of a home. Plan to use Ditra mat which I’ve laid several times before. My only question is do I need to necessarily ‘prime’ the concrete, before then using Thinset into which to embed the Ditra mat. Or, is priming of the concrete substrate unnecessary
Thanks.
Don Bell
Denver, Co.
I have a bathroom that is 4×9 give or take a couple inches my sub floor is 1/2 inch real wood with plywood on top screwed and glued I am going to put down ditra long ways because plywood is running the other way so no one will be walking on the seam so do I have to tape the seam.
Hi Nickolyn,
The direction in which you run ditra does not matter at all. You can even piece it in going all different directions if you want to. You do not need to tape the seams (unless you want it to be waterproof, then you do the seams with kerdi-band)
Hi Don,
Splash some water on the concrete. If it soaks in the drops quickly (within 45 seconds or so) it’s fine. If it just beads on top and doesn’t soak in you’ll need to either scarify the concrete or prime it.
Hi,
I am installing ditra and have attached it to the sub floor and half of the kitchen floor is tiled. However, when I finished for the day I spread thin set mortor onto the top of the remaining ditra and its now dry. So when I lay the remaining tile in two days will the wet thin set mortor back buttered on the tiles adhere to the dry thins set mortor that is already on the ditra? Could there be an adhesion issue?
Hi Kevin,
It will be fine. There is no bonding issue with that. I regularly pre-fill my ditra.
Hello Roger, i am laying ditra over an bathroom osb floor , then tile. Should i use wonderboard or just modified thinset the ditra to subfloor then unmodofied the tile?
Hi Josh,
Just a good modified over the osb then an unmodified over the ditra under the tile.
I know this is an old thread but wondering why premixed mortar is not recommended? Also wondering if after installing the membrane , should I wait 24 hrs before installing the tile.
Hi Kelly,
Because premixed ‘mortar’ is not actually mortar at all, it’s mastic with sand in it (to diminish shrinking). It CAN NOT be used over a membrane. The premixed products ALL require air to fully cure. Placing it between a membrane and a large tile, or between a subfloor or slab and the membrane, is just like placing the lid back on the bucket – it will never fully cure. You HAVE to use a powdered mortar that you mix with water.
Thank you so much for your response. I have all my Ditra pieces cut n was getting ready to start. I’m assuming that I have to wait 24hrs for it to dry before doing the tiles? Sorry I’m new at this , I have only done a bathroom floor before.
Sorry, missed that part. You can tile over it immediately, no need to wait.
Thank you for your help Roger
What is a good way to level a concrete floor prior to installing Ditra? Previously the room had carpet.
Hi Al,
Self-leveing cement, or SLC is the fastest and easiest. Depending on how out-of-level it is you may be able to do it with thinset beneath the ditra by using a large enough trowel and smoothing the ditra out with your level, once cured it should be level.
Thanks. Is SLC something you can use over an entire house? Also would you recut the sawed in control joints after using a leveling cement?
Yes, and yes.
Hi Roger
If I prefill dita cells, should I use modified or unmodified mortar to set tiles?
Hi David,
If you’re concerned about the warranty you need to use unmodified for both. If you aren’t concerned about the warranty you can use either. I normally use modified (don’t tell schluter…). If you use modified give it an extra day to cure before grouting.
Roger, I just finished a recessed (curbless) Kerdiboard shower with Kerdi tray that slopes down from the entrance to a Kerdi-line linear drain at the wall. Next up is a waterproof bathroom floor using Ditra. The question I hope you can help with is about the transition from Ditra to the Kerdi shower tray.
Are there issues overlapping the Ditra floor membrane a few inches onto the top of the shower tray?
Kerdi instructions show butting the Ditra right to the edge of the tray and overlapping both with Kerdi-band. With that, it seems to me that the Kerdi band is decoupling the subfloor from the tray, not the Ditra.
I will run the same 12×24 tiles from the floor, into the shower, The finished tray is about 1/8” lower than the subfloor where they meet at the entrance.
Hey Scott,
There is no issue at all running it into the pan. For clarification, though, the kerdi-band is placed there in order to waterproof the transition only, it doesn’t uncouple anything.
Thanks for reply Roger!
I understand your point about waterproofing with the Kerdi-band at the shower entrance transition. I was thinking that with large format tile running over the transition, using ditra to decouple possible movement between the subfloor and the Kerdi tray would be important.
I will go ahead and overlap the Ditra over the transition joint. If I overlap the Ditra 2 or 3 inches onto Kerdi tray with thinset, would it still be best to also use Kerdi band on top?
I would, just to have a completely waterproof transition. Ditra is waterproof, but the cavities filled with thinset will absorb and hold water. I like to eliminate that where I can.
Hi Roger,
Ditra install question… nuHeat may has been installed and now requires a skim coat of thinset over it before tiling. I will be installing Ditra, and wonder 1) if the thinset to install the Ditra is sufficient as also the skim coat to protect the heat may; and 2) to avoid nicking a wire with a trowel, is it possible to apply thinset to the back of the Ditra, then lay it over the dry heat may? (of course weight is an issue the consider). Thanks.
Hi Tau,
Yes, the thinset you use for ditra will be fine. Trowelling the thinset to the back of the ditra is not a good idea, it’ll be a mess! They make plastic trowels you can use over the wires that will not nick them. If you pay attention while you’re doing it the metal trowels work fine – those cables are a LOT tougher than you may think.