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 am installed my underlayment which is playwood. I am going to apply the ditra on top. The gaps between the sheets, should I avoid getting mortar inside them or is it okay to fill in the gaps with mortar. It will be Laticrete 254 if that matters. Thank you for your help.
Hi Roger,
I’m doing my first flooring installation, and I’ve decided on some 12×24″ ceramic tiles for my kitchen.
I’ve pulled up the old vinyl flooring, and removed the underlayment at the same time, as it was easier than taking the vinyl off the wood underlayment.
I’ve replaced the underlayment with 7/16 OSB board. So that gives me a 3/4″ either plywood or OSB (couldn’t honestly tell after removing the underlayment) subfloor, and then a 7/16 OSB underlayment.
I had originally thought to lay my tiles onto the OSB, but did my research and learned this was a terrible idea. So I’m planning on picking up some Ditra instead. So am I okay to apply a modified thinset between the OSB and Ditra, and then an un-modified thinset between the Ditra and Tiles?
Should I be caulking the edges / gaps on the OSB board before applying the thinset? And anything else I need to think of before I start with thinset?
Thanks for your help.
Andrew
Hi Andrew,
Yes, modified under the ditra, unmodified above it. No need to caulk the edges of the osb, you should caulk or tape over the seams, though.
Thanks for the reply Roger!
If you’re around at the moment, I’m actually just getting ready to secure the OSB to my Subfloor. I had planned on using Staples, but read in the Ditra manual that they don’t support or recommend a stapled underlayment? Do you think this will pose any problems if I staple it down with 1 1/4″ staples. Or should I pick up some Ring Shank nails, or wood screws? If so, what size for the nails or screws?
I really appreciate your help. Thanks again.
Oh and what should I tape over the OSB seams with? I know to use the Kerdi-band and thinset over the seams of the Ditra.
You can either use 3/4″ tape or, better, put silicone in the seams. The purpose is to keep thinset out of the seams to allow expansion and contraction.
Hi Andrew,
You need 1 1/2″ screws at minimum.
Hi there Roger,
So I’m finally getting around to this tile installation. I’ve just picked up my Ditra underlayment, and wondering which modified thinset, and unmodified thinset I should be going with here in Canada? There were a couple of options at our local home depot, but we also have a more specialized tile ideal store here that has more (and more expensive) brands available.
Are you able to recommend a certain thinset? Here are a couple available locally for us:
https://www.homedepot.ca/en/home/p.premiumplus-thin-set-mortar-gray—50lb.1000173397.html
https://www.homedepot.ca/en/home/p.megalite-ultimate-performance-mortar-white—30lb.1000166925.html
https://www.homedepot.ca/en/home/p.speedset-fast-setting-thin-set-mortar-gray—25-lb.1000418372.html
Or should I check to see if the Tile Ideal carries this Laticrete brand that I see mentioned fairly often? And if so, which Laticrete would I be looking for to adhere the Ditra to an OSB subfloor, and then the 12×24″ ceramic tiles to the ditra?
Lastly (Sorry for being so high maintenance), what size trowel should I be using for each stage of the installation?
Thanks so much for your help.
Hi Andrew,
The premium plus or the megalite will work fine to bond the ditra to the osb. None of those are unmodified, but you can use the speedset in lieu of the unmodified. If you choose laticrete you want the 253 for the ditra to osb and 317 for the tile to ditra.
Hey Roger,
Thanks for the info. I went looking for the Laticrete at our Tile Ideal store, I found the 253 GOLD, but they didn’t carry 317. The guy in the store said he recommended 254 for OSB to Ditra, and then 272 for Ditra to Tile.
While I’d like to think he’s giving me sound advice, I wanted to check in with you as well before I started the project. Do you think I’ll be fine with the 254 and 272?
Thanks again!
Hi Andrew,
Yes, he’s giving you sound advice. The 272 is a very good non-modified.
Hi Roger,
I’m doing a small bathroom floor. I pulled up linoleum and scraped every thing I could get off the plywood. still has a little haze of glue on it should I sand it completely off or will the thinset bond as is. also I am putting down porcelain plank tile all thinset directions say I must use modified to bond to porcelin. but Ditra says I must use non modified thinset. which is correct. thanks
You need to use modified when bonding the Ditra to plywood. I think you can use modified when bonding the tile to the Ditra, BUT it takes a very long time to dry and Shluter doesn’t suggest using it – you may want to steer away from Ditra for this project.
Roger?
Hi Dave,
If you splash water on it and it is absorbed then thinset will bond to it. They are both correct, but technically the substrate manufacturers recommendations trump the TCA handbook methods, so unmodified is correct. It works just fine, really.
I’m getting ready to install 18″ porcelin tile over 2 layers of 5/8″ plywood with 1/4″ luan screwed down every 6″ to it. I’m thinking about laying ditra on top of the luan. Is that an issue?
Hi Marcia,
Yes, luan does not have the necessary compressive strength to be under a tile installation.
Dear Floor Elf,
I have been hired to repair an exterior pool deck N of Seattle. So, some freezing in the winter. It was set with Ditra, and, the tiles are essentially falling off. The Ditra is the older striped kind. The tiles are a middle to hard slate. I feel that water is penetrating the slate/grout, then sitting on the Ditra, and freezing, breaking them free.
The owner, who is very nice older guy, but slightly potty, wants me to tear them up, clean them, and reglue them. I am tile contractor # 6. The guys before me were using a Flexbond by Custom. But, after reading up on Ditra, looks like I should be using an unmodified thinset. And, wont’ that have problems this winter?
I forgot to mention this is on a concrete slab with expansion joints.
Are the expansion joints in the concrete followed through the tile installation with a soft joint? Are there ANY soft joints in the installation at all? That could definitely be the problem.
no. The expansion joints run through the tile. I think it is water penetrating the installation, and going through the freeze thaw cycle. Though our winters here are not harsh, we have 10-25 days of below 32 degree weather.
What I want to know is: what should I use to reset the slate to the Ditra? I am concerned about using an unmodified thinset with the slate. Or should I be. There is definitely tons of moisture for it…
The unmodified will work perfectly fine with the slate.
I have never used unmodified thinset, so the idea of it is making me nervous…
I completely understand, but it works very well. Really.
A better option would be to install it with epoxy setting material, it won’t allow water into the cavities and won’t be affected by freeze.
I considered that for about ten seconds. I am getting paid well, but not that well….
Tec makes an uncoupling mortar that is rated exterior, that I can buy on a Sunday. Otherwise will probably go with the Custom Uncoupling if I can make it to the store by 3 on Saturday…. And FYI the 317 is not rated for exterior installations.
Hi Carol,
Unmodified would be your best option. It may have problems this winter, it may not. Unmodified, though, it less affected by freeze-thaw in the ditra cavities. The problem, however, is that there is likely already flexbond filling the cavities. So if you do it I would replace the ditra as well. You may also want to give schluter’s tech line a call and explain the situation, they could help you out a lot.
Thanks!
WWRD?
I’ve got 3/4″ Advantech over 16 OC joists. The 2×8 joists span less than 9 feet, comfortably meeting the L/360 requirement for the porcelain tiles I want to use.
I have Ditra and was planning on using it right over the Advantech, but the JB guys pointed out that many thinsets aren’t approved for use over OSB, and they further scared me into thinking I just might need another layer anyway…So off I went searching for suitable plywood – so far I have had no luck finding anything suitable.
1. I get the impression that you aren’t a fan of OSB, so I figure you don’t like Advantech, either…If you somehow found yourself in my situation, what would you do?
2. Assuming a second layer of plywood is called for, where would you buy it? The best thing I found at HD was this:
http://www.homedepot.com/p/Unbranded-Sanded-Plywood-FSC-Certified-Common-15-32-in-x-4-ft-x-8-ft-Actual-0-451-in-x-48-in-x-96-in-577278/100097501?MERCH=REC-_-NavPLPHorizontal1_rr-_-NA-_-100097501-_-N
I wasn’t smart enough to read the grade stamp (I could on the other plywood panels, but this item didn’t have an obvious BC or CC or whatever marking). The reviews aren’t very favorable either. It is made by “Roseburg” which some people seem to not like.
I really don’t want to go through the trouble and expense of adding a second layer unless I can get the full benefit / no negative effects…Lowe’s also sells a 1/2″ nominal C grade plywood (Roseburg), but it too gets ripped in the reviews for having internal voids.
Hey Steve,
With 3/4 over 9 foot span you can go directly to the advantech with your ditra. I would use something like laticrete 253 or mapei ultraflex 2, but you’ll be fine going directly over that.
I was concerned mostly about bonding the ditra to the OSB. I noticed that Mapei/Laticrete/Custom don’t list OSB as a suitable substrate for the thinset products I looked at. (Maybe this is more about bonding tile to OSB, and doesn’t apply to Ditra?)
I think the FlexBond literature indicated that other surfaces could be used if primed with MBP…is that what I have to do, or is it overkill?
Hey Steve,
I’ve never had an issue with osb, but I’ve also never seen a thinset approved specifically for use over it. I don’t use primer, but it certainly wouldn’t hurt.
Hello, Roger.
I am going to install 12″ slate tile on Ditra. My house, built in the early ’70s, has what appears to be 3/4″ plywood substrate. I am replacing an existing tile floor (already removed). There is an additional 3/8″ layer of plywood that was glued down on top of the substrate. Do you think it is safe to proceed?
Hi Eric,
Provided it is actually 3/8″ ply and NOT 1/4″ plywood or luan.
I just removed tiles that were laid on 5/8″ plywood but homeowner wants to lower the height, (to match height if wood floors) was thinking of using 1/2″ concrete board. Would this hold up ( as far as structural ) or would the tile crack (12×24″ tiles being laid)…?
Hi Santo,
If using cement board you need a minimum 1 1/8″ thickness double layer plywood under it. The thinnest you can reliably go is 3/4″ t&g ply over the joists with ditra.
Hi Roger,
Great site!!
I will be tiling my foyer area which is a 6 inch raised concrete slab framed by 2×6’s (I can’t remove the 2×6’s). I will be using Ditra for this area and covering with 20×20 tile. I plan on using Ditra-set to set Ditra and tile. My concern is the 2” wood border where the slab is framed. This is where a lot of force will be applied when stepping up/down into the next room as well as dealing with expansion/contraction of the wood. Should I put a modified thin-set along this area to bond Ditra to the 2×6’s to account for thermal expansion and compression or continue troweling with the unmodified Ditra-set?
TIA,
Chris
Hi Chris,
You need modified thinset over the wood. I would just use modified under all of it.
Thanks Roger.
Two quick follow up questions…
Should I be concerned of any extra cure times for using modified between slab and Ditra? If so, how long before setting tile?
By using modified over slab, will this void Schluter’s warranty as they recommend the use of unmodified with a slab substrate?
Thanks again!
Chris
You can set tile right away. Yes, it will void your warranty.
Roger,
2″ mosaic floor tiles on Ditra over a super duper reinforced wood floor. I’d like to do that. But I see rumblings on the Web about Ditra not being able to sufficiently support mosaic tile floors. I guess it leads to cracking grout that never quits. The small size tiles and the large Ditra waffle spaces is suppose to be the issue. Is there truth to the matter or just poor installation technique?
Also I’m seeing that Ditra requires a ‘good’ quality of thinset and the big box stores don’t stock a good enough quality for the demands of Ditra. What brand of thinset do you use?
Hi Dennis,
Ditra is fine with any tile 2×2 or larger. Smaller tile than that is not supposed to be installed over ditra. I use mostly laticrete thinsets.
Rodger-
Is it ok to let the modified thinset dry (set) with the mats installed and then come back and set the floor tiles another day?
Thanks
Bill
Hi Bill,
Yes, it’s fine.
I want to tile my 2 year old 500 sqft concrete lanai that is painted. Paint is in very good shape — no peeling. Would ditra installation work? I live in Florida.
Hi Harry,
Yes, it would. But the paint would need to be removed. Your substrate needs to be able to soak in water for the thinset to bond to it correctly.
I’m thinking about using Ditra-Heat as the underlament in the 55 sq. ft. bathroom that I am refinishing. The Ditra-Heat kit I’ve seen won’t quite cover my 55 sq. ft. floor. Can I use some regular Ditra in the areas that I don’t plan to heat to cover the rest of the floor? Thanks!
Hi Doug,
Regular ditra is half the height, you can use ditra xl or two layers of ditra.
Roger,
I’m tiling a small half bath. After pulling up the linoleum and luan, I have a ¾” OSB sub-floor, with a 2’x2’ section cut out and replaced with ¾” plywood around the toilet flange area. This was done as a repair after a toilet leak. Unfortunately the plywood and OSB have a small gap where they meet, and are not exactly level to each other. I would like to use Ditra over the wood floor before laying porcelain tile. I really do not want to raise the floor with another level of ply as it will be much higher compared to the wood floor in the adjoining room. Can I use a self-leveler around the plywood-to-OSB section and then thinset and Ditra? Or, should I level and use backerboard in this case?
Thank you for your site and sharing information.
Hi Matt,
You can actually just use thinset beneath the ditra to level that out. Use a large enough trowel and you can embed the ditra into the thinset using a straight edge. You should end up with a flat substrate, there will just be varying amounts of thinset beneath and around that area to get it flat.
Thanks for all the great info. I will be doing my first Ditra/tile install in my recently gutted bathroom this weekend and am feeling more comfortable about pulling this off without mucking it up.
HI Roger,
I am in the middle of our tile job. At this point I have a heating mat embedded in modified thinset. I also added thinset to adjoining areas to make them a bit closer. One thing I am concerned about is the lack of flatness of the thinset. I have ridges and some bumps and ultimately a floor that was once flat is kind of bumpy.
My first question is how should I deal with the bumping so that is now there. Is that a concern or will the next layer of unmodified thinset fill it
I read your post about making the tile level with uneven floors. Would you recommend those same techniques to get the ditra flat during the ditra install?
Hi Chad,
Yes, the next layer of thinset will allow you to flatten the ditra out with a long straight-edge to embed it into the thinset.
Hey Roger,
Was going to attempt getting the ditra installed tonight but am now having some concerns. Our floor is mostly flat and level except it slopes up roughly 3/4″ in the last 5 feet as the floor approaches the exterior wall (hope that makes sense). This is in a bathroom so we thought we could live with the slope as long as it was flat – it would keep the water away from the exterior wall at least! The problem is in that last 5 feet is also a tub which runs parallel to the slope. The front of the tub lacks an apron so I planned to frame and tile it with subway tile (3 x 6 or 4 x 12, still deciding). I am concerned about how the tile will look in the sloped area. How would you address this?
One more thing… I used thinset to embed the heating wires and did a skimcoat to keep things even. If I wanted to use SLC is there any issues using that over the top of the new thinset?
Nope.
I would level that floor out before I installed the ditra. It wouldn’t be an issue if you didn’t have the tub face across that area, but since you do you’ll be able to see that. It would be very noticeable.
Thanks, Roger! Put two bags of SLC down today. What was i thinking?!? SLC was super easy to use and is quite flat.
Hey Roger,
So I attempted to bond the ditra to the SLC today. I used a 1/4 x 3/16 v-notched trowl with custom building unmodified thinset (the one specifically branded for uncoupling mats e.g. ditra). The thinset dried out incredibly quickly. It dried out so fast that I was not able to get a bond. It became clear to use that after doing two rows we pulled up the ditra, scraped off the floor and hosed off the back of ditra in hopes that we’ll be able to reuse it.
Any recommendations on how the next go can be successful? We started researching at it seems that others have had problems with this thinset. Any recommendations for alternatives?
Hi Chad,
Mix it with more water, mix it more slowly (not full-bore on the drill), let it slake (rest ten minutes after mixing, then remix, then use), work in smaller areas at a time.
Hi Roger,
I am having my bathroom re-tiled as some of the previous tiles have cracked as a result of there being a small section of the floor which is 4mm out compared to the rest of the floor.
My builder and tiler have advised that just putting ditra mat and adhesive underneath the new tiles should solve the problem. Is this correct?
Thank you for your help!
Hi Natalie,
If done correctly yes, that will solve the problem.
Roger – We are having a tile floor installed in a new addition over new concrete. The delivered materials included both regular thinset (Bostick Single-Flex) and mortar for application of Ditra (Bostik Ditra-Set). The contractor has now installed the Ditra, and yet the bags of Ditra-Set remain. My guess is that he used regular thinset for installing the Ditra. He is coming back this week to finish. What is your recommendation? Tear up the just-installed Ditra and start over, or go ahead and finish the job, and, if so, which mortar should be used to install the tile? Thanks. Kevin
Hi Kevin,
Although the ditra-set should have been used to install the ditra over concrete, the single flex will be just fine. The unmodified ditra-set should be used to install the tile over the ditra.
I installed hardibacker board on a bathroom floor before deciding to go with ditra radiant heat. So i am going to put the ditra over the Hardibacker, should i use unmodified thinset for this?
Hi Wayne,
Yes.
What do you think about filling ditra XL waffle holes with thinset, making surface super smooth and then applying tile the next day after it has dried or should it be done all at once? Thank you, lots of good info here.
Beth
Hi Beth,
You can prefill your ditra, I do it all the time.
Hi Roger,
I am installing Ditra in a full bath and a half bath. I pulled up the 1/4″ underlayment in the half bath. In the process I damaged the sub floor. Part of it came up with the underlayment. So now I have a gash in the floor that is about 7″ long 4″ wide 1/4″ deep. What do I need to fix this before I apply the thin set? Also, I am installing a new tub in the full bath. Because the tub has a curved front and I don’t I have the expertise or the patience to cut curved tiles, I am thinking of installing the Ditra and the tile under the tub. Can I install the Ditra under the tub?
Thank you,
Rob
Hi Rob,
You can just float that gash out with regular thinset. Yes, you can install ditra beneath the tub.
Roger
I am installing Ditra for the first time. The tile supply house sold me….
1) Mapei Ultraflex 2 Tile Mortar for applying ditra to the plywood- correct ?
2) Ditra set by Bostik Portland thin set mortar for setting the tiles to the ditra membrane – correct.?
3) I am installing 2 in X 2 in porcelain unglazed tiles that are adhered to a 12 in sheet. I have no idea what size trowel to use. I called ditra yesterday and they recommended a 1/4 X 3/8 deep trowel , but to their credit they continually tried not to answer what trowel to use.
One thing I learned years ago, supply house people are NOT installers. That said, I look to professionals like yourself for the correct answer.
Thanks for taking the time for all of us who are trying to do the right thing.
Dave
Hi Dave,
1. correct.
2. correct.
3. A 1/4″ x 1/4″ square notch trowel should be just fine.
Roger,
I’m planning to use ditra with ceramic, wood grain tile over a new concrete floor [less than one year old] with some settling cracks. I’m learning there may be some question as to the type of thinset [fortified or unfortified] that should be used with tile and/or with ditra. Can you please clarify the issue and let me know which type of thinset is best for this application?
Thank you,
Phil
You mean “Modified” vs. “Non-Modified” right? My understanding is that the only time you would ever use modified thinset with Ditra is when applying it to a ply-wood substrate. You are to NEVER use a modified between the tile and Ditra. That being said, you should check the tile specs for the specific tile you want to install. If it demands a modified thinset, then you need to stay away from Ditra for this project.
All can feel free to correct me if I am mistaken…….
Hi Phil,
Over concrete you use unmodified thinset both to bond the ditra to the concrete and bond the tile to the ditra.
Roger,
I installed the ditra heat mat over osb using modified thinset. Everything seems great except for a 2′ x 2′ area where the ditra didn’t adhere. Last section of the night, guess I got sloppy….
You mention redoing several times here. Am I right to understand this as scraping off the thinset down to the osb and reapply from there?
Also, I read comments elsewhere that the osb has now been “compromised” and must be replaced. Is this accurate?
Thanks!
Hi Mark,
Yes, cut out that section, scrape the thinset off the osb and reinstall it. No, your osb should be just fine. Not sure what context that answer may have been, but it doesn’t apply to your situation.