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 
Common Mistakes and Fixes
Improper substrate
Proper substrates consist of tile or wood. That’s essentially it. Specific types of wood vary, but your substrate should be one or the other – BARE, with nothing installed over it. The images above depict Ditra installed over linoleum. If your thinset will not bond to the surface you are installing over it will fail – every time. If you are unsure whether you can effectively install over something, just ask. I answer all questions.
NOT fully embedding the ditra fleece into the thinset
Spreading thinset and simply laying the Ditra on top does not constitute ‘installing’ it. The fleece on the underside of the Ditra needs to be fully embedded into the thinset. This means that thinset will encapsulate each thread of the fleece and surround it, thus locking it into the cured layer.
Once you lay it down you need to use a flat…something, to push the ditra down into the thinset. I use a concrete float or a straight-edge. You can use your (clean) trowel, a float, even a flat 2/4. Anything to press the ditra into the thinset layer.
You can see whether this is happening after you get it installed. IF it is properly embedded you can see it through the plastic from the top, those areas will appear darker (the thinset is directly against the backside of the plastic, which means the fleece is embedded).
While I don’t recommend it, you CAN use white thinset to install the Ditra, you can still see the color change in the embedded areas, it just isn’t as pronounced. Gray is always better for installing Ditra to the substrate.
Improper type of thinset used for the type of substrate
When installing Ditra you should use UNMODIFIED thinset when installing over concrete, and MODIFIED thinset when installing over a wooden substrate. With this most failures are a result of using unmodified thinset to install ditra over a wooden substrate. That will eventually fail 90% of the time.
Not properly filling the cavities when installing tile
The mechanical aspect of Ditra relies on the dovetailed cavities in the top being completely filled beneath the tile. If they are not, you do not get the full support needed, nor the proper bond to the membrane, which can lead to failure.
Fill the membrane with thinset using the back (flat) side of your trowel, spreading in every direction, THEN comb the thinset out in those pretty little lines for your tile. Properly filled cavities is the only way Ditra works.
Frequently Asked Questions
What type of thinset do I use when installing Ditra to my substrate?
When installing over a wooden substrate use modified thinset. When installing over a concrete substrate use unmodified thinset.
Can I use modified thinset to install tile to Ditra?
In reality? Yes, you can. No, it isn’t necessary. Yes, it will void any warranty. And yes, you likely have access to unmodified thinsets, even if you don’t realize it. Bonus: It’s normally cheaper than modified thinsets.
Can I fill the cavities flat then come back later to install the tile over it?
Yes, you can, with a few caveats:
Do not fill it, wait a week while treating it as a regular floor you can walk all over, then decide to install tile over it. If left untouched (unwalked) you can wait a week. But it’s always better to fill it, then tile over it the next day or two. Filled Ditra IS NOT a ‘wear layer’, meaning that once cured it is not designed to endure regular foot traffic. It’s still an ‘in process’ layer, not a temporary placeholder until you FINALLY decide which tile you want.
If you fill the cavities, try using your grout float. It works much better and takes off the paper-thin layer which will sit on top after you fill it with your regular trowel, then crumble into a dusty mess when you look at it the next day. The float keeps most of the thinset just below the surface, in the cavities where it belongs.
I’ve had the old tile removed in my entryway/hallways. It was 4″ ceramic tile that was installed with something like mastic. After popping off the old tile, quite a bit of mastic is stuck to the plywood subfloor.
I had been planning to have the new porcelain tile installed over Ditra. Will that be okay with the remnant mastic there? If not, how best to handle it? Would it be better to install hardibacker, bedded in thinset over the mastic/subfloor?
Hey Gary,
Thinset will not stick to mastic. You can scrape or sand the mastic off (if it’s white, and indeed mastic, do not try to sand it. It’ll melt and make one hell of a mess! I’d rather not talk about it…
). The thinset and hardi method would be better provided the additional height will not interfere with anything (like opening doors – I’d rather not talk about that either…).
Well, I don’t think it was mastic after all. It easily sands to powder with a coarse belt on my belt sander, with no hint of melting. A chunk of it is unchanged by soaking in water for 20 minutes. I’m not familiar with thinset, having only once seen it being used, but I don’t think it is thinset either. It is gray, and looks plastic sorta like JB weld quick set epoxy.
What could the original builder in the 60’s have used?
It’s probably old-school mastic. It wasn’t the same recipe as the stuff today. What you’re describing sounds like that.
I had the stuff tested, and it 2% asbestos — glad I only used the belt sander for 30 seconds or so.
Since the existing subfloor is only 5/8″ t&G plywood, I’m now considering covering the old mastic with another layer of plywood, rather than removing the mastic. Followed by Ditra and tile. That would end up adding ~ 1″ above the subfloor, which means that I would have to put ~1/4″ on the rest of the floor, which will be getting 3/4″ hardwood.
With the mastic still there, should something else (thinset, other) go between the two layers of plywood to fill any space?
Alternatively, just put the 1/4″ CBU as in my first posting, bedded in mastic and screwed, followed by just tile or Ditra/tile.
I anticipate that some leveling will be needed. Should this be done on top of the added plywood? If I use CBU, should that be leveled by the thinset under it, and then screwed down after it sets?
Thanks so much,
Gary
I would put thinset between the old mastic and the plywood to fill the voids. You can use the 1/4″ cbu only IF your floor is sturdy enough without the additional plywood. That’s a judgement call – I can’t see it from here.
That is exactly how you should level it, thinset beneath the backer, screw it down after it cures.
Roger,
Great site and blog.
Here is my question/scenario. I’m about to tile a small bathroom. I have post and beam construction with 2×6 t&g flooring on 4′ centers. I’m also planning on a wall-less shower with a linear drain running down the center of it. My threshold is 3/4″ hardwood and room is 4′ x 7′ plus 32″x60″ of shower
What system do you recommend that will keep the tile closest to the hardwood height?
Do I need substrate using Ditra over 2×6?
How do I get the 1/4″ per foot pitch in shower area? It’s only 16″ worth of pitch.
Thanks so much.
Hey Ed,
No, you do not need additional substrate over 2×6’s. (To my knowledge, I’ve never had one of those but it meets every standard short of the double layer, but it’s almost twice as thick as needed, so…) Ditra would keep the tile the lowest. For a wall-less shower (??? – I’m assuming curbless?) you need to either lower the substrate beneath the shower and slope it with deck mud from the floor or create a ‘speed bump’ from the bathroom floor to the entrance of the shower with deck mud. You could also slope and waterproof the entire bathroom floor with deck mud, then raise your threshold.
Gary
Good to hear about the substrate. Sorry, I failed to mention I’ll be installing warmwire as well. My concern is a flat surface for the Ditra. Should I embed my wire in a leveling compound over the 2×6’s and what should I use to keep the leveling compound from seeping through the T&G cracks? They might be tight enough, but who knows. Should I use Kerdi? What type of leveling compound if any? How thick of Ditra do you recommend?
Thanks for all the effort
Ed
Your best bet would be to use the leveling compound with the primer. Put a double layer of primer down on your t&g (which will fill in the gaps as well, if they are that small), then your wire, then slc. I prefer the laticrete slc, but most are good. Not sure what you mean by how thick of ditra. There are only two, the regular and the XL, the regular will be fine for your application.
My contractor is putting ditra xl over an existing vinyl floor. The floor is solid and would not come up. He made an error and mixed both a modified and an un-modified thinset together and used that before he put the ditra down. Should he remove this and start again before he starts laying tiles??
Hi Kevin,
Probably. But not because he mixed the two – because he installed it directly over existing vinyl. When doing that it needs to be a modified mortar specifically approved for use over vinyl.
I have 9 x 9 tile over a cement floor in subgrade family room and I want to cover it with tile. I intend to use ditra, but can I use thinset over the tiles and then put ditra down. If so, what is the best thinset to use? Alternatively, should I use self leveling compound and then put thinset then ditra? Thanks.
Mark
Hey Mark,
You can if you use a thinset approved for tile-over-tile installation. Mapei ultraflex 3 is as well as laticrete 254 platinum. Be sure to follow the recommendations of any thinset you use. You may need to scarify the surface of the existing tile. It would be better (and easier) to use SLC. Be sure to use the primer, then install the slc then the ditra with regular thinset. It’d be faster as well.
I am building a new house and will be using Ditra as an underlayment for the tiled area. The floor trusses are 16″ O.C and will have 3/4″ Tongue and groove OSB installed over that. Can I install Ditra directly to that and then lay my ceramic tile? According to the ditra site, it says it is ok, but I am reading mixed messages from people on the internetIt is a hardwood look ceramic tile if that makes a difference. Since it is a large area, it will most likely need expansion joints.
Hey Jon,
For just a 12×12 ceramic tile it’s usually fine, but an additional layer is always best. If you are installing a large format, which most wood look tiles are, with at least one side longer than 16″ then you absolutely need an additional layer of 1/2″ plywood beneath your ditra.
I haven’t seen DITRA with installation of anything smaller than 12″ tile. Is that the smallest tile that can be used with it? We’d like to use 1″ tile but have OSB as a substrate. Will DITRA work? It doesn’t look like it would, but maybe you have more experience at that and can tell me! Thanks.
Hi Cindy,
The smallest recommended tile for ditra is 2×2 inches. I would not try it with the 1×1, it’s simply too small. The reason is that the cavities are only 3/4″ square, you won’t be supporting all corners of the tile, it leads to a weaker installation.
hey roger,
im tiling a bathroom in a century home,
the floor boards are 1×6 inch (actual) pine tongue and groove.
Is this sufficient for the ditra? with some extra screws maybe?
or should i put a subfloor down before ditra?
Thanks!
Hey Cam,
You can go over the 1×6’s with ditra. Use a modified thinset for it.
Hi Roger,
Are there any issues with installing tile directly to 3/8″ gold standard plywood subfloor that is glued and stapled to osb primary floor? (total thickness of osb and plywood is at least an inch).
Hey Allen,
Yes, you’re installing directly to wood. They can call it whatever they want but it’s still going to expand and contract a lot with normal seasonal movement. You need a proper substrate over the wood. Kerdi or 1/4″ backer with thinset beneath it will both minimize the height.
I live in a small town and have very limited options when buying a trowel to put down the thinset…..I can either go with a 3/16 X 5/32 V-notch or a 1/4×1/4 square notch to lay down the thin set between concrete (self-leveler) and the ditra….which one should I use? Also is it recommended to wet the concrete before applying the thinset? Thanks!
Hey Kenny,
Use the 1/4″. Do not wet the concrete (slc), but make sure you mix the thinset properly. (Directions on the bag – really
)
so we ended up tearing up the upper layer of plywood after it became too much of a pain evening out the mortar, and put down a fresh plywood layer. We left the recommended 1/8″ gaps between boards, and I’m now wondering what to do with the gaps. I’ve read places to caulk them and others to just leave them. What is the point of the gaps if we just fill them again with caulk or the thinset?
Hi Mary,
You want to use silicone in those gaps. The purpose of the gaps is to allow the plywood to expand and contract. Silicone can expand and contract enough to compensate for that. It also keeps thinset out of the gaps, which will not expand and contract and will negate the gaps altogether.
Hi Roger,
Another questions concerning those gaps….our skill with cutting the plywood isn’t expert level, and some of the gaps are larger than 1/8″. One area has 1/2″ for about 2 inches. Is this so very bad? I was thinking of wood gluing in a slice of the plywood to fill in the gap a little. what else could I do?
Thanks
That’s about it, it’ll be fine if you do that.
Hi Roger,
So we finally go the ditra cut and down using ultraflex2 last night. I feel we got a good mix and really went over it with the float well, however, this morning, I can see a couple spots that don’t show the “shadowing” through. Does the “shadowing” sometimes not show even though it’s stuck? I’m worried that it’s not adhering in those spots. Will a few spots not stuck down ruin it? Should I cut those spots up just incase and put new ditra down?
There aren’t any large areas, simply places where 1-2 waffle spots have air pockets…I can hear a hollow sound when tapped. Over the 130 sqft, I have found 5 of these areas, so not too bad. everything else seems solid, and short of going around and tapping each waffle spot, is there any other way to tell if there are problem spots? What if I miss a waffle or two that’s not totally adhered to the subfloor? Will that create faults down the line, or will the rest of the ditra that is adhered be able to compensate?
If the bottom of the cavities are not darker, and it does sound hollow, you can cut out any areas and simply reinstall them. If it’s not bonded it’ll come up easily.
Hi Mary,
It’s fine. The thinset is still bonded to the mesh on the bottom, but it doesn’t always touch ALL of the plastic. If you look down into the bottom of the cavities you’ll probably notice that it is darker there.
Hi Roger,
My husband and I have been slogging away at our first DIY tile job since May (yes, it’s now October!) on our rare days off together. We thought we had researched how to do everyof right, but to our horror we discovered last night, AFTER the backbreaking work of tiling our bathroom floor all weekend, that the mortar we’d been assured by the big box store tile guy was unmodified, is very much modified! We had used versabond modified to lay our ditra on the plywood, and specifically bought Megalite and used it to apply our 13×13 ceramic tiles over the ditra because the guy in the store said it was unmodified. There is NOTHING on the bag that says it is modified — I only accidentally found out it had additives when I went on the company’s web site. (I bought it at HD here in Canada) Is our hard work now ruined? We have been doing this ‘by the book’ (or so we thought… Since we also used that megalite to apply porcelain tile over kerdi in our shower, we messed that up too)… We are so upset that we ended up with a different type of mortar than we intended, and instead of being able to walk on our floor 24 hours later to get to our one shower, we don’t know what to do next. Your advice would be much appreciated.
Hi Jo,
It’s fine. Have a beer – relax.
Just stay off of the floor for an extra day or so and grout as normal. Your shower is fine, too. I use modified over both products quite often, it isn’t a problem if you give it extra time to cure.
Hi Roger,
I really have never done tiling before but have undertook to do our front foyer/bathroom. The current tiles are 25 years old. The grouting is cracked and the tiles are scraped with years of use. No tiles are broken. We ripped up the tiles with not too much difficulty using hammers and crowbars to find out what was underneath. There had been a tile floor before and we could see the layout of the grout and fine layer of the original dark mortar.
Looking up at this floor from the basement, joists are 16″ oc, with sold wood sheets (don’t look like normal plywood). Up above, there is another layer of wood sheets screwed in. The tiles had just been laid on this.
After doing research, it seems that laying tile on wood isn’t the best. Is this correct? I don’t want to add any height, so was looking into using ditra.
Couple questions, do we have to sand down all the top layer of wood to get all the layers of mortar off so the thinset will stick?…the original has left a fine layer of mortar in places. would it be best to remove the top layer of wood and just do ditra on the lower subfloor ontop of the joists?
thanks!
Mary
Just to add on, both layers are plywood. The top layer is 3/4″ and I’m not sure the thickness of the lower.
We would be fine with keeping the two layers of plywood if that would make a firmer floor for the ditra. However, would the top plywood have to be totally cleared of mortar? Might be easier to remove it and put a new layer (1/2-34″ plywood) down or just ditra. We just want the best result with the ditra.
Just to add on, What I already said.
I get the questions in the order they are received, except I don’t put you on hold for three hours and play 80’s rock ballads with a brass section. 
Hi Mary,
It would be best to leave both layers of wood. It would also be best to remove all the old thinset, but it isn’t imperative. Thinset will bond just fine to old thinset, you just need to make sure the old stuff is bonded well.
Have done the ditra as above filling the cavities with ditraset, letting it dry and then applying the porcelain tiles with ditraset. Has worked out well. Now time for grout. The room is fairly big that I cannot grout all at once and be able to do all the wiping to take off excess grout in one day. I will have one expansion joint using color matching caulk on the length that is about 25 ft. Are there any tips for grouting when you have to do the room in stages so that you do not see obvious joints.
Hi Karen,
Not really, the best thing to do is measure out your grout and water to that you get exactly the same mix the next day. If you do that it’ll blend in just fine.
I am remodeling my bathroom. I have repaired the sub floor and I am now ready to begin installing the backer board. I have watched several do it yourself videos on the process. But when I got to Home Depot to buy the thin set they are telling me that I have to put a sealer over the plywood before I use the thin set that will go under the backer board. I have not seen this done on any of the videos…what do you suggest. They suggested the Henry Level Pro and the Henry Level Pro underlayment primer. And what the deal with the crazy shaped letters that you have to copy inorder to send this I am having trouble trying to fiqure out what they are.
Hi Debra,
I suggest you ignore them because they have no idea what they’re talking about. Not sure if they simply misunderstood what you’re trying to do or if they’re just trying to sell product. Doesn’t matter. If you seal it you’ll compromise the ability of the thinset to cure correctly – who knows what the chemical properties can lead to. Ignore them.
The captcha image is there so only real people post to my website rather than robots. I have over 11,000 questions (seriously) that I’ve answered. Before I installed that I would need to weed through about 50 posts a day to answer the 12 that were actually asked by people. Seriously. It helps me help you.
You can click on the little audio button and it will read out the characters for you if you need – that’s normally easier.
Hello……I have a question that seems to be vexing to everyone who has seen the issue at my house, am hopeful maybe you can suggest something.
About a year ago I had a contractor, who said he instaled a LOT of tile, replace about 7 12″x12″ squares of marble on the floor in a family room. (I had opened up a wall and needed to redo the opened area). At this persons’ suggestion I bought replacement Crema Mocha tiles that indeed looked like the ones already on the rest of the floor. They were too light though so he did a slight coffee/tea stain and they matched well. Then……he installed them on my concrete slab. 2 days later they were turning yellowish and became ‘mottled’, almost spotted.
He told me it was moisture seeping thru the slab and to wait for the Summer for the moisture to dry and the tiles would then have a uniform ivory finish.
– Now it’s been 10 months and the tiles are still yellowish and mottled looking and clearly stand out from the rest of the floor.
The contractor after many months, finally came to the house the other day and again said there was “nothing he could do”, it was a question of the concrete slab retaining water and seeping up through the tiles.
This seems very odd as none of the other tiles that had been there for years (certainly they were there when I bought the house 8 years ago) and the tiles I recently had put on the kitchen floor, same, contiguous slab, didn’t have this issue.
He said he used Thin Set (?) but didn’t use any kind of moisture backing as he didn’t think it was necessary (which I don’t understand if he now says there is water there).
You can tell by my long-winded note I’m a bit …..upset. There’s got to be a way to correctly put marble tiles on a slab…..I think they’ve been doing it in Europe for some time……. Many thanks for any suggestions!
Hi Jennifer,
It may have been moisture seepage initially which created spots where the ‘dye’ technique released out of the stone. In other words, the surface areas where it is now mottle has had the dye washed out and replaced with water. Does this area have direct sunlight on it at any point in the day? I think the biggest issue is the manner in which the shading of the tile was changed. While coffee and tea staining will work when done properly, it can lead to inconsistent results depending on a lot of factors. Once that staining was washed out of there it’s never going to even out, the tile is like that permanently now. It would be best to replace those tiles.
Any stone can be bonded to concrete, I don’t believe that’s the issue. I believe the issue was with the staining technique.
Hello Roger,
I have Virginia bluestone tiles (12″ x 18″) that I want to install over an exterior concrete slab. The slab is only half under a roof. I live in Virginia, so get freeze/thaw cycles in winter, and lots of rain throughout the year. The slate that was initially thinsetted to the concrete all failed, especially where not covered. How to do it right this time?
I am thinking of Ditra bonded to the cleaned concrete with Mapei unmodified thinset, then unmodified thinset and a 1/4″ notched trowel to set the stone tile onto the Ditra (backbuttering the tiles). Should I tape my Ditra joints? Should I use modified for either or both? Other advice? The slab is about 7′ x 16′.
Hey Frank,
Ditra would work. Yes, you should tape the seams, it will prevent excess water from getting beneath the membrane. I would actually use modified for both of those. You’ll lose your warranty, but you’ll sleep better.
A better choice, in my opinion, would be to coat the slab with hydroban then use modified to bond directly to it. It will give you more crack-suppression and movement compensation in that type of installation than ditra will.
I rarely use ditra on exterior applications here (Colorado – freeze/thaw). Although schluter does have it around the exterior of their Canadian headquarters, I don’t like the idea of moisture getting into the cavities of thinset (which it will do) then freezing. The expansion would be minimal, of course, but why bother with a possible problem when it isn’t necessary? I prefer Hydroban in that particular application.
i have instaled ceramic tile and need to know how to install the flex edging between the tile and the carpet…
Hi Gwen,
Not sure what flex edging you may be speaking of. If it is something like a schluter transition those need to be installed with the leg beneath the tile, in the thinset, as you are installing the tile. If that’s what you have you’ll need to find a different transition which can be installed beneath the carpet side.
Im laying tile and i bought ditra underlayment. it says you can use it on vinyl. do i use unmodified or modified mortar to attach to vinyl?
Hi Kathy,
Where, exactly, did you see that? I’ve never seen that recommendation or method in any schluter literature. Can you tell me where you read it?
To install anything over vinyl (which is NOT a good idea) you need a thinset which is approved for use over vinyl. There are a few, mapei ultraflex 3 i believe is one, but they are all fairly expensive. If you can tell me where you saw that I can do a bit more investigation and let you know what may be required.
Roger – I am considering using the Ditra over existing VCT. On page 9 of the Schulter-Ditra installation instructions, ie. ‘Floors, Interior – Existing Vinyl Floors’ it states that a fast-setting latex portland cement mortar must be used (ANSI A118.4 or ANSI A118.11) over any even and structurtally sound substrate with existing vinyl flooring. The vinyl in my case is laid on concrete. Finding a mortar that bonds to vinyl is the difficult task. And more info that might help me?
Hi Howard,
Mapei ultraflex 3 is approved for use over vinyl provided you scarify (rough up) the surface first.
They may recommend it and say it’ll last, I wouldn’t do it. But that’s me – I’m anal like that.
It’s been done and it has lasted.
I had a contractor do the ditra work. It failed. After investigating I discovered that the plywood subfloor had a black clothlike membrane of some sort. Preventing the thinset to penetrate. I scrapped away the old tile and thinset. The black membrane is not coming up even with heavy scrapping. I do not want to rip up the subfloor. Can I staple, screw or nail the ditra to the subfloor at this point?
Hi Amy,
What you have is called cutback. It needs to be removed before anything is bonded to the floor. Most thinsets will not bond to cutback. Screwing or nailing down the ditra will not work. You’ll only be attaching one cavity at a time and it won’t last long-term. I would call the contractor back and have him fix it or remove the ditra and screw down backerboard over the cutback.
i tried to get the ditra in using the flat side but everytime i would try to trowel with the notched aide it pulled it the thinset away and then i saw holes. i tried a wetter thinset and then it wouldnt hold the trowel lines up. then i began trowelinf the back of the tile. now im afraid to step on the floor!!! it will be 72 hours tonight. should i be worried?
Hi Deanna,
Not too sure what you’re describing. Beginning with installing the ditra – why did you try to use the flat side? The notched side pulled the thinset away from … what? Saw holes in … what? Do you mean backbuttering the tile? (trowel in the back of the tile). If it’s been 72 hours, your floor is in, and solid, it should be fine. But I really don’t know what you have going on nor what you may be having problems with.
Hi Roger,
The installation instructions for Ditra state that we can immediately install the tile after installing the ditra while the thinset is still wet. Do you agree with this? Is one way or the other preferred? Thank you,
Ruthie
Hi Ruthie,
Yes, you can do it immediately after you set the ditra. Makes no difference whatsoever unless you need to pull a tile up for some reason. If so then SLIDE the tile as you’re lifting it – otherwise the ditra will come up also.
Hi Roger
I d like your opinion on this. Schluter calls for modified over plywood or osb and unmodoified over cement. I asked my local supplier why and he said because the cement traps the moisture thus keeping the mortar wetter for longer thus unmodified is the appropriate mortar to use. Basically porous surfaces call for modified and non porous surfaces that do not allow the mortar to escape require unmofified. I recently installed Ditra over osb with modified mortar as indicates in the manual. Used the ditra trowel, worked quickly with a helper and did everything by the book. I have installed quite a bit of ditra and am no rookie. 2 days later i went to lay 12 x 24 tile over top the ditra. I had to lift one afrer laying it to reposition it and the Ditra came up with it. The mortar under the Ditra was still wet. the moisture between the osb and ditra had no dissipated. As a test I poured a couple of handfuls of water over some dry osb and let it sit there. Several hours later the water was still sitting there beaded on top of the osb. None of it had been absorbed. Over a non porous surface like osb shouldn’t I be using unmodified mortar?
Hey Rob,
Certain types of osb used in construction have either a sealer on the face, are pressure treated (which closes up the pores and infuses the product with moisture) or have a lot of exterior grade glue on the face. There is only one or two thinsets specifically designated for use over osb. To guarantee proper bonding there must be a layer of (minimum) 3/8″ plywood fastened to the osb and the ditra bonded to it.
I usually go directly to the osb in many cases, and rarely have problems as well. There is always the 1 in 100 chance that you’ll get that particular type of osb – it sounds like your odds ran out. The best thing to do, although it sucks, I know, is remove the ditra and install a layer of plywood over it. An unmodified may work but without knowing exactly why or how the osb is sealed up you may simply be attempting to bond thinset to glue or sealer – that won’t last either.
Roger: when installing the travertine ( 6×6 ) on the wall that has the shower valve ( 4″ diameter hole in backer board ) how do you secure the tile around that hole ? is there a waterproof membrane that can be installed so the tile will tile be secured ?
Thanks for all your help. I started installing the tile today and so far it looks good.
Hey Chuck,
Not too sure what you’re asking. To ensure the hole in the substrate is waterproof and that no water will get into it you want to place a bead of silicone around the perimeter of the hole then place your tile on the wall, sandwiching the silicone between the wall and tile. This creates a dam of sorts which water will run around rather than into.
Roger: Do I still backbutter the tile with ditra-set on the tile, except for a small section around where you would put the silicone to grab around the shower valve hole or just use silicone on that tile to secure it to the wall
instead of the ditra set ? sorry, I know this is a dumb question but I want it right ( even at the expense of looking stupid !! )
Thanks Again, dont know what we would do without your help.
Roger: I just went back in the bathroom and looked at the hole for the shower valve, I see what your talking about, wont be enough tile on either side of the tile to put ditra set, so I will silicone around the hole and stick the tile to the wall having the silicone hold it. sorry, but I get it now !!!
Thanks for your continued assistance, I ” really appreciate it “
You have smaller tiles? If they are small enough then the silicone will hold it, but at least try to get some thinset back there, if only on the corners. It can overlap the silicone a little bit if needed.
Roger: Thanks !!! thats exactly what I did, thinset then silicone around the perimeter of the hole…..worked great. Another question, do I grout first, then seal and enhanced seal or seal first, then grout.
You are a lifesaver, thanks again !!
None of the above.
If you are using an enhancing sealer (I’m assuming a natural stone) then seal first, grout, then enhancing sealer. If only using the enhancing sealer then use it first, then grout.
Not a stupid question at all. Thinset it as well – you just want the silicone around the perimeter. The thinset is still the only thing bonding your tile, the silicone is for waterproofing purposes.
I actually took pictures of this last week, I’ll have a post up here the next day or so on it.
I appreciated your article and wanted your thoughts on crack isolation membranes in general. We have a modern home in southern Colorado. We hired a tile installer to retile our master bathroom in 18-inch travertine. The bathroom is on a main level with a finished basement below it. Between the basement ceiling and the bathroom floor are joists (about 12 inches I believe) that are 18 inches apart on which 4x8x3/4 plywood substrate forms the foundation of the bathroom floor. The original tile was cemented to a 1/4 inch backer board, which was attached on top of the 3/4 plywood. The tile contractor said he was not going to use 1/4 inch plywood in his installation. Instead, he said, he would lay a crack isolation membrane. Recently we had to replace a tile that had cracked (due to not-enough-thinset under one corner where tile meets carpet). Upon removing the bad tile completely, I discover that there is no crack isolation membrane. We have reason to believe it may not have been installed at all. My question for you is: for the rest of the bathroom floor, how big of a problem do you think it is if there is no membrane between the tile and the 3/4 plywood? Should we expect a shorter life? Cracks in the middle of the tiles? Cracks in grout? Or do you think that 18-inch travertine cemented directly to the 3/4 plywood is sufficient? Thanks for your thoughts.
Hi John,
That is not even close to a sufficient installation method. While you CAN install tile or stone directly to plywood, it is a very DEMANDING installation method which requires specific materials in a specific order with very specific methods. Keep a very close eye on the grout lines. They will likely begin to separate from the tile first and you’ll see a tiny hairline crack between the grout line and tile. Once that happens it’s time to replace it – it’ll go downhill fairly quickly from that point.
A crack isolation membrane such as ditra (when used in that capacity) can be installed over plywood but a liquid membrane requires a backer beneath it. If it was installed with a membrane there would be a height differential between the tile and plywood surface – you would notice that.
Roger,
I think you or someone on this blog mentioned filling the ditra pockets as one step with unmodified thinset and letting that set. Then applying the tile as a separate step using the unmodified thinset. That sounds like a more controllable way to do the job and also ensure the pockets are filled. If so 2 questions. Will the thinset in the pockets and the thinset applied with the tile bond to each other as needed? When the pocket are filled and dried, is there a problem walking on it without doing damage? I am willing to do the job in small steps but if I took this approach I would have to step on the pocket filled ditra while laying the tile.
Hi Karen,
That is a much easier way to do it. Yes, the thinset in the ditra and under the tile will bond to each other very well. You actually have a more solid floor and do less damage to the ditra when the cavities are filled and cured. It’s much more solid.