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 
Alex, you’ve got good product selected, but you’ve got it backwards!
Only UNMODIFIED mortar ever goes between tile and DITRA. (Kerabond mixed with WATER.)
The Ultraflex 2 is modified mortar and must NEVER be used on TOP of DITRA. Because the DITRA is waterproof, and the porcelain tile is practically waterproof in the center, the latex and/or polymers in the modified mortar will internally “skin over” and trap the remaining water under the tile once it starts to dry. Your mortar will remain permanently wet under the center of the tiles, and you’ll never get any decent bond strength from the little amount that does dry.
For almost all uses, modified mortar goes UNDER the DITRA. (Ultraflex 2 or Kerabond mixed with Keralastic.)
There, modified mortar can dry because the substrate will wick out most of the moisture, and the channels above the fabric mesh formed by the waffle indents will also allow moisture to escape, too.
Ultraflex 2 under DITRA, Kerabond over DITRA.
That is technically correct in most circumstances, Michael, but Schluter specifically recommends UNmodified mortar for both when installing over a concrete slab. This is the information DIY’ers get from Schluter – professionals get different advice, everyone’s confused and Schluter covers their ass.
Chemically the mortar beneath even the center of a piece of porcelain will cure – eventually. The polymers will retain the moisture in the mix but the Portland will eventually utilize it all through hydration, and actually become harder. The longer water is retained in the mix the harder the portland cures. The hardest concrete on the planet is concrete which has cured underwater. The problem is knowing how long this timeline is and when it is safe to utilize the installation without breaking the bond in the tile lamination. That comes mainly from experience with specific products.
I’ll defer to your greater experience to advise others, but I’m sticking to the instructions and won’t use a modified over Ditra.
My sole experience with modified over Ditra was taking over a “cost plus” installation where the homeowner had grown unsatisfied with the prior contractor. The 18×18 tiles appeared to be properly installed, but when I took them up several days after they were installed, the mortar was still wet underneath the centers. Sorting through the receipt fiile, I found only modified mortar had been purchased, so I am assuming that was the reason. Apparently that Home Depot location stocked the Ditra, but didn’t stock any unmodified mortars, so that’s what Homer sold them.
Roger said: “Since you aren’t relying on the warranty: I would use UltraFlex 2 for both under and over. Install your ditra on the slab then fill the waffles to the top with ultraflex and let all that cure for a minimum of 24 hours. This allows the modified in the waffles to cure. Then install your tile on top of it using ultraflex. This will allow the modified to cure beneath the tile as well as in the waffles without needing to wait four or five days for a full cure.”
Why would you do that? This seems like you’re just building your own Hardibacker with the added disadvantage of having a less porous surface for the tile mortar to adhere to when you’re done.
Mortar isn’t going to stick to the orange plastic, and the cured top surface of the filled Ditra waffles becomes a shear plane for the tile to delaminate from the Ditra. Modified mortar will set up far smoother than Hardibacker. Is it likely to work anyway? Absolutely. Is it likely to fail? Certainly not. But why add the uncertainty?
It seems to me that you’re adding work, uncertainty, and a lot of extra time to the job compared to just using unmodifed mortar. With unmodified, you lay tile right after the Ditra if you want without waiting for cure.
I don’t mean to sound critical, it’s just that this is how all the Ditra critics that I’ve spoken to assume that it’s used, which is probably why so many of them have never tried it. If there’s a good reason to do it this way, please share.
Earlier, I must have missed where Alex said he was installing on a concrete slab… Of course you’re right that Schluter instructions call for unmodifed underneath the Ditra in this case, too.
Michael, can you please keep all your comments in your comment thread? A lot of DIY’ers come here and discussion of this subject confuses a lot of them. Throwing technical aspects and questions out only increases that. I will be more than happy to discuss this with you – so long as we keep it in this comment thread please – thanks!
I use many different products over ditra – both modified and unmodified versions. The key to choosing what to use comes from proper knowledge of how both ditra and the different thinsets work, alone and with one another. Factors such as time, type of tile or stone, layout, substrate requirements, etc., all play a part in my choice of product. Every time I choose a product which goes against Schluter’s ‘rules’ I immediately contact my rep on the phone and discuss it with him. Another faction is that I never rely on Schluter’s warranty – I give my own. What I want to know when I contact him is whether or not it will work and last. It will and does.
As far as ‘making my own hardibacker’ – hardi cannot be installed over in-floor heating elements. The in-plane movement restrictions for hardi are much less than ditra. Hardi will not act as a crack-suppression membrane (it does to a point, but will still transfer stresses through to the tile lamination). Hardi does not have the vapor channels beneath the installed product – no vapor management or equalization. Ditra has all these advantages in one product. I understand what you’re saying, but I use it for more than simply an acceptable tile substrate.
Water retention in thinset never FULLY utilizes all the portland crystallization method of hydration. In other words, the portland will never fully be used up in a portland-based mix. More water creates further crystal growth – that’s why stronger concrete is found underwater. When you fill the waffles, let it cure, then install tile the next day or the day after the water in the fresh thinset will activate crystals in the cured mix, which will start the hydration process – not to a great extent, but the combination of this small amount plus the full hydration in the fresh thinset will cause the crystals to interlock. So you do have a cold-joint, but not necessarily a shear plane. Modified mortar may set up far smoother than durock, but I believe it is about as smooth as hardi, perhaps less so. It is certainly more porous which enables the fresh thinset crystals to lock in better, in my opinion.
Feel free to be as critical as you want! I have absolutely no problem with that – it’s how I learn too. I’ve found that most of the ditra critics either don’t fully understand the product, nor it’s advantages (and many times I have heard the phrase making your own hardibacker – it is much, much more, which I know you understand) or they simply want to find a reason NOT to use it. I’ve simply stopped arguing with them, if they don’t want to use it Schluter doesn’t pay me to talk them into it.
In addition to the reasons I mentioned above, I can throw 323 square feet of ditra on my shoulder and haul it up three flights of stairs – hard to do that with hardi. 
I am rarely ever installing only a floor. If I am installing only a floor, and the product does not REQUIRE a modified thinset, I will always use unmodified over it – in those cases it does speed me up. I never start with empty waffles and use modified, if I’m using modified I will always let it cure first – or I will use a rapid-set.
Other reasons I do it this way: the floor is normally the smallest aspect of an installation for me. I can use every bit of my thinset – every last drop – as I’m working through a full remodel by filling waffles with the leftover thinset from the day. It’s still quicker for me to utilize ditra, even if I install and fill it one day and set the next. It still takes me two days (with a lot less work involved). I get several products in one: crack-suppression, underlayment, vapor management, etc. without the need to utilize several different, expensive products. There is almost zero waste with ditra – you’ll use the square footage you purchase. The speed aspect helps me stay competitive with other’s price points. Although I’m still normally the highest, not exorbitantly so.
I tell DIY’ers to use the product as Schluter wants them to – both to keep their warranty intact and avoid confusion. When asked if they CAN use modified, or how I would do it in their situation – I tell them. And I make sure they know it will void their warranty. If you’re comfortable only using unmodified over ditra – use it. The example you stated above after tearing out the tile and finding wet mortar was likely set with modified (a highly modified, like ultraflex II or similar) and the waffles not filled, I can nearly guarantee it. I have filled waffles and torn it apart a week later. Nothing scientific, but it was fully cured and a bitch to get apart. I like testing stuff, that simply means finding any way I can to make it fail. It bolsters my confidence.
I think your desire to use every last bit of thinset is false economy, and so is wanting to bring one kind of mortar bag and make up only one mix. Even the most expensive stuff’s incredibly cheap compared to the rest of the job, and I’m pretty sure you’re giving away a large part of the strength of the assembled structure to make the waffle fills a separate cure than doing it as a single cure with the rest of the tile bedding mortar.
If not strength, you’re certainly giving away a lot of time with the double cure, and I’m not convinced you’re getting a fully cured bond before grouting or even walking on it unless your work schedule is measured in months rather than days. With unmodified, you can grout the next day if you want, but with modified, how long are you waiting?
Schluter says the results of using latex modified thinset on top of the Ditra will lead to “unpredictable” results because when using modified mortar, the cure is extended to 14 to 60 days or more, so why not use the less expensive stuff that’s laid in one shot, the way the Ditra was designed to work in the first place? That’s what I still don’t understand.
As far as your Schluter rep saying “it’s OK but don’t put that in writing”, that’s the sort of thing I’d expect a rep to say if he already thinks you’re unlikely to stick to the instructions. After all, you both agreed he’s not going to have to warranty it, so why fight about it with a paying customer…
I think you’ve probably been getting away with “off label use” mainly because of the obvious skill and experience evident in your work shown here on this site. But quite frankly, I think that leading others who are less skilled and experienced to believe they could have the same success as you with an off label use is not such a good idea.
I think I might be this guy’s
advocate here, but if there’s knowledge to gain, I want it. On the other hand, I’d hate to see the less skilled led to try something they’re much less likely to succeed with.
Michael,
I understand you were originally replying to Alex, with information that would further confuse the issue for him. I simply separated your comments from his for that reason as well as the technical aspects of your post. You apparently work with ditra and mortars and began to get into the technical aspects of them – that isn’t why people come here to ask questions. Most people don’t know the first thing about the difference between modified or unmodified, let alone terms such as shear planes and such. I simply don’t want to confuse them any more than the issue already does.
You are assuming quite a few things here. I do not use only one type of mortar, nor do I do this to save money for any type of ‘false economy’ as you put it. If I wanted that I would haul backerboard up and do it all with that. I certainly do not do it so that I can simply take up one kind of mortar bag and make up only one mix. The assumption tells me that you are apparently uninterested in any of the several reasons I listed. You also apparently missed the part where I stated it does not add time to my installation – I’m rarely ever only doing a floor. It takes me ten minutes at the end of the day to fill the waffles, if there’s any added time – that would be it.
With some unmodifieds you can grout the next day, with some you need to allow up to three days for initial cure depending on material installed over it, temperature, humidity, etc. This doesn’t rely on modified or unmodified – it relies on the materials you’re using including the type and brand of unmodified mortar. The ability to get a hold of a GOOD unmodified mortar is virtually non-existent for DIY’ers. The closest I can find here for the general public would be keraflor by mapei – if you use that with floor tile you need to wait a minimum of 48 hours before walking on it or you WILL break the bond and your installation will fail.
Schluter does state ‘unpredictable’ results – when an installation is started with empty waffles and tile is installed with modified mortar. The dovetail cavities are what slow the curing time in a modified mortar. If that is cured that aspect is no longer a factor. You believe that the two different cures create a shear plane – I simply disagree. I have assembled many different mock-ups to try and determine whether this is a factor or not – based on the type of installation I do. I don’t believe it is. If you think my Schluter rep is simply patronizing my methods because he feels there will be no liability for him – fine. Again, you are making assumptions without factual basis. Did you know Schluter does not require unmodified over ditra – in Europe? They changed that over there.
If you read through my answers here you may notice that I NEVER tell anyone they will have the same success as me with my methods – ever. As I’ve already stated, I instruct them to do it in the manner Schluter intends. The main reason, by the way, I moved your comment initially – he was going over concrete for which Schluter wants unmodified. I’ve also stated that if asked if they CAN use a different method or how I would do it in their situation, I tell them.
The term you’ve used, “getting away” with my method is far from what I’m doing. This implies doing something without knowing whether or not it will last – I don’t do that. If you disagree with my methods that’s fine. The bottom line is the different methods or acceptable solutions for each project come with experience – and a LOT of screw ups from which to learn. There are several ways to do anything – many of which would work. The manner in which ditra is utilized, and with which thinset, is no exception. In this case we simply disagree on acceptable methods. While mine is not *technically* acceptable by Schluter’s standards, they work. If you want to stick to unmodified as intended – great! Everyone has their methods. I think that anyone that reads anything here should know damn well I’m not really one that’s gonna follow the rules all the time, eh?
I also believe that eventually Schluter is going to give in on their ‘intended’ specification for unmodified mortar – they already have done away with that requirement in Europe. Couple that with the three companies that have developed an uncoupling membrane with the same advantages and no requirement for unmodified and I think their bottom line is going to start taking a hit here in the states. When the other products are more readily available and do not require unmodified someone is eventually going to notice that.
I think this basically comes down to our difference of opinion on the strength of the installation when two different cure factors are involved. We’ll need to simply disagree on that aspect as I can’t determine a difference in the two for my purposes. If I’m only doing a floor and simply setting and grouting – I’ll use unmodified. If I’m simply setting an onyx floor with glass inserts and ditra is specified – you can be damn sure there isn’t gonna be a bag of unmodified in sight, and the installation will be just as durable in my opinion. It better be – my companies reputation rests on my method decisions – I don’t take that lightly.
You’ve been patient and very descriptive so far, but after all this, my question is pretty much the same.
You’ve stated that you use modified thinset over the ditra most of the time. You’ve recommended the same to others here.
Why would you bother to use modified thinset over ditra most of the time when it directly contradicts Schluter, adding time, expense, and uncertainty when it’s so simple just to remember to use unmodified?
I’ve seen three so far; it’s faster for you, it lets you use “every last drop” of your thinset, and you occasionally set exotic materials like onyx. Call me cautious, but the first two don’t seem like sufficient reason to contradict the manufacturer most of the time. The third I have no experience with, so I’ll be loaded with questions if I ever have need to set something more exotic than ceramic and porcelain.
I’ll watch for your answer, but beyond this, I’ll put down the beating stick. I think the horse is plenty dead by now.
You’re right – dead horse.
It’s not that I don’t remember to use unmodified, Michael, it’s simply much easier, quicker and more convenient most of the time. I am of the opinion that the unmodified requirement is unrealistic for most people – including the DIY’ers who come here asking for advice. I know from experience that modified works if it is done correctly – that’s all. I did lug ten bags of Laticrete’s 317 up two flights of stairs today for a floor with ditra, I don’t always use modified. The other problem is to get one of the three GOOD unmodified mortars I need to order them from the closest supplier – 100 miles away. The only three I’ll use is Laticrete’s 317, Mapei’s kerabond, or ditra-set (the last being virtually unobtainable) – that adds time.
I do it with various stones and because it saves time, the fact that I can use up thinset is simply an added convenience – it does not play a factor in which type I’ll use in specific situations. I will usually choose which to use based on what I’m installing rather than what Schluter suggest. It boils down to what makes my job easier. Sometimes modified makes it easier. The unknown ramifications everyone think exist I don’t believe to be factors, I’ve proven this over time and many, many installations. Yes, it goes against manufacturers suggestions (it is not actually a requirement or specification – they actually only ‘suggest’ it now – check out their faq page). They are slowly and quietly moving toward allowing modifieds in my opinion. The unmodified requirement has not only lost them business, which will become a larger problem in the near future, but it actually makes the contractor consciously choose a different product due to the inability to obtain a quality unmodified mortar – most supply houses will not stock it.
The last swing I’ll take at this horse is the fact that DIY’ers have virtually no access to the good stuff. They see the requirement for unmodified and pick up the $5.00 bag of custom’s powdered dog shit at Home Depot – it meets the ANSI specs, and their floor falls apart after six months. They don’t know modified from unmodified, how do you explain to them the difference in unmodified thinsets? I believe that would end up to be much more detrimental advice than how to use modified thinset with ditra.
Using modifieds (for me) does not add time, the expense ends up being roughly the same, and the uncertainty is not a factor.
You know, you’re actually one of the few people who have been able to discuss a subject on which we disagree with civility?
I appreciate that very much – thank you.
Regarding availability of good unmodified, you might want to try this: Get in touch with a few of your contractor and/or builder friends and pick one local supply house. Come to consensus on a few mortars you’d all be OK with using. Then give the supply house a list of names of people who’d like them to carry what you want. If they see several people want it, they’ll stock it, or call you back with the nearest equivalent they can get from their distributors.
That’s why a supplier near me now stocks Kerdi and Kerdi-band. It helped that the supplier already stocked Ditra, but wasn’t getting much interest until he got a list of people who were looking to buy it somewhere other than just Homer’s place. Now he’s steadily adding other Schluter products, too.
so if i use the 317 instead of the kerabond what size trowel should i use just so you know i will be filling the ditra cavities before i tile if that makes a difference on what size to use.
Same answer Randy, as long as your ditra is flat you can use a 3/8 x 3/8″. All thinsets shrink. If your tile is set correctly none will shrink enough to make a significant difference in the flatness of the tile installation. We do not change trowel sizes depending on expected amount of shrinkage – we would change the setting material. Trowel size is dependent upon both the size and flatness of the tile and flatness of the substrate.
A 3/8 x 3/8 trowel will leave a 3/16″ bed of thinset. Given an average of .05% shrinkage it may drop your tile less than 1/107th of an inch. Unless you’re measuring it with a laser it is not noticeable to the human eye. Since the entire bed will shrink at roughly the same rate (i.e. adjacent tiles will drop roughly the same amount) the actual variance would be a maximum of less than 1/214th of an inch, likely much, much less – my laser isn’t even that accurate.
You’re really overthinking the shrinkage aspect. Guys like me have already studied and argued it to death – it isn’t significant in your situation. Really.
I do appreciate your OCD, umm… – atttention to detail, though. At least I’m not the only one.
thanks – i was just confused because i checked on the kerabond info and here is what it says: Designed for interior and exterior floor and wall installations of vitreous,
semivitreous and nonvitreous ceramic tiles, ceramic mosaics, quarry tiles,
pavers, and natural-stone tiles.
• For installations in freeze/thaw environments or with special substrates,
use Kerabond with undiluted Keralastic or other polymer additives.
(See the Technical Data Sheets for Keralastic and other polymer additives
for details.)
• For installation over smooth, steel-troweled concrete, glass, vinyl
asbestos tile (VAT) (residential and light commercial use only) or old
cutback adhesive (residential and light commercial use only), and for
use over exterior-grade plywood (interior dry floor areas only), use
Kerabond/Keralastic™.
• For setting impervious or glass-like tiles, gauged slate, polished-back
marble, granite or porcelain tile, use Kerabond/Keralastic.
last sentence that got me says for porcelain tile use kerabond/keralastic
Those specs are for installations over flat substrates which rely on a chemical bonding to the substrate. Ditra is a mechanical bond between the membrane and tile. And a good unmodified thiset like kerabond has absolutely no problem attaching porcelain to ditra. (Porcelain tiles are actually ceramic tile which are fired twice anyway – ancient chinese secret!
)
Roger,
I am installing 12×12 tiles on my foyer, which is about 40 sqf and got concrete slab underneath. I peeled off the exsisting lamimate tiles that have orange adhesive as well as black glue on some small spots. I intend to install Ditra on the concrete slab and porcelain tile over it. My questions are:
1. how much do I need to get rid of the adhesive? Until completely no stickiness whatsoever or don’t have to worry about too much?
2. what kind of thin set do I have to use?
3. can i use 1/4-1/4 u-shaped trowel for installing ditra?
In addition to above, any suggestion you could give me will be greatly appreciated.
Thanks!!
Hey Alex,
You need to get as much of that adhesive off as is humanly possible. Thinset will not stick to it. Splash a couple of drops of water onto the slab and see if it soaks in, if it does then you’re fine. If it beads up then you will need to grind or scarify the surface of the concrete to give the thinset something to ‘grab’. Use a good modified thinset sold in a bag to which you add water – none of the pre-mixed stuff, no matter what the salesman tells you. You can use a 1/4 x 1/4″ u-notch trowel for the ditra. The most important part is to make sure the concrete will allow the thinset to adhere correctly.
Roger, thank you for your quick reply. I am kind of confused about the thinset — what is diference between “modified” and “unmodified” thinset? I found Mapei brand products such as Ultra Flex 1 (and 2), are these ones I can use for installing Ditra and porcelain tile over it? Do I have to mix any additives besides water (which the guy at Lowes recommended me). Sorry a lot of questons. Alex.
Modified thinsets have powdered polymers in them. Basically what these do are retain water into the mix for a longer period of time to ensure full hydration for the portland and strengthens the cured product. Some also lend a bit of flexibility to the cured product to allow slight movement without breaking the bond. Adding a latex additive to an unmodified thinset you end up with essentially the same thing. If you buy a modified thinset do not add additional products to them – water only.
Schluter does not want modified thinset used on their ditra. The waffles in the design do not allow the polymers to cure quickly or, in some cases, effectively. I guess they figure the design allows enough moisture retention in unmodified to not need the polymers. Do not add anything to your unmodified thinset between the ditra and tile unless you want to void your warranty. You need modified between your ditra and substrate, either specifically modified or unmodified with the additive, and unmodified between ditra and tile. You can install porcelain tile with either one – really.
Thanks again for your quick and helpful advice. The Schluter site says use unmodified to install Ditra over concrete slab. In fact, it seems there is a lot of discussions about modified vs. unmodified by DIY people and professionals. The only consensus I see is use unmodified thinset for installing tile over ditra. On the other hand, there are two complete opposite opinions regarding which thinset to be used under ditra. Some say modified is just fine regardless of underneath but others say use unmodified as instructed by Schluter. This is where I am so confused.
Since I am installing Ditra over concerte slab, I am suppposed to use unmodified according to the company. What would be the reason for recommending modified thinset against it? Is modified thinset easier to apply compared to unmodified?
Sorry for many questions over and over again. This is my very first tile job and I maybe asking stupid questions?
Hey Alex,
You aren’t asking stupid questions at all. The main problem is that Schluter does not allow any of the reps, customer service, anybody to answer even the very basic questions online concerning their products. It’s absolute bullshit and pisses off every professional tile contractor I know – myself included. It seems to be a veil of ‘cover my ass’ on their part and is, frankly, an unacceptable company position in the year 2011 in my opinon.
Another problem with it is their specification to use a thinset ‘suitable to the substrate on which it is installed’ – that confuses a lot of professionals, DIY’ers haven’t got a chance with their double-talk. Unless given very, very rare circumstances where an unmodified would be REQUIRED for the substrate I ALWAYS use modified under the ditra – cement or not. Now that said, I couldn’t care less about their warranty – I give my own. If you want their warranty you need to use unmodified beneath it on a concrete slab. Reasoning (from them, anyway) I assume would be the same in regards to water-retention in the portland of the mix. Unmodified between a slab and the ditra apparently doesn’t require additional water retention according to Schluter. I may be incorrect about that assumption, since they won’t reply online – TO ANYONE! – I have no way of knowing that.
Strangely enough when I speak to my area rep over the phone or in person about methods that go against every written spec they have for their product they seem to have no problem having those rules broken – they just don’t want it in writing. Again, bullshit cover-my-ass shenanigans. I love their products – I hate their business model.
If you want the warranty – use unmodified (no additives, either). If you don’t care – use modified beneath it. They both work, I never use unmodified beneath it. The choice is yours.
Roger,
If tile professionals like youself have serious debate going on regarding which thinset to be used, I would no way be able to figure that out. Now, I am leaning to do the following for my first tile job. If you could give your opinion I would really be appreciated. I do not care about warranty. All I care is not to use incorrect thinset.
– Use Mapei Kerabond for installing Ditra over concrete slab
– Use Mapei UltraFlex2 for installing porcelain tile over Ditra
In your profesional opinion, what would you suggest?
Thanks again, Roger. You have been so helpful for someone like me who have never done tiling project.
Alex
See Alex, another debate going on about it at the top of the comments.
Since you aren’t relying on the warranty: I would use UltraFlex 2 for both under and over. Install your ditra on the slab then fill the waffles to the top with ultraflex and let all that cure for a minimum of 24 hours. This allows the modified in the waffles to cure. Then install your tile on top of it using ultraflex. This will allow the modified to cure beneath the tile as well as in the waffles without needing to wait four or five days for a full cure.
Wow, there really IS a serious debate going on… And I really appreciate both you and Michael because this is great learning opportunity for me.
Let me ask you some another question Roger.
1. What is the disadvatage of using modified over concrete slab and beneath ditra, if any?
2. What is the disadvantage of using ummodified over Ditra for installing porcelain tile?
3. If modified thinset is used for installing Ditra over concrete slab, how long you should wait before installing tile?
Thanks again for your help!! Actually I feel really fun to learn!!
Alex
Yup, I wasn’t kidding when I said professionals can’t reach a consensus.
1. It depends on how readily your slab will suck in moisture. If it is basically just concrete – without any type of glue or sealer on it, then I don’t believe there is an advantage or disadvantage. Modified will eventually cure harder due to moisture retention in the portland. However, if your concrete is not very porous and has a more dense surface then a modified may not cure as quickly due to the excess vapor needing to dissipate through the vapor channels in the bottom of the ditra rather than moisture being pulled out of it by the concrete as well. In that case you want an unmodified since the lack of absorption in the concrete will help retain moisture in the thinset anyway. This is why you want to splash water on the concrete and see if it will soak into it and how long it takes.
2. The biggest disadvantage is the lack of allowance for a proper cure before walking on it, that breaks the bond. Depending on the size of your tile this could take up to two weeks (or longer) if you simply fill the waffles and set tile at the same time. With larger format tiles if you fill the cavities and let it cure, then install the tile it will mostly cure within 48 – 72 hours. 12 x 12 porcelain normally takes about 24 – 36 hours.
3. Again, this depends on your concrete as far as how long the thinset takes to cure. However, as long as you are not using the ditra to level out your slab (i.e. you don’t have areas with a thick layer of thinset for leveling purposes) you can set tile immediately. You do not need to wait. Thinset forms a mechanical bond to the fleece beneath and the layer is not thick enough to be affected by pressure from above.
Roger,
I am planning a 24″x24″ porcelain tile installation over a 5/8″ OSB subfloor (16″ OC for the trusses). I will also include a radiant heating wire system. I was going to go this way : vapor barrior (room over crawl space), install wires over existing OSB and vaport barrior, pour self leveling compound (is A118.11 self leveling?) on top of it, cure, then portland mortar, ditra, 118.1 mortar then porcelain tiles. Would this setup be sufficien for huge tiles over basically a flimsy 5/8 OSB subfloor? Should I add a layer of plywood over the OSB to make this stronger?
Also, what do you think the total thickness will be (tiles are 0.43 inches thick). Beside, I have 3/4 inch hardwood directly over OSB.
Cheers,
Hey Marc,
Dunno what type of vapor barrier you’re planning on but I would go with a liquid such as redgard or hydroban. It would eliminate height issues a bit. You absolutely need another layer of 1/2 plywood over your osb. The combination of large format tile and in-floor heating will cause a lot of differential movement. I put the barrier between the osb and plywood layers.
118.11 is a designation of a tile and stone mortar – not a self-leveling cement. SLC’s are a different product. You also need to use the primer over the plywood for the slc. Install your second layer of ply, then use the primer, then install your heating wire. Trying to prime it after you get your wire in is inconsistent and a huge pain in the ass.
Thanks for the quick advice. Schluter gives guidance on minimum tile size but had no data on 5/8 OSB with radiant heat and 4 square foot tiles. Will add 0.5 inch plywood over the osb. Unless my english is that bad, am I reading that you recommend installing the radiant heat wire on top of the SLC? Is the SLC what you refer to the ”Primer”?
No, the slc needs a layer of primer which is sold with it, or at least available in the same place, and needs to be brushed, rolled, or flat-troweled onto the plywood. This allow the slc to ‘grab’ onto the substrate and not crack under pressure.
So plywood, then primer (it’s like a thin paint), then heating wire, then slc. Your English is very good, mine sucks.
Hi Roger. I just installed some ditra over exterior grade plywood with some thinset recommended to me by my dealer (servolflex trio supertec by kiesel) I was just looking at the specs on it and it is only ansi 118.4 not ansi 118.11. Do you think I will have any issues because of this? I am quite upset as I did tell him I was tiling over plywood.Your opinion on this would be appreciated.
Hey Ray,
118.4 is a modified thinset suitable for installation of ditra to exterior grade plywood. The difference with 118.11 is the .11 is approved for installation of tile directly to the exterior grade plywood, makes no difference with the ditra to the wood as long as you have 118.4.
you have no idea how happy your answer has made me :)….I was afraid …very afraid ….that i would have to tear out and start again. Thanks Roger!!!
Hi Roger. I notice with the porcelain tile im using that when they get wet, there are rings that show up on the surface—im guessing suction cups or something used to lift them in production that has left an imprint. I noticed this when using the wet saw and I tried washing them with various cleaners but it doesn’t make a difference. I have checked samples from several boxes and its the same thing. Once the tile is dry, you can’t see these anymore. Should I be concerned about this?
thanks Roger
Hey Ray,
Nope, nothing to be worried about at all. They are from the machine’s suction cups while they are being made. They will eventually wear down and not be noticeable anymore.
Thanks Roger. I haven’t installed them yet so I took one back to the tile store today and showed them. They said they’d never seen that before and are going to call the tile rep to ask about it. I have lots of other samples from when we were picking a tile and none of them have this. The Tile store guy even tried a couple of cleaners they use on stains and couldn’t remove it. I guess im a bit worried about putting down a large amount of this tile and it never going away or even getting worse over time. Wife isn’t happy about it either!
Hi Roger. What are your thoughts on leveling systems such as raimondi? Is it a good idea for a diy install of 16 x 16 inch porcelain floor tile?
thanks
Hi Ray,
Haven’t ever used the raimondi. I’ve use the LASH, which is similar and it worked fairly well for its intended purpose, but it’ll kill your thumbs. The raimondi uses a tool to accomplish the same, less thumb strain. I own and use the TLS – it is, in my opinion, one of the best, most useful tools ever made for tile installation. Any of these would work just fine for a small floor or single installation – the TLS, and possibly the raimondi, are more user-friendly to multiple uses.
Hi again Roger. Need your opinion. We (well, my wife)ended up picking a 12 x 24 inch porcelain tile for the floor. I was going to install it in a running bond pattern with 1/3 offset to minimize lippage as the tiles do have a slight bow in them (approx 1/16 of an inch) I can’t eliminate the lippage completely and there is about a dimes thickness worth of it when the tiles are placed on a perfectly flat surface (my tablesaw top!). I was going to use 1/8 inch grout line. In your opinion, will this look ok and is it pretty normal for this type of tile? Appreciate you thoughts—ty
Hey Ray,
With the large format tiles it’s always a crapshoot, so yes, it’s normal. Most are bowed to a certain extent. The 1/3 offset, which happens to be a new TCNA standard for large format tile, looks really good and helps tremendously to minimize the lippage.
Thanks Roger!
Hi again Roger. I was wondering what your opinion is on the mesh/scratchcoat installation method for floor tiles. I’m using ditra but a friend of mine just had his kitchen/hallway tiled and the installer used this method.
Hey Ray,
What you are described is commonly known as a ‘Jersey mud job’. It IS NOT an approved method and I’ve made a whole lotta mortgage payments from tearing those out and replacing them when they fail. Google it – they’re garbage.
I’ve a concrete slab with old cutback adhesive from removed tile. Can Versbond (slightly modified thinset) work under the Ditra? What trowel size would you recommend for a slab application?
I plan to use Ditraset (unmodified thinset) to lay my 16×16 tile on top of the Ditra. What trowel size would you recommend?
Great website & pics!!!
Thanks in advance.
Hey Sam,
Yes, versabond will work beneath the ditra. You want to remove as much of the cutback as possible and ensure that the concrete will absorb moisture. Splash some water on it and make sure it will soak into the slab. If it does not then it will need to be mechanically or chemically scarified in order for the thinset to properly adhere. You can use a 1/4″ x 1/4″ trowel beneath the ditra and a 3/8″ x 3/8″ or larger for your 16″ tiles.
Sir:
I’m considering installing tiles on my exterior sundeck.
The traditional method to avoid problems would be, once the safe load bearing factors have been established static and dynamic- is to install a cement board over the existng plywood floor, then install the tiles….until I discovered this DITRAoduct, which appears to imply that cement board would not be required.
How does this DITRA works on exterior surfaces? have you have any experiences – good or bad – in that regard?
Thank you for your information
Regrads,
CF
canada
Hey Carlos,
Ditra is a good product for exterior applications provided it is coupled with kerdi-band and proper drainage is taken into consideration. You can read about the advantages on Schluter’s exterior balcony installation page. There is a lot of technical information on their site about exterior installations. You can also call their technical department, they’re always extremely helpful with any questions I call them with. I have used it a couple of times on exterior decks NOT over living spaces here in Northern Colorado with freeze-thaw conditions – they’re all still in extremely good shape with no problems.
Hi Roger. I have installed plywood over osb in preparation for ceramic tiles in my hallway. I had intended to use ditra but I miscalculated the height so that I will end up 1/4 inch higher than adjacent hardwood floor. I really wanted to make it level as I think it looks a lot better that way.Is there any other product similar to ditra that you would recommend that would not add as much height? I saw a product by mapei called mapguard 2 that looks thin enough. I calculated ditra will add approx 3/8 (with thinset) and my tile is about 1/4 inch. My height from new plywood to hardwood is about 3/8 of an inch. Thanks in advance.
Hey Paul,
Never used the mapei product – I’m always hesitant about peel-n-stick anything except beer bottle labels. I have used the Tavy thin-skin and it seems to work just fine. No matter which you decide to use you only have a total of 1/8″ for your underlayment and tile (height from ply to hardwood, 3/8″, – 1/4″ tile thickness) – that really isn’t enough for a correct bond no matter which product you choose. You should probably just stick with the ditra and find a transition that makes it less noticeable, you’re only talking about 3/8″ maximum height difference, probably less. That’s not a bad trade-off for a guaranteed proper installation.
Thanks Roger. I’ll take your advise and stick with the ditra.
Hey Roger,
I’ve found your website randomly while searching for information on Ditra installation. You see I am re-doing my bathrooms (about 68″ x 64″) and want to replace the linoleum flooring with nice ceramic tiles. I am leaning toward rectangular tiles (Monet) and believe they are 5/16″ thick (I will get samples from the store next week so I can confirm thickness).
I live in a Condo building to its difficult for me to measure accurately the joiusts spacing but I can pretty much confirm there is a joist right in the center of the room. The subfloor seems to be 3/4″ OSB and there is a 1/4″ plywood on top. Nothing is currently screwed, just gun nailed to the OSB.. (
) The tile store recommended that I pull the plywood out and re-screw the OSB to the joist, and then install a Ditra-XL membrane. My question is regarding cracking and thickness. Next to the future tiled floor, there is a bedroom with thick carpet. My goal is to install hardwood or laminate, not yet sure. What are you recommending ? Is it a good installation with Ditra right on top of OSB? I have a preference to keep the plywood and just screw it to the OSB every 4 inches or so.
Thanks a lot!!!
Hey Louis,
Ditra XL is rated for installation over single layer plywood application (for ceramic or porcelain). You can keep the 1/4″ ply and screw it down provided you can cleanly remove all the residual glue and paper from the linoleum – as long as it is actually 1/4″ plywood and not luan. If you cannot remove at least most of that it would probably be best to remove that layer of ply and replace it with another layer of plywood. You can use 1/4″ to minimize height, 3/8″ or 1/2″ would be better. You should, however, have a suitable substrate for the ditra – that being plywood rather than osb.
Hi Roger.Thanks for the great site and all your help. I have installed 3/4 inch exterior plywood over 5/8 osb with 1.5 inch flooring screws every 4-6 inches. Joists are 2×8 16 oc with span of 10 ‘ 6″. I will be installing ditra over this.As you are a flooring God, can you bless this as being a good base for 13 inch porcelain tile so that I will be able to sleep at night after finishing? :). Also, is there a simple way to mix thinset (by weight or volume for example) to know for sure that its correct for ditra? Thanks in advance.
Pete
Hey Peter,
You’re just fine for porcelain. Your floors deflection is about L/425 – more than enough for ceramic or porcelain and you have all the proper layers on there – well done!
There is no specific measurement for a ‘loose’ thinset beneath your ditra – and schluter won’t give you one. Mix it per the directions on the bag and add about another 1/4 the amount of water it calls for. You want it to be able to still hold the ridge after you trowel it. The easiest is just to mix it and pull up the corner of a piece of the ditra after you get it installed to ensure you have full coverage beneath it. If it is not fully embedded you either have it too thick (if the ridges are flattened on the floor and it isn’t sticking to the fleece) or you need a larger trowel (if the fleece is not making full contact in the thinset on the floor).
I am not a tile god. Minor deity, perhaps …
Thank you Roger…you are too modest! Since you did so well on my first question, I’m going to ask you a bunch more to see if you truly are a deity of some kind.
1. 3/16 by 3/16 notched trowel….is that a good one for ditra on plywood?
2. What size trowel would you use for 13×13 porcelain tile over ditra?
3. Do you recommend any particular brand of thinset and grout?
4. Do you wear a robe and sandals when tiling?
Hey Peter,
1. I use a 1/4″ x 1/4″ but yours should be just fine.
2. 3/8″ x 3/8″ or larger, depending on the flatness of your floor and tile.
3. I’m a Laticrete whore. The 253 is a very good thinset for your application. If you’re concerned with the Schluter warranty you can use the 317 which is an unmodified, it works well.
4. And a cape.
Hi again Roger. I’ve come up with another question as I progress with my install. Most of the floor is quite flat but I have one area of about 8 by 8 feet that has a “hump” in the middle from floor joists being installed out of level. Would it be ok to install ditra normally and then using medium thinset ,screed the area level over the ditra? I would end up with approx 1/2 inch of medium set (at the high point) which I would let dry and then lay my tiles normally over. This seems like the easiest way to fix my problem if its acceptable but I’m always open to suggestions from caped tile setters! ty.
Pete
Hey Peter,
It would actually be better to screed the area with medium-bed thinset before you install your ditra. That way once your ditra is down you have a totally flat substrate for your tile installation.
Hey again there Roger. I was wondering if with the tile i’m using (13 by 13 by 1/4 inch thick porcelain) and unmodified thinset (mapei kerabond) over ditra, there would be any problem with the mortar shrinking as it cures causing the tiles to shift when using the 3/8 by 3/8 or larger trowel size you recommended?
Hey Peter,
There will be shrinkage – not the swimming pool type. The best you can do is either use a medium-be mortar, which is made specifically not to shrink, or make sure you have absolute coverage and smack the tiles down really well as you install them. As long as you get 100% coverage the shrinkage won’t be significant enough to drop the tiles to the point of being noticeable. With 100% coverage the 3/8 x 3/8 will only leave a 3/16″ layer of thinset. With shrinkage an average of .05% that’s about 1/107 of an inch. Put your micrometer away.
Thanks Roger. More questions–sorry!….If you were doing this, would you use medium bed mortar?. The only ones I have seen appear to be modified. Any issue with them curing properly over ditra? Would it be easier to correct “lippage” with it being as I can put down a thicker layer?
I probably would use a medium-bed. When I do that over ditra I will pre-fill the waffles (with either unmodified or modified) and let that cure, then install the tile with whatever you want. I do it almost weekly. Keep in mind that this will void your warranty from Schluter! Properly installed, however, you won’t need no steenkin’ warranty. Medium-bed makes it very easy to adjust for lippage.
If I am using Ditra for tiling my kitchen, foyer, bathroom and laundry room is there a maximum tile size that is able to be used with Ditra?
Hey Nate,
Minimum recommended tile size for ditra is 2″ x 2″. There is no maximum. You can put granite slabs on it if you want to. And if that’s what you’re planning on doing – send pictures.
Unfortunately my ’80’s house has particleboard over plywood floors. Can I put backerboard over the particleboard,then Ditra, and tile over the Ditra or do I need to replace the particleboard with plywood???? I am doing a small bathroom, 5×8.
Hey John,
Unfortunately that particleboard needs to be removed. When particleboard gets any moisture at all it will swell up. This is going to negatively affect your tile installation no matter what your substrate. Particleboard does not belong anywhere near a tile installation (or near a tub or shower, for that matter).
Roger,
I just installed ditra in preparation to lay floor tile in the bathroom. The ditra was attached to plywood using modified thinset as per the manufacturer. I have a feeling the thinset I use was mixed a bit too thin. (I would say the consistency of elmers glue, the thinset held the notch but did start to sag a bit) Will this be ok or should I re-lay the ditra before moving on. Thanks for any information.
Hey Scott,
You should be fine. The thinset should be mixed a bit loose anyway. That’s a bit more than normal but as long as you got good coverage under it and embedded it really well you’ll be fine.
I installed the 2×2 -12 inch mesh tiles over the Ditra with Mapei Type 1 thinset 7 days ago. It is not drying and what is starting to dry does not seem to be sticking. Did I use the wrong thinset? Please let me know if this is salvageable or if I need to start over and just install the tile over cement backerboard. Your advice would be greatly appreciated.
Hi Jimmy,
Yes, you used the wrong thinset. Actually you didn’t use thinset at all – you used mastic unless you meant mapei ultraflex 1 rather than ‘type 1’. Mapei’s type 1 is a mastic – it was pre-mixed in a bucket, correct? When installing tile over ditra (or ANY flooring application in my opinion) you need to use an unmodified powdered thinset sold in bags that you mix with water. The pre-mixed stuff is never, ever going to cure fully, and the parts that do will not remain stable – as you’ve already noticed.
You need to scrape all that tile off of there, replace the kerdi, and install it with the correct product. Sorry, all I seem to be giving tonight is bad news.
Thanks Roger. I suspected as much. I have ripped everything out and started over, an expensive and time consuming lesson. I’ve gotten better advice and have done my research this time. I appreciate the help.
Hi Roger…
After reading the Ditra instructions I laid down an OSB underlayment over my plywood subfloor as directed. Now that I’m about to put down the modified thinset (SpeedSet from Custom Building Products) I read that it not to be used over OSB.
After looking around on the internet for an alternative thinset that CAN be used over OSB I’m coming up empty…any suggestions? What are the consequences of using the SpeedSet over OSB? I don’t want to lay down plywood over the OSB and make my floor any higher and I REALLY don’t want to take up the OSB after all the work it took to put it down in the first place!
My other question is what sized trowel would you recommend for laying 12″ x 24″ porcelain tile? I’ve got a 1/4″x3/8″ square notched trowel but was wondering if a 1/2″x1/2″ would be better or necessary. I plan on filling in all the waffles per your instructions as well as using the smooth edge to back-butter the textured surface of the tile. Also, I’m using unmodified Mapei Kerabond over the Ditra if that’s any help.
Thanks for the great advice!
Hey Gregg,
Oops.
There is no thinset currently available (of which I am aware) that is approved for use over osb. The problem with osb is the individual ‘chips’ in the board. If it does begin to come apart it will be random. It’s just not really a good product to install anything over with thinset. The BEST option is to remove it or install 1/4″ ply over it. Another option is to forget the ditra and install 1/4″ backerboard over the osb with thinset (to support it) and proper screws.
With all that said – I’ve used modified thinset over osb for ditra in the past. Not saying it’s correct, just saying I’ve done it.
A 1/2″ x 1/2″ trowel would be your best bet. If you place two of those tiles back to back (rather than stacked in the same orientation as they are in the box) you’ll likely find that they are ‘cupped’ to an extent. This just means that when sitting flat on the floor the center of the tile actually sits a bit higher than the ends. You should see a space between the center of the two tiles when you do that. It’s completely normal – but the larger trowel will ensure proper coverage.
Hey Roger…
I ended up biting the bullet and removing all of the OSB. The thought of saying “screw it” and just hoping for the best certainly popped in my mind but so did the thought of wasted Ditra, tiles, and time if it didn’t work out…not to mention my wife would probably never let me tackle another home improvement project ever again if I wasted all that money.
Why am I not surprised to hear you say you’ve used modified thinset over OSB? You’re a Ditra maverick! Being a Ditra newbie I figured I’d better play it safe and replace the OSB with 15/32″ plywood… 
Ditra directions!
As for laying the tile, I was originally planning on doing a brick pattern with the 12″ x 24″ tiles but read somewhere that for large tiles (a side larger that 18″) warping or maybe this cupped effect you mentioned increases the possibility of lippage. They recommended overlapping 1/3 of the tile and said not to overlap 1/2 of the tile to make the lippage less noticable. Any truth to that? Seems to me I’ve seen a lot of large tiles in a brick pattern (of course I’ve never closely inspected the tiles either) and it didn’t really make much sense to me. I’m just trying to avoid having my bathroom end up in the “Flawed” section.
Well, gotta go get me a 1/2″ x 1/2″ trowel…thanks again for your pearls of advice.
Hey Gregg,
Good choice! The running bond, or brick pattern you’ve mentioned, with large format tile (which yours is) should be placed with no more than a 1/3 offset. The cupping is the exact reason that standard is now in the handbook. As larger format tiles became more popular manufacturers pumped them out non-stop. This led to not necessarily more warped tile, but led to it being noticed more since it was all over the place and not being installed correctly.
The 1/3 offset will help compensate for the cupping. When you have 1/2 offset you have the lowest point of the tile right against the highest point. It’s difficult to completely eliminate lippage like that, 1/3 helps a lot.
ok i just used unmodified thinset under my ditra on plywood. i know i did wrong, what do i do now to fix this? or is it ok?? if i pull up the ditra is it reusable? thanks jon.
Hey Jon,
You’ll be fine as long as the plywood is in good shape and you used the flat side of the trowel to burn the thinset into the wood before you combed it out with the notches. Unmodified will actually create a stronger bond than modified provided it cures slowly enough. Burning in the thinset first and covering it with ditra helps that.
I have an old 1970’s house that had the 12″ square Linoluem tiles glued down with a black adheasive. I’ve removed the tile, but the adheasvie stayed. I will be laying 18×18 ceramic tile, over Ditra. I’ve decided to lay a layer of plywood over the floor to have a good, clean substrait to install my Ditra on.
My question is: can I use luaun to go over the floor? The Ditra website says not to install Ditra on anything less than 3/8″ plywood. What’s the reason?
My floor is 2×8 floor joist, 16″ OC, 1″ of current plywood, 11′ 3″ span, basement below with constant heat/humidity.
Thanks, Kurt
Hey Kurt,
What you have is called cutback – and it sucks – a lot. You can absolutely NOT use luan beneath a tile installation. It is very light because it is extremely thin and unstable. The 3/8″ plywood requirement is a matter of substrate stability – any less than that (with plywood) and it’s basically a crapshoot as to how stable your floor will be. A better option would be 1/4″ cement backerboard. Place thinset over your current floor and cutback and place the backerboard into it and screw it down every 8 inches or so. You can then install the ditra over that and tile away.
Great info. I am still not totally sure on what thin-set to use. The Handbook states that for Ditra on Plywood application (3/4 on 12″ center) you should use Modified Thin Set Ansi A118.11, then for the porcelan tiles on top use Unmodified Ansi A118.1. In one off your responses you state that you use Modified for both applications, the Ditra and Tiles?
I am getting ready to install on outdoor deck our help would be appreciated. Also does the plywood have to be dry for Ditra application?
Thanks
Jim
Hey Jim,
For indoor applications yes, I normally use modified for both over and under ditra. For outdoor applications I always use unmodified between the ditra and tile – always. The latex or polymers in modified thinset require air to fully cure – If you fill the ditra and put a porcelain tile over the top of it the polymers will take forever to cure. Unmodified will lead to a more secure installation. So for an outdoor deck use modified between the ditra and dry plywood, and unmodified between the ditra and tile. Your plywood should be dry, if it is wet it may add too much water to your modified thinset and actually weaken the mix.
Hi. Is there any thing one can do to successfully prep an old, hard, slick linoleum floor to successfully bond with the thin-set? I cannot remove it w/o tearing up the sub-floor. I was planning to scuff the linoleum with a wire wheel, then cut 1/8″ groves on a 2″ pattern.
Related question: Is it possible to staple down the low parts of the Ditra during while the modified Thin-set is still wet? I was thinking the mechanical bond would be beneficial.
Thanks, Kurt in Indiana
Hey Kurt,
The method you suggest may or may not work – I dunno. What I would do is get some 1/4″ hardiebacker and spread thinset on the linoleum, install the hardi and screw it down. This will give you a nice clean substrate for your ditra and the screws will keep it in place without wonder whether your floor is going to survive.
You can staple down the ditra, but I wouldn’t. You essentially defeat the uncoupling properties of the membrane by mechanically attaching the plastic part to your substrate. The thinset will keep it there just fine – really.
Hi Roger,
Most any plywood (even OSB ) is an approved substrate for ditra. And if you use a thin-set approved for that substrate, there are no problems at all 
.
The bathroom is grouted. The contractor used a nano grout and I love it, it is stain resistant without sealing the grout.
Maybe you should change that one part in your report
Since there is no thin set available for OSB.
Looks like a nice marketing trick
Janet – you’re right, it does seem like a neat marketing trick.
Seeing as I claim to call out companies on their neat marketing tricks (and actually love doing so) I’m gonna look into this a lot more and fire off some emails and try to get some answers. I’m also gonna contact my reps and find out if anyone does indeed have a thinset approved for osb. I’ll let you know what I find out. Thanks for pointing that out!
Hi Floor Elf,
I am just about to install ditra over plywood for marble tiles (18” by 18”), the story guy told me to use ultraflex 2 both under and above the ditra… Do you think that is suitable? I know that you use modified on both sides, it is just the Mapei Ultraflex 2 I am unsure about…
Thx a million
Hey William,
Ultraflex is good stuff, I have no problem using it, and often do. Just be aware that it will void your schluter warranty. It is always best to fill the waffles one day, let it cure overnight, then set your tile. Gives the thinset time to set up in the dovetails.
Really amazing that you take the time to answer to each of us that quickly
I just realized that I am the only prick that wrote is last name in his comment. Can you please take that out ? Thx
I dont want anybody to find me and come steal my Schluter Ditra, it is such an exotic piece of orange plastic. Also, just in case my wife googles me I dont want her to know I didnt know what I was doing!
Have a good one.
Hey William,
All discriminating identifying last names have been removed to protect my readers from over-zealous wives just looking for something to blackmail with and bring up at family reunions – fifteen years down the road.
Hi Roger,
I was looking to find a thin set mortar to use over OSB, but everyone I found(Mapei Ultra Flex 1 and 2), Laticrete thin set unmodified and modified, customs………………………………., all say:
LIMITATION: do NOT use over unstable subfloors like oriented strand board OSB.
I even called the technical department of Mapei and Laticrete and everybody said not to use their products over OSB.
I had my bathroom done by a contractor and was reading the label of the thin set mortar he wanted to use over my OSB floor. I stoped the work and after calling on the thin set manufacturer my contractor put one layer of real plywood over the OSB and used the membrane on the plywood with modified thin set mortar and tiled the floor (porcelain tile)also with modified thin set mortar. I was told to let the floor sit for a few days because the modified thin set mortar takes longer to set.
I wonder what would have happened if I woulded have read the limitations on the thin set bag.
Hi Janet,
You can’t find one because OSB is not an approved substrate for tile or most membranes. It is the ‘chip’ factor in the product which makes it unsuitable – they’re random and may peel off in very thin, random places and layers. I use modified thinset on top of ditra all the time so that isn’t really a very large issue a far as I’m concerned – but it does nullify your warranty. If it hasn’t yet been grouted you may want to make sure he waits a couple of days before doing that – it will allow a bit of extra time for the latex in the mortar to cure.
I’m sure if you wouldn’t have read the bag your membrane would be right on top of the osb. Qualified contractors know better – so he’s either knowingly doing it incorrectly or is simply unfamiliar with the product.
I’m about to gut my small bathroom as I’ve acquired a leak through the old, cracked grout and it’s basically ugly and needs it. My question has to do with membranes. My floor is very small, <20sqft (no free standing shower in the room, just a tub). Do I need to use ditra for this floor or are there any other acceptable alternatives? I will be tiling the floor with 12" porcelin tiles, it is on the 2nd story of the house supported by standard plywood subfloor. And, if I can only use ditra is there anywhere cheaper to get it than a Home Depot. It is shockingly expensive here.
Sorry to be such a cheap ass but the ditra costs more than my tile…just kinda stunned me.
Hey Nick,
It’s expensive here, too. There are many products you can use for a flooring substrate. Uncoupling membranes include ditra and provaflex. The provaflex is a bit cheaper than ditra. You can also use regular backerboard and install directly to that. For a floor that small you don’t really need an uncoupling membrane but you do need a suitable substrate to attach the tile to. You can read about proper backerboard installation here: Installing backerboards for floor tile
What thin set would you use to apply ditra to OSB plywood? Is flexbond okay? Oh, and how do I attach warmwire floor heating wire to a schluter shower pan, hot glue or metal stripping, and do I put the kerdi membrane above or below the warmwire? Thanks dude!
Hey Jonathan,
Flexbond would work just fine. You DO NOT attach warmwire, or any other in-floor heating, to the schluter shower pan – it’s styrofoam, it will melt. Sorry.
If you want the warmwire in your shower you’ll need to create a floor from scratch using deck mud and, once cured, groove out a channel for the wire and install thinset and kerdi over the top of it. I have a series of posts about how to create a shower floor for tile , that’s the way it needs to be done with in-floor heating.
Kerdi and Ditra both are installed over the top of in-floor heating elements.
Well I talked to reps at Schluter and Warmwire, and they both said its perfectly fine to install the wire over the schluter pan and under the Ditra….and its not styrofoam, its polystyrene. Sorry, floor elf
Well, Jonathon, I did not know that warmwire was approved for use with the Schluter pan. So if both your reps tell you it’s fine to use it – use it.
As far as it not being styrofoam, well, it most definitely is. The actual word “styrofoam” is a trademark of the Dow Chemical Company and is made from, wait for it, polystyrene. It was invented (or more specifically, accidentally discovered) around the time of World War II while Ray McIntire, a scientist working for Dow, was attempting to create a flexible electrical insulator. So you don’t need to call it styrofoam if you choose not to – doesn’t change the chemical composition of it, however. The glass transition temperature (melting point) of polystyrene is 95 Celsius (205 F) so as long as the wire of the warmwire doesn’t rise above that it’ll be fine.
Not quite sure why you felt the need for the ‘sorry, floor elf’ comment with the cute little smiley. Are you apologizing for your lack of ability to simply state that your reps told you it was fine in a civil manner or your apparent need to turn everything, even a blog comment, into a pissing match? I don’t have a problem being wrong about something – it’s how I learn. Either way I write this blog in an attempt to help people such as yourself and don’t receive a damn thing for it. Although I realize there are people incapable of pointing out someone else’s mistake without pounding their chest with self-rightousness – I could do without it. Thanks – you rock!
Hello Roger,
I see a lot of good information here. I am in the beginning stages of tiling a bathroom. My substrate is OSB on 19.2 OC. The longest span is 10′ and I am planning on putting 3/8″ porcelain. Because of height restrictions, I plan on using the 1/8″ thick Ditra directly over the OSB. I plan to use Ultraflex 2 (from Lowes) b/w the Ditra and the OSB (not sure what I will use for the Tile / Ditra yet, but I understand that it is unmodified).
Do I just butt the edges of the Ditra up to one-another? What size trowel is appropriate for the
OSB/Ditra and the Ditra/tile. Do I need to tape the joints (I am not looking for a sealed floor)? Do I leave a 1/8″ gap around the perimeter?
I think that I know the answers to these, but I would like some advice.
Thanks in advance.
Dale
Hey Dale,
You really should install another layer of plywood on that floor. But I can’t stop you from here, eh?
Yes, just butt the edges to one another. No need to tape the joints unless the installation needs a waterproof floor, in that case you would use kerdi-band. I use a 1/4″ x 1/4″ u-notched trowel beneath my kerdi. Yes, leave the 1/8″ gap around the edge. You’re ditra isn’t gonna move, but your walls may.
Hello Roger,
Thanks for the reply. You are amazing, and I really respect you for answering all of these questions. I am going to start putting down Ditra, and I have a burning question that I know has been answered so many times here as well as other sites (and the Ditra website).
Is a 1/4 x 1/4 V-Notched trowel okay for Ditra over OSB. I know what Schulter recommends, but I cannot find anything but the 3/16 x 1/4 V-notch anywhere (I understand that this may be too small). I have also seen reference to a 1/4 x 1/4 x 1/4 square notch, but given the Schulter recommended 11/64 x 11/64 sq. notch (area of material = 0.029 sq. in), this seems like a lot of thinset (1/4 x 1/4 sq. area of material = 0.06125 sq. in). Since the 1/4 x 1/4 V-notch is 0.03125 sq. in of material, would this not be better than a 1/4 x 1/4 sq. (the 5/16 x 5/16 v-notch that Schulter recommends is 0.0488 sq. in)
In other words isn’t all about material volume, and therefore the 1/4 v-notch be a good choice ( area of material is between the 5/16 – v and the 11/64 sq.)? Am I way off base here …
Hey Dale,
I tell my wife I’m amazing all the time! (She isn’t buyin’ it) The 1/4 x 1/4 v-notch will be fine. The u-notch that I use does leave a bit more thinset but I’m fine with that. As long as the amount of thinset doesn’t saturate and fill the air-channels (the hollow areas between the squares) on the bottom of the ditra and you get complete coverage (pull up a corner and take a look after you install some) then you’re fine.
I actually quit a fairly lucrative career as a physicist 18 years ago – quit it with the math.
hi roger
thanks for all the great info .. i have a ? for you i want do do a showere with out a curb i plan on using a kerdi shower system with out the curb and lowering the subfloor in the shower area . then use ditra on the rest of the floor outside the shower . my ? is should i overlap the ditra on top of the kirdi or kirdi on top ??? how much over lap ?? i would not think butting them togeather would be good.. thanks
Hey Ed,
You want to overlap the kerdi over the top of the ditra. The minimum requirement to maintain waterproofing is 2″ but in an application such as that I’ll usually go 4″-6″. No real reason to, I’m just a paranoid bastard like that.
dear roger,
quick question – i am using 12×12 and 6×6 ceramic or porcelain tiles in my kitchen, my floor is 1 1/4″ thick 1/2″ ply on 16″ on center joist and will be removing the old 3/4″ osb and installing 3/4″ plywood will be 1 1/4″ thick when done. does the new ply i put down get installed same way the joist are running or the other way or doesnt it matter. i will be leaving 1/8″ gap between boards and 1/4″ gap at walls. i will be using mapei ultraflex 2 to install ditra to the plywood and kerabond on top of ditra to secure the tiles. are these the correct products to use. not sure also i will be filling the ditra waffles one day then tiling the next day – is this ok to do it that way. thanks
Dear Randy,
Quick answer – The second layer of plywood should have the grain running across the joists – perpendicular. And all edges should overlap the seams of the first layer by at least 1/4 the length of the sheet of plywood – 1/3 if possible. Do not screw the second layer to the joists – only to the first layer of plywood. Everything else is perfect! And yes, you can fill one day and set the next.
thanks roger for the quick response – one more question thow – should i also glue the plywood down then screw it. should i use a v notch trowel or square notch to install the ditra to the ply and what size notch to use. and i have 2 open doorways in which i will be installing 3/8 tile to carpet transistion strips i am aware that these get installed by thinset on top of the ditra can i also use roofing nails to secure it or will the thinset keep it in place. thanks
No need to glue then screw – just screw. (Boy – that takes me back to college…) 1/4″ x 1/4″ square notch to install ditra to ply (that’s what I use). You can use nails to secure your transition strips as well, but no real reason to, the thinset will secure them just fine.
roger thanks so much for your advice i will also be installing mosaic 12×12 tile with the mesh backing over painted drywall for my back splash any idea on what brand and kind of thinset to use (modified or unmodified) does the drywall painted surface need to be preped. thanks.
wrong tile kind its travertine tile not mosaic sorry
Regular modified will work just fine. You’ll want to at least rough up the paint on the wall before applying the thinset and tile.
so i could use ultraflex 2 thinset same stuff im using to set the ditra to the plywood. to scuff up the drywall i can sand paper correct or do you have another method of doing this.
Yup, ultraflex 2 is good stuff for most things. And yes, just regular sandpaper is fine. What you want to do mainly is remove the ‘shine’ from the paint, that will open up the sealed pores and allow the thinset to grab the wall.
thanks for all your info saved me alot hassle owe forgot what size notch to use for the wall tile. thanks
1/4″ x 1/4″ square-notched trowel for the wall tile on the backsplash. Make sure you skim-coat the back of the tile if you’re using travertine.
roger, just so know the travertine tile i bought has 3 different tiles in one 12×12 square held together by the mesh square netting so do i skim coat the mesh backing. also to make sure i am laying my sub floor down correctly – the first floor that is down already i can see the grain runs across the joist by looking at it from my basement so my questions are do i lay the subfloor the same way but overlapping the joints on the first floor and 2nd does the ditra get layed down the same way as the subfloor is going or does that not matter. not sure on this. thanks
sorry for all the questions – also i have not purchased the 3/4″ plywood yet should i buy the tongue and groove or the the other without the tongue and groove and leave an 1/8″ gap thanks this should be my last question hopefully. you been alot of help.
Either one would be fine. I prefer the one without the tongue and groove, it’s easier to work with and can be oriented however you wish. But either will work as long as you do not screw them into the joists – just into the current layer of subflooring.
cool you been so much help thanks.
well i got the underlayment up shity osb board – i did some messureing by the looks of it i will be overlapping the 8′ side seem of the subfloor by only 4″ dont know if that is good enough if not i will have to start laying the underlayment in the middle of the floor then work on both sides to the walls – my kitchen is 14’x11′ let me know on this thanks
Unfortunately 4″ isn’t really enough. You will have a much more solid floor if you put the center one down then fill in toward the walls. It will actually be easier to cut enough off of the 8′ side to reach a 12″ overlap, either 8″ or 16″ depending on which side of the joist your seam is. KnowwhatImean? Then you can start at that wall and work the other way rather than starting in the middle.
sounds good so i will make a 2′ wide board x 6′ long at the wall then a full board next to that which will cover the subfloor seem by 2′ on each side and the 4′ ends will also be staggered by 2′. does this sound right. also where should i place the end of the board on the joist beam or between the joist i am aware not to screw it to the joist. just not sure on the placement of the end of the board. thanks
Yup, that sounds good. Place the end of the board between the joists – not over them.
underlayment is in correctly thanks to you – im getting ready to tile need your oppinion would you tile all the way to the walls and put the cabinets on top or would you install the cabinets and tile up to the cabinets what would be better to do. thanks
Hey Randy,
If given the option I always chose to set the cabinets over the tile. It’s quicker and easier with less cuts. The only drawback would be the need to remove them if you ever replace the tile. I don’t install tile to be removed.
roger,
im getting ready to tile i will be using porcelin tiles not ceramic so can i use kerabond or does it have to be Kerabond/Keralastic for porcelin or either one will work to bond to the ditra and is this a unmodified thinset. thanks confused as always to many products out there to choose from
Hey Randy,
Kerabond is fine for porcelain over ditra – really.
Make sure you burn the thinset into the backs of the tile as well with the flat side of the trowel. This will ensure proper adhesion, then wait a minimum of 24 hours before walking or crawling on it.
thanks – i was just confused because i checked on the kerabond info and here is what it says: Designed for interior and exterior floor and wall installations of vitreous,
semivitreous and nonvitreous ceramic tiles, ceramic mosaics, quarry tiles,
pavers, and natural-stone tiles.
• For installations in freeze/thaw environments or with special substrates,
use Kerabond with undiluted Keralastic or other polymer additives.
(See the Technical Data Sheets for Keralastic and other polymer additives
for details.)
• For installation over smooth, steel-troweled concrete, glass, vinyl
asbestos tile (VAT) (residential and light commercial use only) or old
cutback adhesive (residential and light commercial use only), and for
use over exterior-grade plywood (interior dry floor areas only), use
Kerabond/Keralastic™.
• For setting impervious or glass-like tiles, gauged slate, polished-back
marble, granite or porcelain tile, use Kerabond/Keralastic.
last sentence that got me says for porcelain tile use kerabond/keralastic
roger due to some shrinkage with kerabond what size square notch trowel should i use – i am going to back butter the tiles and smack them down into place. thanks
As long as your ditra is flat you can use a 3/8 x 3/8″. With backbuttering that will ensure coverage and not so much thinset as to shrink considerably.
i know your a laticrete thinset guy does the 317 shrink and could i use this on the ditra
317 is the unmodified I use over ditra most of the time. All thinsets shrink, the 317 doesn’t shrink enough that you’ll really notice it if your tile is properly bedded. It’s good stuff.
hey roger
went to lowes today and the associate that helped me did not know anything so basically i was on my own – the only laticrete product i saw that they had was laticrete multi-purpose pro, megabond, 253r gold rapid thinset mortar did not buy anything not sure which one was modified or unmodified. they did not have the mapei brand but she says she can order it. whats your suggestion. also if i am doing a hopscotch tile pattern with 12×12 and 6×6 tiles can i start at my open door entry at the 3/8 transistion strip so that i know how far to push the tile into the thinset so that its even with the top of the transistion strip. thanks
The associate didn’t know anything??? Say it ain’t so…
The megabond is the one you want – it’s real close to the 317 from what I understand. The others are all modified. Yes, you can start at the door to gauge the height.
hey roger,
finally found a tile store that sells the ultraflex 2 and the kerabond – 50 lb bags – do you know of any formula for mixing because i dont want to mix the whole 50lb bag at once want to take my time on this project.
example how many lbs of thinset to gallons of water to get the correct mix.
Specific liquid amounts should be listed on the bag. Each product is different and I can’t remember what the mapei stuff requires, but I do know it’s on the bag and for the entire 50lbs.. Just divide that accordingly.
Oops.
No, do not skim the back of that unless you can do it without making a mess.
Lay the new layer perpendicular to the current layer. The grain will run with the joists, in this case. Overlap the joints on the current floor as I’ve described. Doesn’t matter which direction you lay the ditra, whichever works out better for you.