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 
waterproofing a kitchen floor with KERDI-BAND on joints and wall/floor transitions:
Over kill?
Would you do it in your kitchen?
(you know, to deal with spilled beer and Rudolph’s accidents when the elves forget to let him out.)
I did not do that in my kitchen. It is overkill to me, but I have a dog who drinks beer, so…
It never hurts, though. Completely a personal choice.
Is there magic pixe dust in the Schluter DITRA-ROLLER? or can the heavy floor roller available for rent at Home Depot work?
Only if you sprinkle some on there.
The regular roller will work, but it’s really not necessary. You can use a small linoleum roller if you want.
Hi there,
Beginner here, so please forgive me!
We are looking at adding tile to our 100 year old home in a bathroom/laundry addition. The addition is over the basement, and we will be taking it down to the original subfloor aka, diagonal boards (which has hardwood floor covered by particleboard covered by epoxy covered by laminate tiles!) I understand we need to add plywood to strengthen the flooring, and with washer vibration the dirta seems to be a good idea. Do we need to add a cement backer board between the plywood and dirta or can we just use it over the plywood (it seems so give your previous comments but want to be sure)? Will the dirta provide adequate water protection near a shower? We are in this mess due to water penetration, so this must be avoided at all costs!
Hi Ann,
Yes, you can install the ditra right over the plywood with modified thinset. Ditra is waterproof, but if you want the complete installation waterproof you can install kerdi-band over the seams. It isn’t normally necessary, but it will give you a completely watertight floor.
Room is 9 ft x 28 ft
How much thinset is used below Ditra?
How much thinset is used above to fill the Ditra and bond the tile?
This question is asked to estimate how many bags of thinset needs to be budgeted for and on hand…
Kinda distressing to run out of supplies after the stores are closed….
Thanks for all your information.
And…. (and never think of everything the first time…)
looking at the Ditra-Heat version.
Found on page 15 of Schluter Ditra-Heat Installation Handbook 2015
http://www.schluter.com/media/ditraheat-handbook.pdf
ESTIMATED THIN-SET COVERAGE
To bond DITAR-HEAT to the substrate:
Use on 50lb bag of mortar per 100 sqft
To bond the tile to the DITRAT-HEAT, using a 1/4 x 3/8 square notched trowel:
Use on 50lb bag of mortar per 40-50 sqft
To bond tile to the DITRA-HEAT, using a 1/2 x 1/2 square notched trowel:
Using one 50lb bag of mortar per 30-40 sqft
Oh, in that case double the thinset over the ditra. It does depend on your trowel size, but you’ll get about 25-30 sqft out of each bag.
Hi James,
You will need one bag of thinset for every 50 feet of tile over the ditra (approx.). You will get approx. 150 sqft of ditra down for every bag of thinset.
Oh no! We used Mapei Ultralite thin set mortar with Ditra. It had the correct AINSI level. Someone told us that it was not unmodified after we laid the tile – didn’t say on the bag and thats what they gave us at the local store. Will it cure even if we grouted? Help!
Hi Bill,
Yes it will, you’ll be fine.
I just purchased a house and have an isolated area of failed ditra (cracked grout, loose tiles, (now) a cracked tile a a general sound of loose mortar underneath. The subfloor appears 5/8 on 12″ centre 2×10’s. Deflection does not seem to be an issue. The area that is failing is in a high traffic area but n particular any worse than other areas. I recently found the tile (12″x12″ porcelain) so now I have lots of spares. The install is only 3 years old. I am wondering if I am dealing with a mortar screw up in one area (one mix?) or a bigger subfloor issue than I cannot see. Ditra failure in the short term due to deflection- is that possible or is that a longer term problem. Thanks for your input (PS this is a 30 year old, VERY solid house)
Hi Jeremy,
The problem is only having 5/8″ subfloor beneath the ditra. While that isn’t always a deal breaker, you need to have a minimum of single layer 3/4, optimally a double layer 1 1/8 total thickness. That, coupled with obvious issues with the thinset (either watered-down, allowing it to skim over, using past the post life, etc.) will lead to what you’re seeing.
Removing it and adding 1/2 ply over the 5/8, then properly installing it would be the way to go.
Hi There,
I am installing 12×24 porcelain tile over ditra heat. The company i purchased the tiles from recommended Mapei LFT (Modfied) to lay the ditra heat as well to lay the tile. Schluter recommends unmodified to lay the tile.
The company also advised me to lay down a skim coat on top of the ditra, let it dry and then lay the tile.
I have laid quite a bit of tile using the traditional wire mesh, and obviously and extra skim coat would be fine. But my concern is, will i loose ditra’s uncoupling effect adding another layer of mortar than the recommended?
Perhaps that is why they sold me modified to lay the tile?
Hi Anthony,
Nothing wrong with that method at all. I do it a lot. You’ll lose schluter’s warranty due to the modified component, but it works very well.
Roger- feeling like an idiot, but I am installing 3/8″ thick marble 2″x4″ tile in a bathroom, over a cracked 3″ thick concrete floor. After tearing out the old tile and mudpack base on top of the concrete, I needed to build up about 1″ to come in level with the toilet flange(at all costs, I don’t want to disturb that or have to install a new one), and during process of leveling up of the low 2/3 of the room(the flange is at the high side, so I didn’t do any delf leveling there), I was watching the height in relation to the door threshold, which would be close as it is now, but I neglected to pay attention to the height of the flange(the old floor was about an inch out of level, and not flat either, so the flange is about 1/4″ higher than the threshold). I laid in my Ditra with unmodified thinset, then looked at my flange, now sitting 3/4″ above the Ditra. It was then that I realized that I need to gain an extra 3/8″ to bring the tile in at the flange height. Can I build up the thinset really thick on top of the Ditra to make that up, or possibly lay 1/4″ Hardibacker set in thinset(without mechanical attachment) on top of the Ditra to try to gain that 3/8″? Any ideas you may have (aside from changing the flange) would be appreciated. Thanks
Hi Dana,
You can not use hardi over that. The only two solutions I can come up with would be either self-leveling cement or a mud bed over your ditra. Medium-bed mortar (some of them) can be built up to 3/8″ or so, but thinset can not.
I am considering using ditra over a vinyl tile floor To install 13×20 porcelain tiles. There is 5/8 plywood with 1/4″ underlayment and vinyl tile on top of that. The floor is solid and the tile well attached. Is this something you would consider ok? What leveling compound would you use to fill a few small voids in the vinyl tile?
Hi Snuffy,
I wouldn’t use any type of leveling compound. The vinyl needs to come up, at minimum, the 1/4″ ply should also come up as it’s likely luan and does not have the compressive strength to support a tile installation.
Hey Roger, Ditra Heat question:
Small bathroom, in basement, on concrete. Plans changed with in floor heat option, had True Comfort, turns out I bought too small of a package for the size of room. Read a lot (after the fact) about a thermal break (not to mention what I had bought would not of heated all the area I wanted to), as a result decided to go with the Ditra product.
Initially I picked up all products from tile store, thinset (modified), leveling compound, and ceramic tile (True Comfort bought at orange box store). I am guessing, with my previous plan that the modified thinset would suffice. Now I’m about to install the Ditra Heat and tile, and read that unmodified is recommended, it’s Sunday, and they are closed. Yikes! There goes my progress for the long weekend. So I am going to make the hour drive to the city to pick up unmodified thinset, so as not to screw up my investment in the Ditra Heat, and tile, and time. I’ll likely find out after the fact that unmodified may have been ok, but I am going to err on the side of caution here. Would still love to hear your answer, hopefully before I make my purchase of unmodified, if by chance the modified will work.
Eagerly awaiting your response.
Hi Brent,
Modified would work, but I would use the unmodified on it if I had a choice.
Hi I’m installing 12×24 Floor tile over Hardy backer board should I put down detra first?
Hi Roger,
No, you don’t need it over hardibacker.
Hey can I lay Ditra on a concrete basement for to install rubber floor tiles on top of it as a vapor barrier for air to move freely? Or is this a unapproved substrate?
Hi Russ,
I imagine you can, don’t see why not.
Hi,
I just removed 1×1 inch tiles from my small bathroom area and laying 16/16. The plywood seems brittle in places where water probably leaked in the past. In some places, the top layer of wood broke up (came up with the tile) leaving an uneven layer of ply. I’m putting Ditra down before the tile, but wondering if I need to anything to make the plywood smoother (like fill in some of the areas with putty or something) or will the thinset for the Ditra take care of that?
Hi Jen,
You actually need to cut out and replace that plywood. If the top layer is compromised due to water damage, the subsequent layers beneath it are the same way. That means the plywood does not have the compressive strength it once did, think of it like very thin layers of wood with nothing bonding them together. It will move – a lot. That is no good under a tile installation.
Finding your forums very useful, thanks for offering up so much advice! I’m tiling my bathroom and I’ve already put down warm-wire and about to embed it with SLC. My question however is if Ditra is particularly necessary in my case as I have 2×10 joists spaced 12 inches resting on a very sturdy beam (that we also just installed), 3/4 T&G on the joists, and 3/4 plywood over that, so inch and a half total subfloor, with a layer of SLC to come. To me that seems like a stable enough subfloor to not have to use Ditra but I would definitely appreciate your thoughts.
I’m also finding it hard to accept (at a physics kinda level) that Ditra will transmit the heat from the warm-wire effectively enough through it as it essentially acts as an insulator between to the two layers. I’m sure it will still work but won’t it require a lot more energy to reach and maintain temperature? Thanks!
Hi Double-A,
No, it isn’t required at all, you can just set straight to the slc. Ditra is not an insulator of any sort, it is simply plastic. If you hold a hair dryer against the bottom of a piece you’ll still burn your hand holding it on top of it. Makes no difference at all.
Thanks very much for your advice! I thought about it a bit more though and I went ahead and put in the Ditra anyway as I live in a maritime climate with a LOT of summer humidity, so despite the stability the floor the wood underneath is still likely to swell/shrink a bit. Several of my interior doors won’t close during the summer months so I figured better safe than sorry.
Thanks again for the response! Cheers
Hi Roger,
I am planning to install 12×24 rectified porcelain tiles on top of 3/4″ tongue and groove plywood, 10″, 16oc joists. Just to be safe, I plan to use a self leveler to make it as perfect as possible. Do I still use modified thinset to attach the ditra?
Hi Chris,
Unmodified thinset over slc.
Sooooooo, I may have neglected to wet the plywood before installing the ditra. How screwed am I?
Cheers
Hi Greg,
You shouldn’t be screwed at all. You don’t need to wet plywood, you just need to have it clean. You need to wet cement backerboard.
I am about to tile a 330 sq ft sunroom built on an existing pressure treated wood deck. (1″ deck boards over 2×10 joists 16″ o.c.) The 3/4″ TIG exterior grade plywood subfloor is down and is smooth and level. (How far apart should the screws be?)
Over the subfloor, I am planning to add the following, in this order:
1. Latex modified thinset with 1/4″ x 1/4″ trowel (Tec Full Flex, or which one?)
2. Durock 1/2″ cement board underlayment
3. Unmodified thinset with 1/4″ x 1/4″ trowel (which thinset?)
4. Progress Profiles / Prodeso Uncoupling Heat Membrane and SunTouch heating wires
5. Unmodified thinset with 1/2″ x 1/2″ trowel (which thinset? I think you recommended Bostik Ditra-set or Laticrete 317)
6. Cerodomus Cherokee tiles, 6″ x 24″ x 0.4″ thick
Please correct anything I have that is wrong, and advise on best thinset selections. The store where I’m getting the tile sells only Bostik and Mapei products, but another nearby tile store carries Tec products, and another carries Laticrete, and maybe I can find other brands if I look at other stores. I’m in North/Central NJ.
Thanks!
Hi John,
Screws should be 8-12″ apart.
Full flex is fine, tec uncoupling mortar, mapie kerabond, laticrete 317 (or 272) or bostik ditra set are all good, unmodified mortars. However, I’m not sure prodeso wants unmodified? You may want to check that, I think they recommend modified, but I’m not certain.
I am considering Ditra for a kitchen back splash to help level out some of the issues I have with a wall that is not straight. I realize that going by the book backer board is the way to go but everything I do that is not simple creates several more projects that must be done first so I am trying to reduce and simply the work if possible. I also realize this is not the purpose of the product but might be OK for my job unless you or others from your blog can recommend something else that is cheaper and works as well or better with a lower profile? This product is about 1mm and the smallest backer boards is about 4-5mm (1/4 in) ? I have invented a self perpetual machine with my renovations…. i only wish I could patent and make money… any takers?
The only other ideas I have would be some kind of cabinet/counter top/ vinyl back splash covering I would secure to the wall using screws to the studs and then apply the thin-set/mud/glue but I have no idea about trying to attach tile to those products or if it is a bad idea.
Back to the wall…
The wall is not straight and this product seems like an option to smooth out the wall so my 12″ X 4″ tiles fit better. I like the thin size of ditra compared to 1/4″ backer board but finding any information using google has been tough or I am just not searching correctly. I see pictures on the manufacture website showing it used in the back splash area but nothing devoted to that specific area showing correct application and is why I have the question.
How should I prepare the wall and is the back splash an OK area for this product? I have recently found out that the mud used trying to level my wall is questionable because I added water to make it apply better… yes I am am an amateur (dumb) looking for answers on the internet when it comes to mud but I am trying. I plan to sand off the questionable mud but once that is done how to prepare the wall for ditra? It ditra OK for a back splash that runs about 8′ X 16″ or should I go ahead and use the backer board and prepare for the additional projects that I will find.
–Desperate…
Hi Bob,
You really don’t want ditra as your substrate on vertical surfaces. 1/4″ backerboard would be your best bet. You can also use 3/16″ kerdi-board or 1/8″ wedi (you can google either of those).
Hi Roger,
Will we have a huge problem if the ditra that was installed over 3/4 sub flooring is not totally level? We plan to use 12 x 24 inch tile. Can the floor be saved using extra thin set over the ditra?
Thanks,
Kevin
Hi Kevin,
Do you mean level or flat? It doesn’t have to be level, but it does need to be flat. The flatter the better. Yes, you can put extra thinset beneath the tiles, over the ditra, where needed to keep the tile flat.
Dear floor elf.
Thank you so much for all of the information on your site.
We are about to lay shluter ditra down. Our substrate is 3/4″ plywood and our chosen tile is 12×24 volakas 3/4″ thick marble tile. We know that we have to use an unmodified mortar ON TOP of the ditra, that is between the marble and the ditra. But shluter recommends ‘latex portland cement’ for BELOW the ditra, between the ditra and the 3/4″ plywood underlayment.
Waaaah? Which product do you recommend. A ‘latex portland cement’ is a ‘modified mortar’ correct?
PLEASEEEEEEEeeeeeee which brand do I use for this?
Thank you
Hi Ava,
Over a wooden substrate you need to use a modified thinset to bond the ditra to the substrate, then an unmodified thinset to bond the tile to the ditra.
I have an old marble floor on a send fl bathroom that has thermal heating under it. The heating still works great but the marble has developed some cracks. I know there is a 3/4 inch ply sub fl and 1/2 inch concrete board on top of that. My guess is there is still enough movement that the marble cracked. It does not sound hollow under the cracks so it appears that the marble is still bonded. Can I tile over this fl to save the thermal heating? If so would I use Ditra or RedGuard under the tile?
Hi Tony,
Yes, you can. I would rough up the marble and apply a proper thickness coating of redgard, then tile directly to that.
hi roger,
i have a question about floor height, on a concrete slab, what would be the best approach to add more height to the tiled area? i will have laminate and the underlayment at about 13/16″ height against the tile floor. Should i use multiple layers of ditra in order to try and get close to level or should i go in a different direction and use a different product?
thankks
Hi Anthony,
Ditra XL is twice the thickness of regular ditra. You can do two layers of ditra if needed, but the xl is a better option.
The Schluter manual says to use unmodified thin set to install tile. The supplier where I purchased by Ditra heated floor roll said that since I was using travertine I could use modified thin set because travertine is so porous there would not be a problem with the thinset drying. What is your opinion?
Hi Don,
Unmodified thinset works fine on travertine. Modified also works fine, but you’ll lose your warranty.
While Ditra can be installed on OSB, I’ve never seen a thinset approved for OSB, do you know of one that is approved in writing for OSB?
Hi Gary,
No. The problem isn’t the thinset, it’s the WILDLY varying types of osb and coatings. A good modified thinset will bond just fine to the UNSEALED side of any osb, but you’ll never get it in writing.
Hi:
I am planning to use Ditra in my kitchen. I wanted to use it because it is thin and I have a short ceiling. After demoing the kitchen, and tearing up a layer of osb I was left with a 15/32 layer of plywood. I don’t want to add too much height so that the new cabinets will fit, but I am worried the plywood Ina’s too much flex. I am reinforcing all the joints with 2x6s between joists. Will this suffice, or am I asking for trouble. Also with the thin plywood, am I better off just going with backer board instead, would that stiffen it up enough?
Will,
I’m not a pro (and I don’t even look like an elf), but that 15/32 plywood is not sufficient for tile. I believe the bare minimum is 5/8″ nominal thickness plywood or OSB, but I don’t think you will find many people voting for the bare minimum. If I were a betting man, I’d bet a six pack that Roger is going to tell you that 3/4″ is his minimum recommendation.
So, you probably don’t want to hear this, but rip up your 1/2″ floor and put down 3/4″, followed by the Ditra – that’s probably as thin as you will get. (there are other products thinner than Ditra that serve a similar function (greenskin is one) but I don’t know anything about them. I believe Ditra is 1/8″, so not a lot to save there anyway)
– backer board does not appreciably increase your floor stiffness.
– The plywood must be exterior glue and have all face grades C or higher (no CDX / sheathing allowed – internal voids cause problems)
– The plywood should be tongue and groove or blocking should be added to support the seams.
– Blocking should be added at the perimeter of the new flooring (where there are no tongue/groove joints)
– The single layer of 3/4″ is based on a 16″ joist spacing – I don’t know if it would work / is advisable for 19.2 or wider.
– The single layer of plywood assumes you are using porcelain / ceramic tile. If you were planning on using stone, stop and research it thoroughly before you do anything else.
– Have you evaluated the joist structure and verified that it meets the L/360 deflection requirement for ceramic tile. Determine the size, spacing and unsupported span of the joists – look for a grade stamp if possible. There are span tables available on the googlebrain – look it up and find out if your floor is rigid enough – if not, be prepared to stiffen it by adding (sistering) joists or reducing the unsupported span. (or be prepared for cracked grout and or cracked tiles at some point in the future.)
Just one DIYers two cents.
Steve is correct, but it needs to be a minimum 3/4″ under ditra, not 5/8″. Thanks Steve.
Hi Will,
You need another layer over that. A 3/8″ layer would suffice, but I would REALLY use another 1/2″ layer. Cement board will add zero dimensional stability to your floor, it is simply a proper substrate to bond the tile.