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 
Hi again,
Some more random questions:
-I pulled several layers of linoleum off my subfloor, and there is a lot of fixative left on boards (About 25% of total surface area has this fixative on it, and I can’t get it off). Fixative does appear to absorb water. Wondering if it’s worth cutting out and replacing subfloor, or just throwing down thinset and Ditra and going for it.
-How much weight can bare Ditra handle once it’s been affixed to the floor with mortar? This is before more mortar to fill in waffles and tiles are applied. I’m about 130lbs, and don’t want to point stress the membrane by tiptoeing around and cause membrane failure.
Thanks!
If you’re using the rapid set just burn it in really well to ensure it bonds well. If my 200lbs. doesn’t bother it your 130 won’t.
Just don’t go trompin’ around in them high heels.
Like literally burn it? I have a propane torch, and need excuses to use it when I’m not plumbing…
You only need an excuse if your neighbor does not own a cat.
By burning it I mean skim-coat it with the flat side of the trowel.
Hi there,
Can I use Mapei Rapid-Set Tile Mortar (it’s a polymer-enriched thin set) between my subfloor and Ditra, and then again between Ditra and slate tile? Or do I have to use a regular setting polymer-enriched mortar under Ditra and an unmodified mortar over Ditra?
Thanks,
Andrea
Hi there Andrea,
Yes, you can use mapei rapid set for both. Work quickly, it’s fairly unforgiving.
Thanks!
Hi Roger,
I have a subfloor of 5/8 ply on 2×10″ @16″ centers. I don’t understand if gaps are necessary for the subfloor. The 5/8 sheets of ply are t&g and are tight together. Should I be making cuts with a circular saw along all the ends and edges.
I will be adding 3/8″ underlayment (or maybe 1/2″) ply over the 5/8, for this I will leave an 1/8″ gap.
Thanks.
Hi Kevin,
Yes, it does need expansion space. However, most t&g ply will have a gap on the underside of the groove when the tongue is placed into it. So no, you don’t need to take a saw to it. Use the 1/2″ ply – with the gaps.
Hi Roger,
I have a multilevel home and am about to install a back yard patio along with french drains and replace old rail road ties with a more suitable retaining wall system or natural stone/concrete. That’s to control drainage as wall as foundation shifting. The walls show signs of movement and I’m concerned about installing tile on the second floor as a result of the potential problems I feel are valid. In order to make the home livable and living on a reduced income I used a combination of available materials that helped “flatten” as opposed to level the floor. The materials are a combination of 3/4″ plywood, self-leveling compound, chipboard, concrete fiber board. I used thin set to install the CFB and plywood. The house was way off level and in order to fix it at the joist area I would have had to spend a fortune I don’t have. I’m a disabled vet and have limited ability to do things in a timely manner. That’s why it’s taken me 12 years just to get to the point I can install a better flooring product than the laminate flooring installed over the mixmash of subflooring choices I felt stuck to using.
Can I install ditra over this and if so, what should I do? Any details would help greatly. I’d love to hire someone, but am afraid that most if any would not work with this mess. Please advise.
Eagerly awaiting your response.
Hi Mark,
Yes you can install ditra over it. If your top layer (you didn’t mention) is the cement backer then you want unmodified to bond the ditra, if it’s one of the woods then you want to use modified.
Problem is the floor goes from plywood to leveling compound over plywood.
Hi Mark,
What problem??? If you’ve already asked a question about something and I’ve given you an answer you need to post this under that answer as a reply. Right now the above is the only information I have at all about your project. I answer 30-45 questions a day and have over 15,000 questions and answers on my site. I have no idea at all which one may be yours. If it is a new question I’m gonna need a lot more information about your project.
Hi Roger,
Regarding your message that you write “What problem??? If you’ve already asked a question about something …” i believe there is a problem with the functionality of you website. I asked a previous question and then selected “Reply” under your response and apparently it started a completely new post. I apologize for upsetting you. But it was not my doing. It’s one of the glitches in the sites functionality that split my message off from the original. Have a good day.
Mark
You didn’t upset me at all Mark. I’ll see what’s going on with it.
Okay, found it on page 9 of my comments.
In that case you can use modified to go over all of it – concrete included.
I apologize if you thought you upset me. Sometimes when I’m in a hurry I realize it can sound as if I’m being short, I’m not. It’s difficult at times to translate into written word without something getting caught in the translation. Not sure why it didn’t post as it should have, sometimes the elves who run the site drink too much.
Roger, I am coming into the middle of this project to pick up where the previous homeowner left off. Its a 10×8 laundry room, ext grade 3/4 inch ply over 16″ joists with 1/2 Durarock glued/screwed already to the floor – seams not yet taped.
I don’t want to pull the Durarock up but prefer a waterproof and problem-proof install – can i use Ditra over the Durarock and if so – unmodified thinset?
Thanks, Dennis
Hey Dennis,
Sorry for the delay, been out of town at the new Schluter facility all week.
I’m concerned by what you mean when you type ‘GLUED and screwed’. But yes, you can install ditra over it, tape the seams with kerdi-band to make it waterproof, and tile over it. You want unmodified beneath and above the ditra.
Hi
We used modified mortar to set our wire heat. Now we are setting ditra mat so can I use Mapei kerabond for this stage?
Hi Mona,
Yes, unmodified is fine.
Hi Roger,
Thank you so much for your advise! It have helped a few times!
I have a problem installing ditra over gypcrete. I called Schluter and they told me to use unmodified thinset over gypcrete. I did, using the right size trowel (V-1/4 x 3/16). The thinset adheres very well to the gypcrete, but doesn’t adhere to Ditra! It just doesn’t hold it at all. I have the feeling that the amount of the thinset (that is determined by the size of the notches in the trowel) is simply not enough. Or maybe the gypcrete takes too much moisture out of it (although I moistered it before putting thinset)?
So what I do with the ditra sheets that I already installed (and that are coming out easily, not bonded with the gypcrete)? And how to ensure the bond in the other sheets that I still have to install?
Should I use a trowel with bigger notch? Or what?
Hey Alexander,
Certain types of gypcrete sucks the moisture out of thinset ridiculously fast. You should be working in areas of about 6 square feet at a time. It’s that bad. The good news, if there is any, is that since the initial coat of thinset bonded well to the gypcrete it essentially ‘sealed’ it so that won’t happen again. You probably want to switch up to the next largest sized trowel you can find and install it as you normally would. It should bond just fine now.
Hi Roger,
I did just that: switched to 1/4×1/4×1/4 trowel, and it made all the difference! Thank you!
Hi Roger, 1.5 years ago with had the foyer and front hall redone from ceramic to marble (12×12 tiles). The flooring company did not use Ditra, but only cement board. Now whole rows of tiles are cracking along the cut lines of the cement board. A $12K job shot to hell. The flooring company and Home Depot who subcontracted the work will be coming over to take a look. Luckily, we took some photos and videos during the installation. Any advice on what we should be asking them to do to remedy the situation?
Hi Nicole,
Yes, tell them to remove it and replace it. The seams are likely not taped and mudded as they should be, and there may not be thinset under the backer either. Both are completely wrong and they are liable for it. It’s an improper installation, period.
I have the Ditra installation manual open to page 9 where it addresses the installation over clean and “well adhered” vinyl using fast-setting latex p.c. mortar. The home builders added 1/2 inch plywood over 3/4 inch OSB as a subfloor. I have some of the leftover vinyl to test the adhesion of the Ditra. If I have to remove the vinyl, wouldn’t it just be a better solution to lay backer board over it?
I know. The easiest solution is to clean the vinyl and wax it. I’m a bachelor – and likely to remain so, and could live with it.
Hi John,
Yes, it would be better (and easier) to install backer (with thinset beneath it) over the existing vinyl.
Roger, I have a elevated covered concrete porch (18’x18′) that I am going place tile on. The problem is the row lock(?) of brick that creates a pocket of approx 1 1/4″ to 1 1/2″ deep. Would it be better to place the Ditra on the slab in usual fashion, then float a thicker slab on top and tile? Or float a slab and go with the Ditra? Due to the block masons error, removing the row lock(?) is not a viable option. Any advise will help get this monkey off my back….
Hey Chet,
You should float a slab with deck mud and tile. You can use the ditra if you want, but if you build a mud deck you can tile directly to it with no problem.
Roger,
Can’t get matching silicone for near the shower but did get matching acrylic caulk. To better protect this area, should I apply the caulking so that I have color match, and then run a thin bead of clear silicone over it? I assume the silicone will stick to the caulking.
Not on my home computer and for some reason it did not reply properly. Main question is will sili stick to caulk?
Yes, silicone will bond to caulk, but there’s no reason to do that. The acrylic will work just fine, it will just lose it’s elasticity sooner than silicone would. You may need to replace it in 3-4 years rather than 5-7.
Any idea as to how to read the expiry code on the caulking? Mine says A110942. Trying to figure out if it has expired as the expiry date is all blurred out.
Also, any idea how long the grout is good for? I have a sticker on my buckets that have 06/12 on them which I assume is the manufacturing date. Stuff is already 15 months old at this point if that is correct.
thanks again,
As long as the caulk hasn’t been opened it’s extremely rare to find a tube that is old enough to be unusable. I don’t know how to read that particular code, the manufacturer will be able to help you out with that.
The grout is 15 months old, it’s just fine provided it’s unopened and hasn’t been exposed to moisture.
thanks, Non have been open. I’ll call them tomorrow morning. I’ll post back any pertinent information.
Hi Roger,
Got the Ditra installed and even though I left space at the walls for expansion, 1 side and some of the other walls in partial have no space.
How much of a problem will this be if I still make sure I have proper spacing for the tile?
Should I try and remove some of the Ditra along the edge? Won’t be an easy job for sure.
Thoughts!
It won’t be a problem with your ditra, just make sure to have expansion gaps there with your tile.
Any chance the ditra might move with either the floor or tile and then buckle up resulting in a popped tile?
No chance at all. IF that happened (and it likely won’t) then it’s only one row of the cavities that will deform, you have 15 more entire rows of ditra holding it in place. The cavity will deform before the tile even notices it.
Question for you. I am in the middle of a kitchen renovation. I have already put ditra down using modified thinset to attach to the plywood, this went well. I then started laying the tile using unmodified thinset, this process is time consuming having to back butter every tile to ensure coverage. I made sure I packed the thinset into every square first then used the notch trowel and applied the tiles.
so, after going the first 50lb bag of thinset, I ran out of steam and decided to call it quits. I made sure i filled and leveled any exposed thinset on the ditra. I’ve read you can prefill these squares in advance and then come back another day and run the notched trowel over them. what are your feeling on this, does it provide just as good a bond? I personally feel it would be best to do it all at the same time to assure the best bond.
I’m asking because I did end up leaving some area with just the squares filled and leveled off but no tile on that area yet.
In a way I guess its the same as a piece of backer board at that point.
Hey Ray,
As long as you tile over it within a couple of days or so you’ll get the same bond as setting it the same day. I do it quite a bit.
Hi Roger,
I am tiling my dining room and kitchen and attempted to install ditra yesterday. My subfloor is particle board. Instead of reading the directions carefully, I followed the guy at Lowes advice and used unmodified thin set. A few hours after I put it down it started bubbling up and peeling away from the floor in a few places, especially at seams and corners. I am guessing that this is due to one or a combination of these factors: 1, using unmodified thin set; 2, the thin set was not wet enough to soak into the ditra; 3, I didn’t moisten the subfloor. What do you think? Is the ditra ruined? Is there anyway to fix this?
Thanks for your help.
Hi Josh,
The problem is the particle board. The glue that holds particle board together is water-soluble. When exposed to moisture it explodes! Well, it doesn’t explode, but it falls apart and disintegrates. When the particle board was exposed to the moisture in the thinset it began to delaminate, it causes bubbling in the substrate.
You need plywood or backerboard, particle board won’t work. Over wood you need modified thinset. The use of the ditra will be dictated by how much thinset is on the back of it. If the thinset did not bond to it you can likely reuse it. And it may not have. Your thinset may have been plenty wet enough but the particle board will SUCK the moisture out of it.
Moldy smell coming from bathroom following porcelain tile install. 2nd floor condo, not on slab. Tiled area = about 35 sq ft. Replaced vinyl tile over gypcrete substrate. Gypcrete was cleaned with citrus degreaser and NAC 101 applied to harden and retard flash drying of unmodified thinset.
This was completed about 3 weeks before Ditra install of porcelain tile. Toilet checked for leaks after reseating. No leaks. There was no moldy smell until after the toilet was reseated. While toilet was off during curing of thinset, and before grouting [epoxy] no moldy smell.
Bathroom completed about one week ago. Any suggestions? Does ditra
promote mold?
Hi Emerald,
No, it doesn’t. Nothing on a floor tile installation will. The smell would derive from moisture in some manner, nothing with ditra, unmodified thinset, epoxy or the NAC would cause that. My guess would be either something with your specific degreaser or the toilet – since it was cleaned and sealed 3 weeks before with no issue, and only began after the toilet was installed that would be my guess.
Is the entire base of the toilet caulked? All the way around? Or was grout inserted under it – in a manner which would seal it to the floor? If so you may have a leak and not know it because water can not escape out the sides of the base, it would find another way out or simply continue to fill the sealed space. That would be about my only guess.
Roger, my contractor did my floor 9 months ago. We use Gazzini Italian tile – level 5 porcelain. Not cheap. The Ditra was not installed correctly. Grout keeps cracking. Turns out, the Ditra is not adhering to the plywood. This is now my 3rd repair (over 40 tiles replaced) due to this moving of tiles. I have 6 tiles left, I have 9 more coming up. My tiles are discontinued. Should I be fighting my contractor for a totally newly installed floor by now? It seems with every repair, the adjacent tiles now come loose. If the floors been installed incorrectly, what is your professional opinion of having it all replaced right. Why now should I use less than perfect porcelain tiles that the contractor wants to find and continue to use as a Patchwork Fix. What do you feel………….
Hi Dor,
I feel that if it was incorrectly installed then it should be fully replaced. If the problem lies in the substrate, as you’ve stated, then nothing short of that will solve the problem, they will continue to have cracked grout and come loose.
Roger,
When installing the Ditra, I’ve heard that many pros, when time allows, will fill in the waffles as a separate step then the next day will lay the tiles. This ensures that they can work hard in getting the proper waffle fill. Would you suggest that extra step if time allows?
Hey Ed,
I do it when I have time. Not so much to ensure a fill but it sure speeds things up the next day because I’m working on setting the tile and getting it flat only, rather than adding the filling waffles part as well. It’s easier and quicker to set over a flat surface than it is over the open waffles.
Got a slight problem and wondering on how to deal with it. I used SLC to level the floor over top of my heating pad. Well, in one place I have some of the mesh of the heating pad sitting on top of the slc level ( about 1/8″ higher than the floor) and similarly in another spot I have what appears to be a heating wire just at the surface stting about 1/8″ above the floor.
The rest of the floor is flat pretty well, not necessarily level but flat.
Can I somehow put on the thinset a bit thick below the Ditra and try to use that to hide the little bump OR should I put the Ditra down and try to work out the bump when installing the tiles OR do you have any other suggestions?
thanks,
Hey Ed,
Yes, you can do that with the ditra. You may also be able to slice the mesh on the bottom of the ditra so that the wire sits between the cavities in the bottom. If it’s not too large and it’s straight that usually works. You can actually cut the mesh from the heating pad out of there if you want. It serves no purpose once the slc is over it.
Thanks Roger,
I’d like to get the floor as level as possible first then the Ditra and tile install will be much easier. What about just laying some thinset over the SLC and then use a long straight edge to get it flat and to pull off the excess?
You can skim over it with a long flat trowel. Thinset will stick to a straight-edge.
Looking at Ditra installation instructions and it shows 1/4″ expansion gap around the walls with sill seal in the gap. Now that is fine for the walls that will have baseboard or tile above it to which you can fill the gap with caulk. What about where the Ditra meets up with a drop in fiberglass shower base? I’d have to run a 1/4″ wide bead of caulk to seal and cover up the sill seal. Any suggestions?
BTW, do you have a section of tiling and how exactly to manage the tile cutting, laying of tile, thinset work time, etc.?
I normally leave a 1/8″ gap there and silicone it. I have bits and pieces of the things you’ve mentioned, mostly in the comments. Thinset work time is on the bag, it varies by manufacturer. The problem is there are so many variables on the cutting and layout/laying of the tile that it’s impossible to write things that will cover anything significant. I do have the basic layout and design manual in the library which covers a LOT of the basic stuff with layout.
So 1/4″ around the walls, 1/8″ at shower base?
Of course I will have all the layout already planned before I start with the tiling. My concern was over how to manage the install of the tiles when you have lots of cuts to do, are using a leveling system and are bound by the thinset work time. My thoughts are to just break the job into much smaller spaces and only mix enough that you think you can get done in the thinset work time.
I check into the manual in the library.
Thanks, this website and your help is very valuable and appreciated.
I leave an 1/8″ gap everywhere. You should only mix as much as you can use within the allotted time. No need to break it down into smaller spaces, just mix up as much thinset as you can use and use it until you run out, mix up more and continue.
Hi,
I am installing 2’x1′ ceramic tiles in a fairly narrow hallway. The subfloor consists of 3/4″ x 3″ diagonal pine planks. Over those, I’ll be installing 5/8″ exterior plywood, then Ditra over the plywood. How can I correct a 1/2″ x 6′ long dip in the middle of the 16′ long x 3′ wide hallway?? It seems such a frightful proposition to use SLC – can I shim the dip using wood slats prior to installing the plywood?
Thanks.
Hey Rob,
You can provided you ensure the plywood layer above it is screwed down tightly and there is no movement over it before your ditra goes down.
Hi there,
Thank you for your website.
I think I just made a boo boo.
When installing the Ditra on my plywood floor, I grabbed the wrong bag of thin set. I used unmodified with the Ditra instead of the modified I had purchased. I did wet the plywood before putting down the unmodified thin set if that maybe helps my mechanical bond due to the plywood not absorbing much moisture out of the unmodified thin set.
I am about to lay down the tiles over the Ditra using unmodified but haven’t yet for fear of issues due to using unmodified on the Ditra. Tiles are interlocking 16”x24” and use no grout.
What to you think? Last thing I want to do is waste the Ditra and start over, but I’d like the install to last a long time.
Thanks so much, Ed
Hey Ed,
I’m not concerned about the unmodified under the ditra, it’ll be fine. What I am worried about is the part that says ’tiles are interlocking 16×24 and use no grout‘. Exactly what type of tiles are you installing there???
Hi Roger, thanks so much for your advice and help.
These are the tiles I’m installing:
http://www.lowes.com/pd_398406-40398-CARMEN1624BROWN_0__?productId=3799699&Ntt=brown+tiles&pl=1¤tURL=%3FNtt%3Dbrown%2Btiles&facetInfo=
I’ve never seen them before nor have I been able to find any installation instructions (not even from the manufacturer or in the box). Actually, I have no idea what the best procedure is to install these. Just read the comments on the Lowe’s website. I’ve done standard tile in the past, but there doesn’t seem much room for mistakes with these.
Thanks again for your input – the wife will feel much more comfortable knowing I’m not just winging it!! Regards, Ed
I believe that would (or at least should) still require grout between the tiles themselves. If it is indeed porcelain then no matter the pattern on the face it’s still considered a full porcelain tile, and as such requires a grout space between each individual tile.
Thanks Roger. I’ll grout it out. Thinking unsanded Gray.
Regards, Ed
I have removed two layers of linoleum from which a concrete floor (apparently) exists beneath my basement floor.
Its on 3rd level of a 4 level split. My intention is to tile the floor , first using ditra with a modified thinset (versabond) however the Glue residue from the Linoleum cannot be completely removed. I think I am at about 50 % bare concrete flo0r. I have tried a heat gun , 4 inch scrapper with new blades, also tried to loosen the residue with water and dish soap.
Im also a newbie at Tiling but before i start anything I want to know how much glue residue is too much for the Versabond to stick to? Im sure i could keep at it for a few months and go completely insane but I need some benchmark advice to work to.
can you help?
Hi Dave,
It depends on what type of residue you’re speaking of. IF (that is a definitive if) you splash water on it and it soaks in then the versabond will bond to it just fine. If it does not then you need to either chemically scarify it until it does or switch to a thinset approved for use over cutback like laticrete 255 or mapei ultraflex 3.
Thanks for the quick response roger….I just poured water onto various spots in the room
will let you know
Thanks
Roger i have a follow up question… After 20 minutes the water seems to have absorbed into the residue as it makes it easier (but not too simple) to scrape off. Obviously this is in an area where there is still quite a bit of the paper or backing. In areas where the glue residue is very very thin (where it looks like the trowel ran across it) it is the same and scrapes off with some effort. Both shavings are damp on top in both my examples but the bottom shavings are dry
I know this is critical as Uncoupling membranes are not cheap so if by your reply you stated that if water is absorbed than Versabond will work fine…does this fit the criteria?
Get as much of the residue off as you can by soaking it first, then yes, the versabond should bond just fine.
Thanks again Roger I planned on allotting 2 more days of scrapping
…peace of mind goes a long way for the DIY er
you have provided that thanks
Hi Roger,
Thanks so much for the info. I’m going to be installing Ditra for the first time in my kitchen, and have a few random questions:
1. When laying Ditra over a plywood subfloor, should the subfloor panel expansion gaps just get filled in with the thinset as you go? Or should they be covered by something or caulked before laying down the thinset? Just wondering, since I thought those expansion joints had to stay open.
2. I saw a video online of someone installing Ditra over plywood – they moistened the floor a bit before laying down the thinset. I believe they said it was so the wood wouldn’t ‘suck out’ moisture from the thinset too quickly. Any value in doing this? I didn’t see any mention in the Ditra handbook.
3. I have a 5/8″ t&g plywood subfloor, 2×10 joists, 16″ oc, all in good condition. I read in the handbook that installing Ditra over this floor is acceptable, but I’ve read elsewhere that the floor should really be beefed up a bit. Especially since I’m going to be laying 18×18″ tiles. I had considered adding 1/2″ plywood, but I’m wondering if I can get away with 3/8″ and still get some decent additional structural support out of it. The 3/8″ would lessen the height transition a bit to the adjoining vinyl floor. Any thoughts on whether 3/8″ would be beneficial, or should I stick with 1/2″ ?
Thanks!
Just to clarify on 1. – I’m referring to the expansion gaps between panels, not the gaps along the perimeter which I know need to stay open.
Thanks
Hey Chris,
1. They need to be covered with something. Thin tape works, silicone works better.
2. It’s not normally necessary but it certainly doesn’t hurt. I don’t do it with plywood, I do with backerboard.
3. 1/2″ would, of course, be better, but the 3/8″ will help.
Thanks for the info on the site. I am tearing out a vinyl floor and replacing with ceramic. What are your thoughts on whether I need to remove the luann under the vinyl? I’ve seen differing opinions…but most say it must go.
Thoughts?
Hey Ryan,
The luan must be removed. It should be nowhere near a tile installation. It simply isn’t sturdy enough.
Like your site, a lot of good information. I am currently tiling ( hired a “contractor” ) my living room, kitchen, hall. All linked and a total of 800 sq’
The contractor said that 1/4″ or at max 3/8 over my existing 5/8″ ply on 2×10 @ 16″ would be a good sub floor for this application. I called the engineering firm that build my home they said use 5/8″ with proper over lap.
So I made him do this………
He then said he was ready to tile, I asked if detra was worth using due to the high cost of the tile ( 16×24 ceramic ) and the electric floor heating. After looking at your web site and calling just about every dam manufacture of mortar/tile I came to only one conclusion
Use the dam ditra
So I made him do this ……..
They all said basically that if we use the ditra the floor will have the best chance at a trouble free life and if there is a problem with a cracked tile that the likely hood of damage to the heat cables would be reduced.
Now my ” contractor” wants to put the ditra done on unlevelled floor due to the heating mats
The question I have is
If the floor which is very level has heating mats and due to the mats has a differential of about a 1/8 within a foot ( mat does not cover areas were furniture will be ) can he install the ditra and level / back butter the living crap of of the tiles or just level the substrate and be done with it so I can get back to watching tv and drinking my beer
It could be me over doing it or
He is either stupid or screwing with me and he better not be screwing with me
Thanks for ur input
Hi Jason,
The substrate beneath the tile should be leveled, you should NOT make up for that with thinset beneath the tile – especially if your floor is currently level and flat. There is a method of doing it as he installs the ditra, it’s here: Installing heat wires beneath ditra. It is always best with an installation that size to level it using slc, though. Either will work.
I am about to put down some white marble mosaic tiles that come in 1’x1′ sheets with a mesh backing. My substrate is a concrete floor below grade that has been treated with level quick rs self leveling underpayment. There is a spot in the levelquik that had formed a little funnel shape due to draining into a crack in the concrete floor. I intend to ether fill it with a patching skim coat compound or just with some thin set when I install the ditra. Not sure if filling it with mortar will cause problems down the road. The funnel is about 4″ across and 1″ deep.
I intend to use un modified Mapei Kerabond mortar to set the Ditra and push it down with a wooden float or a 2×4. My main question is about which mortar to use under the marble. I heard that marble can stain and its recommended to use white mortar with white marble etc. There are also marble specific mortars that are modified. I understand that modified mortars are not recommended over Ditra because they don’t cure properly when stuck between two moisture barriers.
Hi Jimmy,
Technically you should use a white unmodified mortar. Laticrete 317 is available in white and works well. That said, I use modified mortar over kerdi – a lot. You’ll lose your warranty from schluter, but it works just fine as well. The marble-specific mortars are all normally medium-bed mortars. They have more sand in them and it prevent it from shrinking as much as it cures. No matter which you choose it needs to be white.
I am doing my first tile installation using Ditra on a bathroom floor. The substrate is 3/4″ plywood. Would it be foolish to skip the Kerdi-Band on the seams? The bathroom is over a dirt crawl space so I am not too concerned about a completely waterproof floor.
Thanks
Hi Marc,
We rarely put kerdi-band on ditra unless there is a specific reason or need for waterproofing. It will be just fine without it.
Thanks so much. That will save me some bucks and time.
Hi Roger,
Great site btw. I’m doing my bathroom floor with infloor heating and then Ditra as per heater manufacturer instructions.
Here’s my plan:
Floor tile: 12” x 24” porcelain tile
Patch floor spaces between plywood, caulk around perimeter of wall, dam around drain holes in floor and at door to bathroom.
Install floor heating.
Apply primer to floor and then self level flooring to cover floor heating.
Use a non-modified thinset on top to attach Ditra.
Use non-modified thinset to attach Kerdi-band to seal between Ditra joints.
Use non-modified thinset to set porcelain tiles.
Does this seem right to you? I’ve read that some use modified thinset to set the tiles on Ditra. Would that just mean longer dry time and if so how much longer?
One other question…if you wait too long the thinset will not adhere as well to the tile. How can you tell the thinset has reached this level and about how long is that (5 min, 15 min, 1 hr???)?
thanks,
Did you miss my question little Elf :-)?
Nope, it just didn’t post before I moved to the next question. Sometimes I just need to slow down. Your order is correct, except it’s easier to install the primer before the heating wires – in ensures complete coverage of your substrate. If you look on the bag of whichever thinset you use there will be what is called the ‘pot life’, that is the usable amount of time once the product is mixed. It varies from thinset to thinset.
Thanks Roger.
For the primer, I’ve heard that the SLC needs to be down within a certain amount of time after the primer is applied. Not sure why that is as I would think the primer is to make for better adhesion and once it is dry it is dry so….
I’ll definitely check with the primer instructions but have you ever heard of this?
Roger,
Looked into primer that I’m using: Mapei Primer T
Here’s what they say:
If Primer T sits for more than 24 hours, re-apply a
second, undiluted coat and install underlayment within
correct application window. If application window is
missed again, remove primer mechanically and start the
installation on clean substrate.
So have to get primer down, install heating and lay SLC within 24 hrs.
Being my first time I’ll have to be well prepared and get it all done in one day. Maybe I’ll layout the heating and have it all cut (it’s in a mesh roll) and laid out in advance so that the install after the SLC will be quick.
I’ve still never heard of that.
I would get the primer down in the afternoon and lay your heating wires, then slc first thing in the morning.
Never heard of that at all.
If you have any more questions can you PLEASE post it as a reply to my last answer? I have over 15000 comments on this site, I really can’t keep track of everyone all the time and if I don’t know which project you’re asking about I really don’t have time to search 15000 comments to find it. Thanks.
Just thought of on other thing. My floor with the 3/4″ plywood installed is rated for L/720 (12″ OC 2 x 10’s, 12′ max span) so I’m NOT thinking of adding another layer to the subfloor. My subfloor was installed with 1/8″gaps between the boards and between the older boards (that are now just extending into the room about 1″ from the walls).
Before I pour the SLC I’ll have to patch these gaps up otherwise the SLC will drop into the 1st floor ceiling. If I patch these holes with some kind of cement based patching compound, does this cause any expansion problems with the subfloor? If so, could I just apply some duct tape over these spaces so that the spaces stay open and the duct tape prevent the SLC from entering, or just fill with caulk, OR, is there something else I could use?
thanks again for all your help,
Caulk is the easiest and most durable.