Photo 1
I am not writing this to tell you why your tile is cracking or why your grout is cracking – I have other posts that may tell you that. (Click on the pretty little links 
) If you happen to have Schluter Ditra as your substrate, this post will tell you why either one of the above may be happening.
While Ditra is my preferred membrane for floor tile installation (as well as countertops and tub decks) it absolutely needs to be installed correctly. The two main techniques for this are fairly simple:
- Make sure the cavities (waffles) are filled correctly
- Install it over an approved substrate (and with the correct type of thinset mortar)
Photo 2
There is a lot more to ditra than those two items but if either one is incorrect I can nearly guarantee a failure. See photos 1 and 2 there? The tile was cracked and it was a direct result of a) not getting the waffles filled correctly and b) improper coverage on the tile. Now b may be due to not backbuttering the tile, an improperly-sized trowel, letting the thinset skim over or set too long before installing the tile or simply incorrectly mixing the thinset. All three of those things will cause any tile installation to fail – whether you use ditra or not.
Not filling the waffles correctly, though, will cause the tile to not be fully supported and/or not ‘locking’ the tile into the ditra. Because it is not correctly locked into the ditra you will lose the mechanical bonding properties of ditra and you may as well install it directly to particle board at that point (That was sarcasm – don’t do that!). For more specifics about exactly how ditra works you can check out Provaflex vs. Ditra wherein I describe exactly how the mechanical bonding process works – and rant about a particular jackass. But the mechanical thing – that’s what you want to concentrate on.
You need to use the flat side of your trowel and spread thinset in every direction over the ditra to ensure that all the little waffles are full. Since the cavities are dovetailed (that means they go down and away from the opening) you need to ‘force’ thinset into the bottom corners of the cavities. Simply running the trowel over the ditra will not do this. Simply running the trowel over the ditra did that (photos 1 and 2).
Photo 3
Installing ditra over an approved substrate is much, much easier. In fact, nearly every bare substrate you find in a modern house would be considered an approved substrate – shiny linoleum is not one of them (Photo 3). While there are thinsets that ‘say’ they will bond to linoleum (and some of them will) apparently the jackass who installed that particular floor was not aware of that. 
See photo 4? I lifted that up with my pinkie – literally! It was not attached at all. He may have had correct coverage beneath the tile and all the little waffles filled – I have no idea. There was not enough stuck to get enough leverage to tear one off and find out.
Photo 4
Most any plywood (even osb 
) is an approved substrate for ditra. And if you use a thinset approved for that substrate, there are no problems at all. Photos 3 and 4 had an unapproved substrate and, apparently, incorrect thinset (and a shitty tile job, but that’s a whole other post). It was nearly guaranteed to fail.
When you buy ditra for your installation every roll comes with a handy little instruction booklet. You can go to Schluter’s Ditra Page on their website and access the instruction booklet (This link is a PDF!). They even have a flash video about the proper installation technique. You can leave a comment below and ask. You can email me. You can send up smoke signals – I’ll answer.
Given the 17 ways to acquire correct ditra installation information above there is absolutely never a reason to do it incorrectly. Ditra, in my opinion, is the best membrane for most floor tile installations. The only time I’ve seen it fail is due to incorrect installation. And that isn’t just the common BS everyone accuses failures on. Me, personally, every one I’ve seen fail is incorrectly installed.
If you use ditra, and if you have an approved substrate, and if you have the correct thinset mortar, and if you fill the waffles correctly, and if you use the proper trowel and get proper coverage it will not fail. Yes, that’s a lot of ifs – when you read it. In practice it really is not that many things to get right. It’s just common sense, mostly.
So here’s one more if: If you have any questions at all about correctly installing ditra and using it for your tile installation please, for the love of all the marble in the Sistine Chapel, ask me below in the comments. I WILL answer you. I’m just super-cool like that 
I laid ditra 1/8in mat over osb with modified thinset and 24 hours later some of the edges and corners don’t seem to be adhered ( aren’t secured into the thin set) is this normal?
Hi Andrew,
No, it is not normal. You need to cut out any unbonded areas and redo them. Make sure you embed the ditra into the thinset fully so the mesh on the back is completely saturated.
Hi,
I’ve got 5/8 plywood subfloor with joists at 16″. Everything I read says Ditra directly over that is ok, but adding another layer of ply would be preferred. I’m trying to f stay close to the height of the hardwood in the adjoining room. What do you think? Can i run with the existing subfloor and 1/8 Ditra?
thanks!
Hi Tom,
Yes. Except everything from the manufacturer states 3/4″, not 5/8″.
To answer my own questions, I called Schluter this morning and they said yes, unmodified over the SLC part and modified over the wood part. Also 24h is long enough for the SLC to cure before Ditra is applied, just as it is for tile directly over the SLC.
Yes, you will maintain your warranty that way, but modified will work.
Hi. I’m tiling a 5’x6.5′ bathroom floor. I had to use self leveling compound over about 2’x5′ of the plywood subfloor. Do I need to use un-modified thinset to adhere Ditra to the SLC portion and modified for the rest, or is modified OK for 1/4″ deep SLC? Also, is 24h long enough for the SLC (Mapei) to cure before putting down the Ditra? That’s what it says for tile but I don’t know if it would be longer for Ditra since the SLC will be sealed under the plastic Ditra.
Thanks,
Ben
Hi Ben,
Yes, you can use modified for all of it. 24 hours is just fine.
Hi there,
Just did my first tiling project ever — our two bathrooms. Used Ditra and at the advice of the Lowe’s flooring guy, we used a pre mixed adhesive both below the Ditra (plywood sub floor) and between the Ditra and clay tiles.
Now after reading I’m realizing this is against the manufacturer’s recommendations. Do we need to wait an extra long time before walking on the floors, grouting, etc.
Hi Ryan,
What you need to do is go back to lowes and speak to a manager to find out how long it’s going to take before they can cut you a check to completely rebuild your floor, because the ‘premixed thinset’ (which is NOT thinset, it’s MASTIC) will NEVER cure. EVER. It needs to be replaced, all of it.
And I’m serious, it will never cure. Even waiting a very long time. It’s just like putting the lid back on that bucket.
Roger
I’m installing tile on an in closed deck that has 16 inch spacing with 5/4 deck boards. I have 1/2 hardy backer glued and screwed to the floor and the floor seems extremely solid. I have put under neath 2 inches of closed cell spray foam insulation to tighten up all and any cracks or seems then placed r30 insulation and 1/2 plywood underneath to close up and insulate the floor even more. my question and worry is should I put ditra? my to friends who are tile guys and looked at the floor say it would be over kill and a waste of money that the hardy backer is more then enough, however with it being a porch I am worried about expansion and contraction going from summer to winter cracking the tile or grout. The ditra sounds like it would help stop this from happening. however it would put a lot more cost and work in to the job. along with the fact that neither of them have had much experience putting it down. Also I am not sure what type of thin set to put down before and after the ditra I was looking at versa bond but could really use your help. I would also be doing the job alone so it would take me a day to do the ditra and then a few days to do the tile is it safe to walk onto the ditra for a few days before putting down the tile. thank you for any help Lew
Roger
thank you so much for getting back so quickly. I see a lot of the “how to” videos are using modified underneath then un modified on top is that because they are setting it on plywood and not hardy backer? also I see that some guys wet the floor so that It does not dry out the thinset so quickly do you think this would be necessary for hard backer. I was going to go with Laticrete 317 if possible, however if I can not find it I think lowes cares Mapei Karabond but i’m not sure. will straight ditra work fine for me or do you recommend just going with the LX. I don’t have much room on height if I’m going to match up the floors however would any one really notice the 1/8 difference .
Yes, that is because they are going over plywood. Yes, if you go over hardi you should sponge down the hardi first, leaves a better bond. Hardi sucks moisture out of your thinset quickly if you don’t. Regular ditra will be fine, you don’t need the XL.
Hello,
I am installing 12 X 24 porcelain tile in two locations during a renovation/addition to my home. I want to install it in the laundry room, an existing room with plywood underlayment. There were “sticky tiles” installed previously, but I’ve removed them and scraped the surface of the plywood clean.
I believe the 1/2 plywood will be an acceptable substrate. What do you think?
In addition, I’m putting the same tile 12 X 24 in a new small bathroom addition. The floor is 3/4 OSB. I’m worried the OSB won’t be acceptable. Do I need to add a layer of plywood before Ditra?
Really appreciate your advice and info.
Thanks, Sandy C.
Hi Sandy,
Neither of those are a suitable substrate to bond the tile to. You need something like 1/4″ backerboard, ditra or another tiling membrane, greenskin, etc. Tile does not get bonded directly to wood.
Hi Lew,
Ditra is always a better solution. It has many benefits over the regular backer board. The backer would work fine, but with ditra would be better.
If you are concerned about a warranty from Schluter – you need unmodified thinset both under and over the ditra if installing over backerboard. If you aren’t concerned with the warranty you can use versabond for both.
Yes, you can walk on ditra before it is tiled.
So I have never used Ditra before and am planning on doing so for the tile in our new house (builder is letting me set the tile). I want to get it right and who better to ask than the Ditra guru you. I have a few questions about it,
1. for heated floors install the heat mat under Ditra?
2. The substrate for the floors is OSB in all areas what thin set should be used to attach ditra to substrate.
3. It says to use unmodified thinset to set tile on the floor and I understand the reasons not to use a modified thinset, but could a product like ardex x5 be used which is a chemical cure modified thin set?
Thanks for your articles with the smidggen of sarcasm thrown in for entertainment.
Kent
Hi Kent,
1. Yes, heat mat beneath the ditra.
2. Any good modified thinset will work fine. Use an slc primer on the osb before you place the heat mat down, that way the thinset will bond well to all areas.
3. Yes, x5 is actually an approved thinset for use over ditra, although they don’t advertise that.
Roger,
I’ve in closed my porch to add on to my house, the porch has 5/4 deck boards screwed on , and then 1/2 hardy backer glued down with tight bond sub floor and screwed down with hardy backer screws, I have 2 inches of closed cell insulation sprayed on underneath along with r 30 and 1/2 plywood screwed in to the bottom of the floor joists to seal the floor, I didn’t have room to put down plywood first before the hard backer if I was going to match up the floor to the current floor & hope it doesn’t screw me in the long run. The floor is sold as a rock and nice and flat. My buddy is a tile guy & said I’d be fine laying the tile right to the hardy board but I’m worried about expansion and contraction going from summer to winter.I was reading about ditra and thought this material would reassure I have less problems of cracking do to the change of seasons. I would like to get your thoughts on this project and any recommendation you may have to help me. This is my first tile floor alone. I was also wondering how much the ditra would raise the floor and if I should use the XL or reg. what type of thin set to use below the ditra and what type above. Also doing it alone can I lay the ditra down one day and then walk on it the next few days laying out and setting the tile? Any information you can offer I’d greatly appreciate thanks Lew
Hi Lew,
I would absolutely use ditra in that scenario. Schluter wants unmodified thinset both below and above the ditra. Regular ditra will raise your floor about 1/4″ and ditra XL about 3/8″.
Hi Roger,
We have been asked to install a 1″ thick blue stone floor over an overly structural Plywood floor with infloor heat and a 1-1/2″ lightweight concrete over pour. I have 2″ remaining to bring everything to the correct height. The bluestone is 1″ so I have 1″ for a bed and would like to install ditra first before my thick bed.
Is there a preferred thickness of ditra i should use? I have 1/8′ in stock and was going to use this.
Mike
Hi Mike,
The regular ditra (1/8″) will be fine. I would put a 3/4″ mud deck over that before the ditra.
Hi Roger,
I recently applied the ditra….and was getting ready to lay tile later that week over the ditra when I had time. To make a long story short I mixed the non mod thinset to apply over the ditra and changed something with the tile and did not install it……But………..I already mixed the thin set and did not want to waste it. So I filled (ONLY) the waffles on the ditra. And will have time to tile it in a few days. Is that okay ? So in a few days when I get time I will only have to notch trowel over and set tile ? I was thinking it would not change anything the ditra or the way it works ?
What do you tihink ? No worries ?
Thanks & Happy Holidays!
Hi Kevin,
No worries at all. I regularly fill the cavities so I can work over it the next day before I tile the floors.
Hello. Do you suggest a second layer of ply 1/2 over existing 3/4 OSB before ditra.
Hi Chris,
Yes.
I laid DITRA over concrete with unmodified motor and the next morning you could pull the DITRA off the floor fairly easily. Is this normal or does it need a better adhesion?
Hi Jason,
It is normal because your tile will not be pulling your membrane up off the floor. After 28 days you won’t be able to do that.
Stop pulling your membrane off the floor.
Couple questions…using porcelain time that looks like wood (6×24 pieces)
1) I’m tiling over a sunken living room that’s on a slab. There is a crack (runs the length of the room) in the slab that an attempted repair was done on. Is there anything I need to do to the crack before putting the ditra over it and is it smart to even tile over such a crack?
2) what is the best way to cut the tiles on a curve (fireplace in sunken living room is curved)
3) how do I transition from the top of the step down? I’m putting the tile both above and below the step what is the best cap or piece to place on top of the tile. I was also told to place drywall on the riser and paint it. How do you attach drywall to cement?
Thanks
Hi Ryan,
1. It depends on how a repair was attempted. Normally the ditra should be fine with it.
2. A grinder with a diamond wheel.
3. Schluter has the best stair transitions I’ve ever used. Just google it, they work VERY well. You don’t attach drywall to concrete, but you can attach tile to it.
Hey Roger, I have old wooden planks in the laundry room. I have 3/4″ remaining before the floor is level with the kitchen. Do I need to put down plywood before installing ditra?
Hi Derek,
Yes, you need plywood over planks. Too much differential movement between planks to go over it with any membrane.
If I have a 3/4″ substrate of Plywood, is there a reason to apply a 1/4″ layer of Hardibacker before laying down the Ditra? Is there a real purpose in this situation to using the Hardibacker at all?
Another pro I spoke with always puts down Hardibacker before the Ditra but that seems like duplication to me.
Hi Tom,
No reason for the backer, you can go directly over the plywood.
Hi Roger. 1 I need to install nuheat blanket on new 3/4 plywood first before my ditra, 2 what products modified or not from ply to blanket then blanket to ditra? 3 Can I leave thermostate sensor wire above finished tile floor?(have a problem with this in a different room.)
Hi Jeff,
1. Is not a question, is it?
Yes, nuheat first, then ditra (if that is the question).
2. Modified under nuheat as well as under ditra.
3. No, it must be embedded in the tile lamination at the same level as the wire to work correctly. What kind of problem?
Hello it’s my first time using Kerrdi membrane I have installed it on walls and in shower floor over a Nu heat mat that was installed with modified thinset then skim coated with modified then kerdi adhered with unmodified
I’m using pebble style on floor in shower and porcelain tile on wall .im concerned that the unmodified thinset bonds to wall tile -should I be ? Also wanted to useI think it’s called LFT from mapei for pebble in shower because I’m concerned that pebbles (actually like 1-2″rocks) are not completely flat on bottom so I was hoping for some bite on side . Please advise. Thank you
Hi Anthony,
Pebble tile is an extremely bad choice over kerdi over a heating wire. If they were flat bottom they would be fine, the rounded bottom are not. They create a point load on very small areas, it could end up damaging the wire, puncturing the membrane or causing the stones to come loose. LFT would be a good choice, but you NEED to get them fully embedded up to the halfway point of the stones.
Not sure what you mean by being worried that unmodified bonds to wall tile – yes, it bonds to all tile, if that’s what you mean.
Roger,
Thanks so much for your efforts and insights. Some thoughts on a master bathroom reno…
Do you think Kerdi-Band over Ditra compromises tile adhesion in anyway? I mixed my Ditra-Set as suggested on the Schluter videos and filled the cavities. Pulling back, I seemed to have good coverage. I’m not sure if I pressed too hard when setting the Kerdi-Band with knife, but 12 hours later it seems a bit easy to lift a corner (my impatience with the unmodified?).
Your input is much appreciated.
Steve
Hi Steve,
It is a combination of impatience and worrying about something that will never happen to your kerdi-band. It is always easy to ‘lift’ a corner, but your tile installation will never be subjected to pressure like that. If you can grab a corner of the kerdi-band and slide it off the ditra – then worry.
It does not compromise the bond of the tile in any way.
Great site, lot’s of useful info!
I would like to install the Ditra heat system on a concrete slab in a main-level laundry room and half bath. Currently there is linoleum glued to the concrete. I can get the linoleum itself up fairly easily, but there are 2 things left behind: the tan-colored linoleum glue and what appears to be a black substance under that. The house was built in 1972 and I have been told the black stuff is likely an old (possibly asbestos-based) glue from the original linoleum that must have been removed to install the linoleum I ripped out. I think I can get most of the tan glue up with some scraping, but the black stuff is on there pretty good. One thing to note is the black “glue” isn’t uniform across the entire floor… it covers about 60% of the floor from what I can see so far. The Ditra instructions indicate use of non-modified thinset to install the Ditra to the concrete. I have two concerns with that: 1) will the non-modified thinset adhere well enough to the black residue, and 2) will there be an issue from the Ditra heat system warming up the residue and possibly causing the bond to weaken? Basically I am trying to avoid having to remove the black stuff for both health reasons (possible asbestos) and just because it looks like it would be a pain!
Hi Chris,
It’s called cutback, it isn’t asbestos, but you don’t necessarily need to remove it. Unmodified thinset will not bond to it, modified thinset will bond to it (and you can use it beneath ditra) if you prime the floor first with something like Mapei Primer T or a similar product.
Thank you for the quick response. I wasn’t familiar with the term “cutback” but now that I am, I did a little internet research. Most references state that it may contain asbestos, so I will err on the side of caution and assume it does. I will definitely look into using the Mapei Primer T. One last question: you state that I would be fine using modified thinset beneath the Ditra, are you certain? Just want to double-check since the Ditra instructions state otherwise. Do you know why Ditra recommends unsing unmodified thinset?
Yes, I do know why they recommend it. They are of the opinion that using modified thinset between the ditra and concrete is not needed because the plastic base of the ditra will help retain water in the thinset long enough to get a complete cure – the reason modifiers are added to thinset. Yes, I’m certain you can use it
I’m also positive you’ll lose your warranty from schluter if you do.
Can I use a 100 pound roller to set the ditra on a plywood floor.
Thank you
Hi Alan,
You can, but you shoudn’t. That’s extremely heavy for ditra. It would be better to flatten it down with a block of wood.
Hi Roger,Does plywood have to be completely clean for modified thinset beneath ditra. tore off old vinyl floor,it’s smooth but has some old thinset still on it. thanks bob
Hi Bob,
Ideally yes, but realistically as long as it soaks in water when you splash some onto it they thinset will bond just fine.
Hi, I am set to install new porcelain tile 12×24 in my powder bath on main floor tomorrow afternoon and I have purchased ditra, I was informed that I put down thinset, then ditra, then more thinset and that’s what I’ve also read. So with that information and the information here I want to make sure I’ve got the right thinset….I’ve purchased porcelain thinset that was recommended for my tile at the store. I just need to make sure I’m using the right thinset.
Thanks
Hi Jason,
If your bathroom floor substrate is wood then you need to use a modified thinset to bond the ditra to it. You need to use unmodified thinset to bond your tile to the ditra. I have no idea what porcelain thinset you have so I can’t really tell you whether it is modified or not, it likely is.
Hi Roger – Great info here! I’m doing a small-ish downstairs bathroom. I had a shower added, so the contractor had to cut the slab to tie in the drain. The screed job is ok, but there are a couple spots where the new concrete meets the old that are a little high, and the new concrete is a little low in spots. I’d say no more than about 1/4″. I still have to grind off the rest of the adhesive residue from the vinyl that used to be there, so I can probably knock off the high spots while I’m there, but would you say I should spread some self leveling compound across the floor before proceeding, or will the thinset/Ditra/thinset be enough to even out the slight irregularities? Also, in speaking with Schluter, my understanding is that non modified is correct for use on a concrete subfloor, right? My plan is to use Custom Building Products’ Uncoupling Mat Mortar both below and above the Ditra. Schluter was happy with this product (and in fact it even says it’s “made for Schluter-Ditra and other uncoupling membrane systems”. Do you have any opinions on this mortar? If I do need to use self-leveling compound, how long do I need to let it cure prior to laying the Ditra? Finally, I plan to seam the Ditra with Kerdi Band, but is there any use in running the Kerdi Band along the perimeter to waterproof the transition from floor to wall? the wall will be just drywall (not tiled), with a baseboard to cover the kerdi.
Sorry to hit you with so many questions, and thanks in advance for your time!
Hi Steve,
Provided it is relatively flat, as in withing 1/8″ or so when you’re done grinding, you don’t need to use slc. If you do use slc you need to wait 24 hours before going over it. Yes, the uncoupling mat mortar is the correct product both below and above the ditra, it’s good stuff. No reason to tie the ditra into the walls with kerdi-band unless you want a completely waterproof floor, which is kind of a moot point if you happen to have a doorway into the room.
Our tile installer has never laid ditra or tile on ditra. I am installing the ditra heat system myself. The installer will be tiling my shower and then the floor tile. They suggested I do a layer of thin set over the top prior to their arrival 1 to protect the wire and 2nd to ensure there are no voids. If you agree that this is OK, which thin set should I use. Saw a site where a guy used his left over modified he used to lay the ditra but did use unmodified to lay the tile.
Hi Doug,
That’s fine. I would use modified, but that’s me and I’m not concerned with the warranty.
Roger,
Thanks for your site and your willingness to respond to questions! Awesome to say the least. Couple of quick inquiries:
Recent Beach Condo Purchase. I have an original 19/32 Plywood Subfloor as my 1st layer. I recently installed a 2nd layer of 19/32 Plywood and attached firmly with screws. (I did this to provide extra stability as the Floor Joist are 24″). I do have a few “rises / humps” that related to variations in the joists in spots. My plans are as follows:
Use MAEPI Kerabond thinset (not sure if it is modified or unmodified) below Ditra XL to adhere the Ditra to the Plywood and to level out the joist rises.
Use Ditra XL vs Non XL due to the spacing of the Joists.
Use MAEPI Kerabond thinset (again – not sure if it is modified or unmodified) between the Ditra XL and the Tile to set the tile.
Questions:
1.) Is the above plan consistent with what you would do (correct thinsets specs?)?
2.) Can I do the thinset below the Ditra one Day, and do the thinset/tile installation above the Ditra on a different Day?
3.) I am laying a pretty large area of tile – are there any concerns if I completed 1/2 of the tile installation on one day, and had to pick up from where I left off the next – In other words, what would you do around the area where you are ceasing installation of tile after the first day (as it relates to the thinset)?
I believe the Ditra is the perfect answer for my Tile installation as it is more forgiving in my particular installation.
Thanks so much for your help!
Ed
Hi Ed,
1. You’re good up until the kerabond. It is an unmodified thinset. While it will work well for between the tile and ditra, you NEED to use modified thinset between the plywood and ditra.
2. Yes
3. No problem at all. You can split it up in any way you want.
Thanks Roger for your response. I really appreciate it. Do you have any recommendations on which modified (between the plywood and ditra) and unmodified (between the ditra and ceramic tile) thinset products and modifying agents are the best to use? What specific products would you use if you were doing my install?
Thanks again!
Ed
Hi Ed,
I would use laticrete 252 or 253 under the ditra and 317 over it.