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 just got done installing Ditra over plywood that was primed with B.I.N. A shellac based primer. Is this going to fail if I proceed to tile it? Was told (too late) that the thinset would be bonded to the primer and not the plywood and would pull the primer right off the plywood. I used versabond thinset
Hi Chris,
My guess would be that yes, it will fail. If water beads up on your primed plywood then the thinset can not bond into the plywood itself, it may bond to the primer for a period of time, but that’s it. You NEED to bond into the plywood.
i am planning on tiling an area of my basement (100sqft), and plan to use ditra. There is a 4inch pvc pipe sticking out of the slab in the corner, which i plan to cut flush and put a cap. my concern is when installing the ditra, do i need to apply something to the cap so that the thin set will adhere to it?
thanks.
Hi Anthony,
I would cut it 3/4″ below the slab, then cap it and fill it in with deck mud. Let that cure then go over it with ditra.
RE: Finished Height
What should I assume using the correct trowels and 1/4″ thick tiles.
(Thinset/Schluter/Thinset/1/4″ Tile).
Thanks Roger
Hi Tom,
Normally 1/2″ – 5/8″ or larger, depending on the size of your trowel.
What do you do when you have different tile thicknesses? ie main floor tile vs thinner border tile? Hence the question can you adhere Ditra on top of Ditra for the border tiles with modified thinset to match the heights? Please help!
You can use the search box at the top of the page or just click this link. Setting different thickness of tile
Can you adhere Ditra on top of Ditra? What do you use? modified thinset?
Hi Paul,
Yes you can, with unmodified thinset.
Couple of questions:
1) Any issues with walking on the Ditra after install, but before we tile without damaging the baffles?
2) We have 1225 sq. feet that will have natural slate. Do we need expansion joints through the middle, or just space on the edges?
Thanks so much!
Hi Cheryl,
1. It depends on how hard you’re stomping and whether or not you’re wearing heels.
You can walk normally over it without damage, for a while. Eventually it will wear down. That’s one reason I pre-fill the ditra once I get it down.
2. Expansion joints need to be placed every 20-25 feet on interior applications, and every 8-12 feet in exterior and interior with direct sunlight exposure. It depends on your longest run, not the total square footage. If you have a run longer than 25 feet then it needs to be placed somewhere within those parameters.
Hi – I’m planning to do a DIY porcelain 12×24 tile on a small bathroom with only 30sq/f. The floor is concrete but has vinyl or linoleum (either one) and Im going to use Ditra as a membrane. Can I use TEC 380 unmodified thin-set without removing the vinyl/linoleum? Just like what you stated –NO NO
but ditra instruction say If the vinyl still in good shape and free from any wax then it’s a Yes… 
Soo Should I remove the vinyl or not??!! IF yes how hard it’s to remove the vinyl? IF no, why not? TIA for replying……..
Hi James,
I ALWAYS remove it and begin with a bare substrate. Any time you install tile it is only as solid as what is beneath it. If I didn’t install it, I don’t want it under my tile. You CAN go over lino, but you need to do it with a thinset approved for that use, which means highly modified like ultrflex 3 or laticrete 255. Unmodified it not it.
How hard it is to remove varies wildly.
Natural stone. Correct thinset, read over instructions many times.
My question is when i laid my 1st sheet of Ditra and flattened it out there was a small 1×1 area that had a wavy look.. In the middle of the sheet. It bonded properly, the thinset may of been just a tad uneven. Is that ok?
Hi Nate,
Yes, that’s fine.
I just finished laying a large room of polished travertine tile with Bostick Urethane grout. The floor is covered with a milky residue, enough to take away the polished look. The product Blaze works to remove it, but is expensive and takes a lot of effort to get it back to a polished look.
Do you have any different suggestions for removing this haze?
Hi Wes,
Just a better cleanup as you install it. With some tiles you don’t really have any options other than the blaze. Try to only do about three square feet before wiping, that usually helps.
What type of thinset should I Use to install the ditra I have leveled the floor with laticrete nxt level.
Hi Doug,
Unmodified.
I have red pine floor in my bathroom and want to install tile over it. I’m looking at the Ditra installation directions and it says that plywood underlayment is needed over plank subfloor. all the tile contractors that have bid on this job say that plywood isn’t necessary because the pine is in such good shape. Should I trust their advice, or insist that they use plywood, or look for a different installer? I would love to have the shortest transition into the tile but also want the job to last 100 years.
Hi Jacob,
Insist on the ply or find someone who will do it.
Hello,
A local company is installing tile in my 1982 home over plywood.
The whole house is almost finished and is now coming up–I have just read your information. Please let me know, our problem is that the detra is sticking to the adhesive, but the adhesive (white material) is not holding the tile in place. It did hold for a few days and is now coming up. My second question is… what recourse do I have as a home owner. Thank you so much. Carol Ruot
Hi Carol,
Is the white adhesive thinset or mastic? Did it come premixed in a bucket, or did they mix a powder with water? Your recourse would depend on your local laws, I’m not a lawyer so I have no idea. You can call your local building department, they’d be able to help you out with that part.
Roger,
Thanks for the info.
I’ve got a brain picking question for you…
I’ve used for every time as substrate when tiling; bathrooms, kitchens, entrance, laundry. ~loved it.
Now I’m on to the next house and next set of projects.
I’m thinking of pouring a concrete floor rather than tile this time around.
For concrete floors a substrate composed of asphalt and steel screening is used. Then, a 1-1 1/2 thick concrete floor is poured.
Thick we can get away with a thinner floor using Ditra as a substrate?
This might not be your area of expertise but i thought I’d bounce the idea off you.
Thanks!
Hi Bruno,
I would think so, but it would really depend on how large the room is and whether or not you are installing relief joints in the concrete layer.
thanks roger!
i’ll look into relief joints.
i appreciate the help
Hi Roger,
Can I install Ditra over vinyl tile adhesive residue? This thing is still sticky after 13 years and I can’t find a product strong enough to remove it.
Is there a particular thinset I should use that sticks to that old adhesive?
Thanks
Hi John,
If you can splash water on it and it soaks in the substrate then any thinset will work just fine. If it doesn’t then you can either prime the floor with something like mapei eco prim-grip or use a thinset intended for use over things like cutback – mapei ultraflex 3 or laticrete 255 are two of them.
I beg you, please amswer, stupid me started installing ditra today and i set it on unmodified thin set… I know, should have read more before, can i take it off wash it and put it back on modified? Or should i leave it alone, but ditra would not bond to plywood, with unmodified? Is it salvigeable, the ditra? Please help.
Gio
You should be able to remove it and reinstall it with modified thinset. You need to have modified between ditra and your plywood substrate.
Thank you so much
Hi Roger,
Am retiling a bathroom set on slab foundation. Surface is a little uneven so I am thinking I needo do lay down a selfleveler, like levelquik first, then go with the crack isolation membrane install. Do you see that as a problem in terms of adherence between the levelquik and the unmodified thinset for ditra. Why ditra over laticrete strata mat? Thanks. You sound like a super guy.
Hi Madelaine,
No, there are no bonding issues over levelquik. I like both ditra and stratamat, you can use modified thinset over stratamat if you go that route.
we are doing a kitchen remodel and there is an adjoining dinning/sitting room The total area is 400sq ft. We are installing a porcelain plank styled tile. My question is related to the substrate preparation. Part of the area we are doing is on a concrete slab with previously installed ceramic tile that is rough to touch (not shiney at all) and another area has cement board covering the cement slab to bring the two surfaces to the same height. We plan on using the ditra heating system. Do we need to do anything to the tiled surface prior to using the un-modified thin set to place down the ditra? (some one told us we should use a primer on the tiled areas but I am not sure this is necessary or should be done under the ditra. ) Again the tile is very solid and is not slippery or shiny in fact it has a slightly rough feel to the tile surface.
Hi Shelly,
You do need to prime the tile if you’re going over it. If you splash water on the tile and it is not soaked in immediately then thinset will not bond long-term to it. You can use a primer over it or scarify the tile.
Hi Roger,
I was wondering if you can put a second piece of ditra on top of another to give the height at a threshold? I’m tiling up to 3/4 hardwood.
Hi Sean,
Yes you can.
Can you use this product over sheet vynal
Hi Frank,
No, the vinyl must be removed. The thinset needs to bond to whatever substrate is beneath the ditra, it will not bond to vinyl.
Hi Roger. I’m learning some good stuff on your site. One question I have started tiling my bathroom with ditra, using the right trowel and mortar to get the ditra on the floor but I’m pretty sure the tiles in the first small section I laid don’t have the coverage they should have, found out a few hours later I was using the trowel, (v-shaped, small notch) that I used to lay the ditra. It was stupid and I was rushed. Should I try to redo this section? or wait till they crack and tackle the problem when it arises? what’s the best way to fix this mistake?
One more question – what’s better back buttering or back trowelling?
thank you
Hi Christine,
It’s always best to fix it now. You can take a drywall knife and ‘sheer’ the tile off the ditra – that’s the easiest way to get it up. Back trowelling is normally not required unless your tile is REALLY warped or you aren’t using a large enough trowel to get the proper coverage.
Problem. Custom Flexbond Thunder not approved for OSB UNLESS you use redguard layer first and thinset tile to redguard over OSB. Can you flexbond thinset the Ditra heat coupling membrane to OSB or would you flex membrane to redguard over OSB to prevent failure?
Hi Jared,
While it would definitely be better, you should be just fine bonding the ditra heat mat directly to the osb with flexbond.
I recently moved into a block foundation house (~1930s) in London, Ontario. The walls are lined with Ditra, which diverts moisture from the block foundation into a weeping tile system below the concrete floor that goes to the sump pump. From what I can tell, the system works decently well, though I am noticing in a few places around the basement that there are moist areas (sometimes a small amount of actual water accumulation, as well) where the concrete floor meets the Ditra. I am guessing this is a result of times when the water accumulation is too much for the weeping tile to handle? Regardless, what can I do about this? Put some sort of sealant where the the Ditra meets the floor? Is this bound to fail? Thanks!
Hi Graham,
I have no idea, ditra is not intended for use on walls and I’ve never seen the system used like that before. Are you sure it’s ditra? Is it kerdi?
I’m wanting to install porcelain tile in my bathroom and the current subfloor is gypcrete. Can I still use ditra? Will thinset adhere?
Side note (not at important at question 1) I’ve heard of people tiling over Mapei Mapelastic AquaDefense membrane. Do you have insight into this product?
Hi Daniel,
Yes, thinset will bond to gypcrete. It would be best to prime the gypcrete first with something like eco-prim grip from mapei.
Aquadefense is a very good product, similar to the other elastomeric membranes like redgard and hydroban.
I have a question, my contractor used unmodified thinset between my plywood subfloor and the Ditra membrane. I noticed prior to tile inttallation and had him pull up the Ditra and scrape off the thinset. Hewould of course like to reuse the Ditra membrane. The felt is intact although feels stiffer then new. Our tile will be natural stone in sizes from 24×16 16×16, 8×16 and 8×8. Can I allow reuse or must I insist on new Ditra? We are doing a kitchen, a out 300 sq ft so the membrane cost would be significant.
Hi Bill,
It can be reused.
I plan to put Porcelain Tile in a 40 ft. Diesel Pusher Motorhome over plywood after installing Ditra Heat. What should I use for thinset to apply the Ditra pad to the plywood and what should I use between the Ditra and the Porcelain tile. I plan to use Reflective Premix Urethane grout.
Thanks.
Hi Joe,
A good modified thinset like Mapei ultraflex 2 or laticrete 253 for the mat to the ply, then a good unmodified like Laticrete 317 between the mat and the tile and you’ll be good to go. I would call the tech line and make sure their urethane grout can be used over heated floors – some can’t.