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Why Schluter Ditra is so cool: Uncoupling membranes explained

How tile uncoupling membranes work

Schluter Ditra waffles uncoupling membranes
mmmm…. Ditra Waffles

You’ve probably seen uncoupling membranes before. It’s not unusual to see these orange waffle-y products on some DIY television shows. You may have even wondered “Do those things really work?”


In my quest to keep up on tile installation products I took one of Schuter’s training courses some years back. I wanted to learn about Kerdi board and the Kerdi waterproofing membrane.

However, to my surprise, the product I came away most interested in was Ditra uncoupling membranes. Here’s why:

Please note:  This post has several updates, as recently as 4/30/2017, noted below. I try my best to keep the information up to date.

ancient tile floor
Did ancient tile floors use uncoupling technology?

A (very) condensed history of the world

There are many ancient tile installations still around today and they were installed without the cement technology that we have. How did they do it? According to Schluter, the old Europeans would have a layer of sand that separated the substrate (the ground) from the tile. They (Schluter) claims that this sand layer acted to uncouple the tile from the ground.

Even today, in older homes, you may be familiar with the mud beds with chicken wire that are underneath tile floors. This again is a sand layer separating the tile from the substrate.

Nowadays we have thinset that bonds the tile directly to a concrete floor. Since concrete expands and contracts at a different rates than the tile this can result in stresses on the tile installation and can lead to cracking. So although today’s thinsets have more flexibility they still are no substitute for the uncoupling layers of old.

Enter modern day uncoupling membranes

Uncoupling membranes, such as Ditra, work to let the tile layer move independently of the substrate. It’s much thinner than a two inch mud bed. Look at this Youtube video of Ditra in action:

Four interesting facts about Ditra

Thinset doesn’t adhere to Ditra:

The waffles work like dovetails so when the thinset dries it’s stuck inside the cavity. On the underneath side, there is a fleece that is bonded to the Ditra mat. As a result thinset embeds itself into the fleece layer. So Ditra doesn’t adhere to either the tile or the substrate but is mechanically fastened to both.



When used with 5 inch Kerdi Band (KEBA 100/125), Ditra can be a waterproof installation.

Modified or unmodified thinset

Schluter wants unmodified thinsets used to bond the tile to the Ditra. As a result this can cause conflict due to some manufacturers wanting you to use modified thinset for the larger tiles that are more common today. To get around this, you can use rapid-setting thinsets mortars (I don’t know if Schluter will warranty this). Furthermore many companies now have their own uncoupling mats and typically modified mortars are preferred.

Thinset Decoded: A buyers guide to purchasing the right tile adhesive

dovetail joint
Ditra works like a dovetail joint

Update: Schluter just recently came out with their own line of Schluter branded thinset mortars. They have both modified and unmodified and approve their use with Ditra.

edit: Spiderweb is now Redgard Uncoupling Mat and, as a result, comes in a new snazzy red color! There’s also Stratamat, Flexbone, and others that allow for modified mortars.

Tile floor heating systems

The Ditra layer should be directly underneath the tile layer. Due to this it needs to be installed on top of an electric floor heating system such as Nuheat mats or Sun Touch heat wires. It also gets installed on top of any self-leveling or other prep that is done to the floors. This is the opposite of how you would prep a floor using something like cement board, for instance.

Update:  Schluter has  a new uncoupling product called Ditra Heat. You can find out more in this post:

Ditra Heat Mat Installation: Ignore these two rules

Schluter Ditra (1/8″ thick), Ditra XL (1/4″ thick), and Custom Spider Web Redgard Uncoupling mat (1/8″ thick) are lightweight membranes that are used to prepare floors for tile installations. With the uncoupling aspects, along with the other advantages, you would do well to consider them for your tile floor.

For additional reference, please see this Schluter Ditra Installation Handbook pdf.



  1. very helpful to consumer.

  2. is this system acceptable for “green homes”?
    thank you,

  3. Suntouch’s installation directions say to put their mat on top of the isolation mat.
    Who to believe and who takes responsibility if there is a problem?

    • Short Answer: If you are talking about installing Schluter Ditra with a Suntouch floor heat system then the heat goes under the Ditra. Suntouch addresses this very question in their FAQ on their website.

      Long Answer: The makers of uncoupling membranes (Schluter Ditra is in the category of uncoupling membranes) feel that their products perform better when they are as close to the tile as possible. So they want all floor prep and leveling, along with heat systems to be done first and the uncoupling membrane directly under the tile.

      Crack isolation membranes are different altogether. Schluter Ditra or, at this point, any other uncoupling membrane do not fall under the category of crack isolation membranes as defined by ANSI A118.12. If you are asking about crack isolation membranes then Suntouch wants their product to be installed above the membrane as addressed in the FAQ section of their website.

      For those that will find this comment later, always read the manufacturer’s instructions for the products that you will be using and don’t take anything off of the internet as the final word… not even this blog. 🙂

  4. Great explanation, glad to have found it. A question: Can this type of waterproofing membrane be used on a vertical surface, specifically the rear wall of a tub surround? I normally use cement board but I have a tricky project where product thickness is an issue and I can’t remove existing wall surface to inset cement board. Thanks for any observations or advice!

  5. I have 6X36 and 6×24 (3/8″ thick) Florim Ecowood tiles which I am placing in a bathroom over OSB sub-floor. I am also installing a “Warm Tiles” heated floor. Due to plumbing changes for the new shower, I had to pull up sections of the OSB and replace them leaving me with old and new OSB thickness differences of roughly 1/8″ (I know). My question is what system would you opt for.

    My thought was to use a feather finish to level the low spots on the OSB and then cover with 1/4″ hardibacker to attempt to obtain the flattest surface possible for the large format tiles… then the heat system and scratch coat (recommended by Warm Tiles), and tile.

    Originally, I had planned to use (and purchased) Ditra before I realized floor flatness would/could be an issue. Would you opt for a “feather finish/heat/Ditra/tile” system or the “feather finish/Hardi/heat/tile” system? Or something different all together, or a combination? (The doorway tile/carpet transition will allow 3/4″ which I know I will exceed a bit, but I’m trying to maintain an inch or less).

    • I prefer Ditra Heat or Prodeso heat these days. Using the feather finish is a good way to go.

      I think either the Hardibacker or Ditra would work. Going with 1/8 inch thick Ditra would probably work best for the transition height.

  6. I was curious about how ditra, if used as an underlayment for a waterproof installation in lets say a wetroom floor with a preslope and the seams are sealed using kerdi band, how does it drain when moisture falls into each cavity? Versus having used kerdi instead which theroretically would be able to send any moisture towards teh drain. Wouldnt you risck having stagnant moisture sitting in the low end of each individual cavity of the ditra?

    • Ditra isn’t supposed to be used in flooded areas. It’s waterproof but to a point. Schluter wants their Kerdi product to be installed for the shower pan floor and sloped to the drain- not Ditra. But Ditra can handle water and is a great product for outside of a shower on a bathroom floor. Moisture will migrate out over time.

  7. Can you put Ditra over Hardiebacker 1/2″ which has been fastened with screws and mortar?

  8. What kind of mortar would you use for 6×36 plank tiles over Ditra? Is there an unmodified medium-bed mortar available or is it safe to use regular unmodified thinset because of the decoupling effect of the membrane?

    • There’s a mortar called Ditra Set by Bostik that works well for your application. It’s unmodified and medium bed.

      Additionally, Schluter has a modified mortar called AllSet that would work but I have no experience with.

      If you can get Kerabond T from Mapei then that would be a good one.

      Otherwise, I would use a medium bed modified but that does violate Schluter’s terms for their Ditra product.

      • Wow, that’s a new record for answer promptness. Thank you.

        I’m interested in the Ditra to save some floor height. With the tile that will be replaced, there’s already a noticeable step up from the neighboring floor and the radiator cover is grouted in at the bottom. There may be too many layers underneath like old tile but I won’t know until I pull it up. How much height would using Ditra save compared to 1/4″ cement board and redgard?

        • Should be about 1/8 inch difference.

          If you’re in an older home there’s a good chance that you have an old mud bed tile floor where the floor is recessed. This would mean that you’ll have the opposite problem of needing to fill space- assuming the different flooring layers are removed.

  9. Schluter bases their recommendation for using unmodified mortar to set tile over Ditra on moisture impermeability of both the Ditra and the tile itself. I’m wondering why is it acceptable to use modified over other impermeable surfaces like something treated with RedGard for example? Wouldn’t that be the same situation? Even if it’s cement board with plastic sheeting behind it, doesn’t that still keep the moisture in?

    • Yes and this is a very much debated topic. Think about this: There’s a lot of companies that have impermeable membranes and Schluter is one of the few (maybe only) that require unmodified mortar.

      I’m not saying that there’s nothing to their science. But I do think that this guideline is a little outdated. Mortar technologies have advanced to where they are self-drying now and, frankly, I don’t think this was ever a big enough deal for all the fuss. But that’s just my opinion and I’m not a scientist.

      Furthermore, Schluter has their own modified mortars now that can be used for their products. What’s the secret to their mortars? Are they magic? Do they have special technology that other mortar manufacturers haven’t figured out?

      Schluter claims their mortars are “specifically engineered” for their products. People may choose to believe this, or not.

      Custom Building Products has manufactured mortars for years and they don’t require unmodified mortar for Redgard. Same with Laticrete. Same with Mapei. Same with others.

      Modified mortars are where the technology is being applied. They are just better mortars that are able to grab a hold of hard and impervious surfaces like today’s porcelain and glass tiles.

      If you want Schluter’s warranty you’ll have to abide by their rules and use either their modified or an unmodified. This is one of the big reasons why I typically choose other manufacturer’s products for my installs. I think modified mortars are better.

      • I got the impression that you were a fan of Ditra. I was considering going with it for a small bathroom floor install of 36″ plank tiles because it sounds like it would give additional safety from cracking (and a height reduction) but the comlexity is getting me worried. I don’t even know that I can get Schluter’s mortars around here.

        • I like Schluter and they make good products. But I like to use modified mortars so I end up using competitors products because of that. When I use Ditra I use Bostik Ditra set which is a good medium bed unmodified mortar. I don’t know how easy that is to get? I believe Ardex gives a warranty with their mortars for use with Schluter also.

          • I’ve seen Bostik Ditra Set on amazon but it looks like it’s almost the same price as the Schluter Set and All-Set mortars. Seemed like if all else equal, I’d just go with Schluter’s mortar, especially since it’s modified and I can use it both above and below the Ditra (going over plywood). A bit hesitant since I’m using 36″ plank tiles but Schluter says All-Set is good for it. I’m having trouble finding a good unmodified large format tile locally: Lowe’s doesn’t sell the Mapei one that you recommend any more. This will take some planning.

            I only have a ~30 sq. ft. area to cover. Could you please tell me what your experience is of mortar coverage with laying Ditra and putting large format tiles over Ditra?

          • For installing Ditra, you’ll need at least one bag and maybe a bag and a half. That’s to install Ditra, fill the top side, and install large format tile. Usually, around 35-55 ft is what I get out of a bag for installing large tile. The waffles will eat some thinset up also. Installing Ditra itself usually doesn’t use a lot. It’s been a while but I would think that you would get 100+ sq. from a 50lb bag.

  10. I like how you are straight to the point and provide your sage expertise. Now, I just removed old tiles (5 year old) that was laid over Ditra. I am left with the fleece… and this was over OSB floor. So, my question. Can I ditra over that or would it be best to use 1/4 wonderboard, hardiebacker or wedi, over that membrane that came out in some parts and others is just laughing at me. I also thought of removing the subfloor and putting new 3/4 plywood but… the subfloor is not only nailed it is glued… dangit!
    so recap:
    Removed ditra, left with membrane not easy to remove.
    apply ditra over that? OR Wonderboard, Hardiebacker or Wedi? I am using wedi on the shower surround.

    • They say that demolition isn’t supposed to be easy but that’s not very helpful when you’re tearing out flooring that doesn’t want to come out.

      The best way is to sand or grind off the fleece and thinset that’s on the floor. It’s not as bad as it sounds if you have a way to manage the dust.

      Short of that I don’t know how well it will work to go over the existing. I’d probably try cement board or hardbacker as they don’t require a bond to the flooring to work properly (they still need to be thinset and nailed/screwed to the floor).

      But loose and extra fleece could still cause problems with those backer boards.

  11. Thanks! In the words of Charlton Heston… Damn (fleece) to hell, damn you fleece!!! 🙂 Thanks. I am going to have a busy weekend, and the box stores will love me. Hardbacker here I come!

  12. I’m about ready to redo the tile floor in my bathroom. The current tile is installed to a 1/2” plywood underlayment on top of the subfloor. The transition to carpet is almost level & I’d prefer to keep it level or carpet above tile, not vice versa. I’m not sure what I should do for underlayment. Remove the 1/2”plywood underlayment and install 1/4” Ditra to subfloor? or 1/4” backer board + 1/4” Ditra? Any advice would be appreciated.

    • It depends on why the 1/2 inch plywood underlayment was installed. If it was installed because your subfloor spacing is more than 16 inches on center then you will need to either keep the underlayment or replace it with the same.

      However, in some parts of the US, it’s common to install 1/2 inch plywood underlayment and tile directly to it. This is simply the practice and there’s not a better reason than that.

      If this is the case then you can remove the underlayment and replace it with either backer board and/or an uncoupling membrane such as Ditra.

      Another thing to consider is the kind of tile that’s in there now and what will be installed in it’s place. It’s quite common nowadays to replace standard 12×12 tiles with 12×24 tiles which tend to sit a bit higher than their 12 inch counterparts.

      What I have found is that 1/8 inch Ditra adhered directly to the subfloor and 12×24 tiles over the top will sit flush with the neighboring carpet. At least flush enough.

      By the way, if you find that you need to keep the 1/2 inch plywood underlayment I recommend going over it with a thin membrane. A liquid membrane such as Redgard would be OK but I would rather see a sheet membrane such as Noble CIS or similar.

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