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Ditra Heat Mat Installation: Ignore these two rules

Schluter Ditra Heat mat
Schluter Ditra Heat mat


Typically I encourage people to follow the manufacturer’s instructions. Especially when the company has clear, detailed, and easy to understand instructions. But when it comes to both Ditra Heat and Ditra Heat TB [edit: now called Ditra-Heat Duo] there are two things that I just don’t feel are good ideas.


This post deals with the installation of Ditra Heat mats only and not the heating wire.

So what are the two things you want to avoid when installing Ditra Heat mats?

Ditra Heat prefill
Ditra Heat is pre-filled or skim coated prior to installing tile

Pre-fill the waffles. Always.

I feel strongly about this. Now the Ditra Heat Handbook doesn’t specifically tell you to not pre-fill the waffles. However, it does tell you to start setting the tile right over the mat. Also, the photos show the tile being set right over the orange heat mats. This is not a good idea, in my opinion.


Why Schluter Ditra is so cool: Uncoupling membranes explained

First of all, what does pre-filling the waffles mean? This is where you skim coat thinset mortar over the top of the heat mat. You do this with the flat side of your trowel. Then you let it dry before installing the tile. This skim coat layer helps to protect the wires below. This is important for two reasons.

  1. It helps to protect the wires from the teeth of your trowel when combing the mortar.
  2. If you have to pick a tile back up after it’s been set (during installation) you won’t drag the wires out with it. The wire can stick to the underside of the tile. You can prevent this by pre-filling the waffles.

An additional benefit of prefilling Ditra mats is being able to snap chalk lines and actually being able to read them.

Ditra Heat TB
Ditra Heat TB has additional insulation for going over concrete floors.

Don’t install tile the same day over the Ditra Heat mat

Schluter says this is ok. Again, I think this is a bad idea. As mentioned, when you have to pick a tile up and re-install it (you will have to do this- more than once) the suction of the tile can pull up the Ditra mat causing it to break the bond to the plywood.

My advice:  install the Ditra Heat mat one day and install the electric heat wire a different day. Then skim coat the wires with thinset and let that dry. You have enough to do just installing the tile properly. You don’t need additional challenges or things going wrong.


  1. Did you have any issues getting the DITRA HEAT-TB to stick to the thinset? When I layed the pad down it didn’t seem to grab the fuzzy backing.

    • I have noticed some issues with getting the heat membranes to stick. Not just Ditraheat tb, but also ditra heat, prodeso heat, and suntouch heat matrix. When I’ve peeled them back to check the coverage it hasn’t been what I would like. They have all been fine the next day and well adhered.

      But currently I don’t know if this is a something to be concerned about, or not. I just don’t have enough information about it.

      If yourself, or anyone else, has any feedback on this I would love to hear it.

    • This happened to me on the first few sheets. Always peel back the first sheet to make sure that the thinset is completely “soaking into” the fleece. When the thinset is too dry, it will not be absorbed properly by the fleece and when you peel it back you will see white areas. Make sure that you use enough water to make it drip from the trowel and yet stilll hold a trowel pattern on the floor. Then when you examine the fleece it will be soaking it in, as desired. Good luck!

  2. When running the lead wire from the membranes to the thermostat box, I have a lot of extra lead wire. Can I rim that and how do I deal with the cable sheathing?

    • Yes, you can trim it down. The sheathing just gets stripped down enough for the wire ends to show. It’s similar to wiring in a light switch or an outlet in that regard.

      One cautionary note: The wire leads can be cut but the heated portion of the wire can’t be. I know this isn’t what you are asking about but just want to make it clear for anyone that may happen onto this comment.

  3. Please help!

    1. Modified or unmodified thinset to smooth over the ditra-heat mat?

    2. Small mosaic marble tile going over the ditra-heat mat (with thinset smoothing coat), modified or unmodified to install this type of tile?

    • 1. Schluter wants unmodified over the top unless it’s their own brand of thinset. Then it’s ok to use modified.

      FYI, Schluter now has their own line of mortars and some are even modified. They say that they are “self curing”. The thing is that this self curing technology is not unique to them and has been available in other manufacturer’s mortars for some time now. This is the reason other mortar manufacturers have required modified over Schluter products. So going over Ditra with a modified mortar is probably not just ok, but could likely be a better installation. But I want to make it clear that it would void Schluter’s warranty.

      2. Same answer as above with one caveat: Schluter has a 2 inch minimum tile size for going over their uncoupling membranes. The reason is because they are worried about point loads- too much pressure concentrated in one small spot. For a residential bathroom you may choose to take the risk but at least you’re informed enough to take the risk if you choose.

  4. Unmodified. You may get away with a modified if you use very small format tile and give a long curing period, outside of that. unmodified or your thinset will never cure.

  5. Not finding the info I need, and want to make sure I don’t screw up now…….help this poor sole!

    1. Bathroom floor has 1/4 ” Hardiebacker attached with thin set and screws to plywood subfloor.

    2. I will be putting Ditra Heat mat down to it.

    3. I have 18″x18″ Porcelain Tile.

    Not finding what thin set to use between Ditra and Hardiebacker. Modified or not???

    I have Un-modified for the Tile to Ditra, but I may need special for the large Porcelain tiles?

    I know Schluter wants unmodified between tile and Ditra, but what under Ditra to Hardiebacker?

    Have both HD(for Custom products) and Lowe’s (Mapei) close by to purchase.

    Any thing I am forgetting?

    • You can go with either modified or unmodified with the Hardibacker but I would push you towards modified. There’s too much riding on it with the wire and the tile to have something go wrong.

      One thing about the unmodified mortars is that they tend to be the cheapest mortars available and they aren’t all created equal. I would encourage you to stay away from any thinset that is under $10. A good unmodified mortar for large tiles is what you want. I’m pretty sure Lowes carries something. If you have time to order Mapei Kerabond T from Lowes that would be what I recommend.


  6. I am trying to tile river rock on my bathroom wall. The stones are approximately 2″ x 3″ and anywhere from 1/2″ to 2″ thick. So some are pretty heavy and some are very light. At the store, they recommended to me to use TEC large stone mortar-latex modified. I am combing it to 1/4″. However, the stones are seriously sagging. I know I should work from the bottom up, but I am working around a mirror so I really do not want that as my bottom.

    Do you have any suggestions for me? I would really appreciate it. It will really save my sanity. thank you!!

  7. Why can’t prodeso be or ditra be glued down over hardwood?

  8. I am planning to install Ditra-heat-tb over concrete in a bedroom and Ditra in hallway leading to that bedroom. The difference in height is 5mm or 3/16″. Is this going to be a problem? How do I transition? Thank you.

    • It would depend on what you are transitioning to. But typically people will use wood or metal profile reducers to go from a higher tile surface to a lower surface.

      • It will be the same tile in the bedroom and in the hallway. I was hoping to get away without metal profile reducers by sloping the tile and make the transition from area with ditra-heat-tb to area with regular ditra invisible. Would sloping the tile work? Or I still should use the reducers? Thank you.

        • I didn’t realize that you were going to transition from Ditra-Heat TB to more Ditra. You might consider the following options:

          -Running Ditra-Heat TB (now called Ditra-Heat Duo) throughout the whole installation and only heating one section of it.
          -Using Ditra XL for the non-heated part. Ditra XL is the same thickness and is probably a little bit less expensive than Ditra-Heat Duo.
          -Running regular Ditra-Heat mat (not Duo) in the non heated section. This is probably the most impractical of these three options.

          If you still want to use 1/8 inch thick Ditra in the non heated section then you could transition with just a tile. If you were using, say wood plank tiles, then you could always run the plank a different direction at the doorway which is something often done with real wood flooring installations. You would have to angle the tile but it would still work.

  9. I am considering skim coating over Ditra Duo heat cables over concrete substrate. It could be 3 or 4 days before I start installing the tile. Will the skim coating crack or crumble when working on it to install the tile.

    • Yes, there will be small cracks and crumbling. This is fairly normal because there is a small amount of movement in the plastic before the tile is installed. As long as you clean and vacuum it well prior to installing tile then you should be fine.

  10. I am planning on installing Ditra heat in a bathroom. I wish to install a redundant heat wire(not connected at same time) in case the initial one fails at some point in the future. Can the heat wire be dual run right next to each other? Can the non powered backup wire cross over the live one? Please advise?

    • Unorthodox for sure but I’ll speculate: I don’t think it would be an issue to run a redundant heat wire nor cross over the real heat wire as long as only one or the other is working- and not both. However, crossing the wires could lead to an uneven floor and also leave the high wire exposed to potential damage.

      That’s my speculation but this isn’t something that you should take people on the internet’s word for it. A call to Schluter’s tech line is advisable. 🙂

  11. I realize that following the manufactures specs to use the Schluter Ditra membrane is recommended and the best option. I personally dislike Schluter mats in general so is it possible to not use the membrane at all and pour a self leveling layer over my wires instead of going through a triple layering process?

    • Yes. If you’re going over a wood subfloor then Ardex makes a product called Liquid Backer Board that works great for this purpose.

      Otherwise, your supposed to use lath for reinforcement and Mapei makes a plastic lath that won’t cut into the electric wires. Then you have more options for self levelers by using that.

      If your going over concrete then the self levelers doesn’t need reinforcement.

  12. Hi, I have installed my Ditra membrane and realized after the fact that it would be cool to have heated floor. Can I install heat cable on top of the regular Ditra membrane? Thanks for your help!

    • I don’t know whether Schluter would approve, or not. You would have to call their tech line to find out.

      But I don’t see a reason that it wouldn’t work. I don’t think self leveling over it would be a good idea but you could probably coat it with thinset.

      Here’s a little known and probably against the rules tip: before the days of Ditra-Heat some guys would slice the ridge part of the waffles on normal Ditra and stuff their wires in there.

      But you didn’t hear that from here. 😉

  13. Just had this installed and the guys out down durock, then ditra heat mats, then put self leveling concrete on top of that, and finally the next day laid down stone. Is this a completely botched job? I stopped payment on their check…

    • Not necessarily.

      I don’t think Schluter wants self leveling over the top but some self leveling companies will warranty the installation. But it depends on which brand, which product, and how it was applied.

      If you can find out which product they used then you can contact the manufacturer and see what they say.

      But there’s nothing wrong with Ditra Heat over cement board.

  14. Can I do like a stair pattern with my electric wire to go around curves of a shower base ?

  15. Hello,

    Im using prodeso heat membrane and it was mentioned to use an unmodified thinset on top of the membrane – but what is recccomended to use to adhere the membrane to a concrete basement bathroom floor that has been already levelled with self leveller? Modified or unmodified? Thank you 😉

    • I don’t know that Prodeso requires unmodified over the top. I’m almost positive that they don’t but I can’t seem to locate their install instructions online anywhere- including on their own website- which seems quite odd to me.

      However, unmodifed should be ok over concrete but I hate encouraging anyone to use unmodified mortars as there are much better modified choices out there.

      edit: Nuheat sells a relabeled version of Prodeso Heat membrane and they have installation instructions on their website. They mention both modified and unmodified are ok.


  16. Hi

    I have installed ditra heat with modified thinset. The heat cable was put down and the flooring guy installed unmodified thinset from mapei (kerabond) over the ditra and cable before starting the ceramic. It as been 48 hours since the thinset was installed and i notice this morning when i was walking on it that there was cracking sound. I puch down on the thinset over the round in the ditra and the thinset is caving in at mostly every round but over the chanel for the heat cable everthing seems ok. Is this normal?

    • This sounds normal. The crunching sounds and even thinset coming loose, especially in the centers, if something that happens with prefilling.

      Working on it, kneeling on it, and putting pressure on certain areas can cause the mortar to come loose.

      The loose areas should be removed prior to installing tile and should be refilled in the process of tiling.

  17. I put down Ditra Heat mat in a small bathroom and it adhered great to the subfloor(plywood) except for a 6″ spot near the wall. How can I fix this bubble and make it adhere without lifting the entire mat? The thinset has hardened. Installed 40 hours ago.

  18. Hi, and thank you for your wonderful blog. What is the “thinnest” heat plus uncoupling system you know of? I’m trying to stay close to adjacent floor height!

    • I think 1/4 inch is pretty standard and about as thin as you can get. Ditra Heat, Prodeso heat, Suntouch’s Heat Matrix (actually from MD Building products) are all about the same.

  19. I am thinking about DIY’ing the install of Ditra Heat Duo in my basement. I’ve read everything I can find on the process and watched endless videos. My fear is that I do something that may void the warranty.
    Just read your comment re: prefilling the waffles. Sounds logical to me, two questions:

    1.after this pre-fill dries does it not crack when walking about tiling the floor?

    2. I am tiling over concrete and plan on concrete/modified/Ditra Duo/unmodified/tile. Should you use the unmodified to fill the waffles and then another unmodified to tile?

    Thanks in advance!

    Trying to save $3000 :))

    • Yes, prefilling can crack.

      I’m not sure why you would want to use two different unmods above Ditra-Heat Duo. I would use whatever you are going to use to install the tile.

      If you meant that you wanted to use modified to prefill then that’s fine. At least I’ve heard that Schluter doesn’t have a problem with that.

      Congrats on saving $3000. This is what’s great about DIYing.

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