Installed WarmWire Heating Elements

Figure 8

Now that your floor looks like a can of silly string exploded (figure 8 ) its time to add more layers to it and cover up all your hard work.  If you have not yet done all the hard work then your floor doesn’t look like that. Check out How to install WarmWire Part 1. I’ll wait.

Okay, you may want to check out a speed reading class. Just sayin’. :D My preferred method is Schluter Ditra underlayment installed atop the WarmWire for your tile installation.

The best method of leveling out your floor for your Ditra would be an SLC or Self-Leveling Cement. This product is mixed with water and poured over your WarmWire. When mixed properly (follow the instructions to the letter – really) and poured it will – wait for it – level itself. When cured you will be left with a level, flat floor.

You can actually install your tile directly to this layer if you chose to do so. I do not chose to do so. I prefer to have an additional uncoupling membrane above these layers then my tile. That’s just how I roll.

For purposes of speed (SLC requires 24 hours to cure) I will level the floor above the wires with mortar. If you chose to do this, and you may since it is considerably less expensive, you need to take absolute care with the method I utilize. You risk damaging the wire while leveling the floor so be careful.

To level the floor above the WarmWire with mortar I use a 3/16″ square notch trowel. Mine is metal – yours should not be! You can purchase a plastic trowel make exclusively for use over the wire. These are much safer to use as they would require actually trying to damage the wire with it. It’s difficult to damage it accidentally if you use the plastic version.

The most difficult thing to do over in-floor heating elements is installing tile properly. That means flat, flat, flat. By using the ditra and utilizing my method you will begin your tile installation with a flat substrate over the heating element.

I mix my mortar (thinset) fairly thick for this. It assists in holding the shape and getting absolute coverage with the mortar – no voids in the mortar bed. I will first use the flat side of my trowel and fill all the areas between the wires moving my trowel in the same direction the wires are running.

Leveling over WarmWire for Ditra Installation

Filled and Combed

In the left side of this photo I have the flat troweled the mortar and in the right side it is ‘combed’. Notice everything is running the same direction as the heating wires.

Once that is all filled in I will flip the trowel over to the grooved side and ‘comb’ the thinset in the same direction by placing the trowel so the wires are between the grooves. That is so that the teeth of the trowel are actually touching the floor beneath the wire and the wires are between the notches. This will give you a grooved bed on which to install your Ditra.

Once that is finished place your Ditra over the top and grab your straight edge, level, 2 x 4, or whatever you have that is straight and handy. You want to press the Ditra into the thinset to it fully embeds into the fleece on the back of the Ditra. Place your straight edge on top of the Ditra and run it back and forth in the direction the wires are running or at a slight angle (figure 9).

Leveling Ditra over WarmWire

Figure 9

If you run your straight edge in the other direction you will end up with a roller coaster for a floor. Your straight edge will go from the top of the wires into the spaces between the wires and back up. It will not be flat if you do it in this manner. By keeping it either in the same direction, or a 45 degree angle, as the wires it will always remain on top of the wires for the length of your straight edge. Your floor will come out flat and even.

You need to make sure you put enough pressure on the straight edge to embed the Ditra fleece into the thinset. If you stand back you can see the difference in the color or shade through the top of the Ditra. It will turn a darker shade when embedded correctly and you will be able to see any spots you may have missed.

Leveling Ditra over WarmWire


Installed and Uninstalled Ditra over WarmWire

Embedded and not

The left side of this photo shows the Ditra embedded correctly and the right side the Ditra is simply laid onto the mortar bed without embedding it yet.

Simply keep doing this as you walk (crawl) your way out of the room. Be careful not to step or kneel on any parts you’ve finished. There is a considerable amount of mortar beneath your Ditra so it is easy to put ‘dents’ in it with your foot or knee. As long as you use a fairly decently sized straight edge you should end up with a perfectly flat, stable floor ready for the tile of your dreams.

After 24 hours you can walk on the Ditra to wire your heating element and install your tile. You need to wait this long for it to cure. Stepping or crawling on it prematurely will create dents or otherwise compromise your membrane and substrate. Be patient.

Ensuring Ditra is flat over WarmWire

Contractor Glamour Shot!

Wiring the control box and heating element is fairly straightforward. Read through the directions included with your control box to ensure it is wired the same as I am describing – it may not be. I don’t know, I can’t see it from here.

There are three things you need to attach to the control box: the power from the house, the heating element, and the temperature control probe. The power from the house gets wired to the ‘LINE’ side of the control box and the heating element is wired to the ‘LOAD’ side.

The power from the house consists of three wires. They should be white, black, and ground which is usually green or a bare copper wire. The ground is not wired to the box, only to the heating element. The black is wired to the black and white to white of the control box. Easy enough even if you’re colorblind. The black wire is the hot wire! It’s not like your car radio in high school. The white is neutral. This is the way it should be but I didn’t wire your house so don’t blame me if it isn’t.

WarmWire Control Box Wiring

Figure 10

The probe has two wires – red and black. They are wired into the appropriate clips in the back of the control box.

The heating element has white, black, and silver. White to white, black to black, silver to ground. Please note that in this photo (figure 10) I have two different heating elements running to the control box so there is twice as much electrical spaghetti hanging out of the wall. You should not have that many wires.

After it is all wired up just stuff all the wires straight back into the box and

WarmWire Control Box Pins

Figure 11

attach the control box to the wall. The face of the control box is plugged into the back plate with the pin cluster in the back of it (figure 11). Place it in there carefully. They bend easily if not lined up correctly – just trust me on that one without testing it for yourself.

Now you can turn the breaker back on. Oh, you turned that off, didn’t you? Sorry, if you are currently sporting a Yahoo Serious hairdo, it may be my fault. You want to turn the breaker off before you start wiring.

Now the bad news – after you install your tile you must wait 3 – 6 days before turning on your underfloor heating. You need to give your mortar plenty of time to fully (relatively) cure before zapping it with unnatural heat. Doing it sooner may compromise the strength of the mortar and, in turn, your tile installation. You don’t want to do that now, do you?

WarmWire Control Box Installed

Warm Tootsies!

You’ll just have to wear socks for a couple of more days.

There you go – warm tootsies for those cold mornings. If this has helped you out please send me a photo of your completed tile installation. I would love to see what people do with my good advice and bad humor. I keep tellin’ my wife that it is a great combination but, you know, she doesn’t believe me. Oh well, extreme jackassery is getting to be my tag line so why quit what works?

Send me your photos at and lets see what you can do.

{ 223 comments… add one }

Leave a Comment

  • Tracy

    :shades: I love all your How-To’s, Roger! They have expanded my tiling knowledge and added laughter to my life (insert tears of joy). I plan to install WarmWire in the bathroom floor area and shower floor and have some questions I hope you can answer…
    1. The tiler who sold me the WarmWire mentioned his technique for installation and it sounded smart, but I wanted to check with you first. He installs a 6″ border of hardibacker (properly of course) around the room, underneath vanities, underneath toilet, etc to make a damn/border for slc and space the wire from the wall properly. Then he installs the wire on slc primed plywood subfloor and pours slc to embed the wire and level the floor with the hardibacker at the edges of the room. Once set, tile is installed. Does this sound like a good idea for dry areas of the bathroom floor?
    2. What is the best way to attach the wire bracket/strips to the shower pan (following your shower pan creation instructions, of course). Hot glue? Then burn in thinset to embed wire before tiling? This is assuming you con :shades: done the use of WarmWire in wet locations…. do you?

    Keep up the killer job you do!

    • Roger

      Hi Tracy,

      1. Yes, that technique works great.
      2. Yes, you can do it in wet locations provided the wire is rated for it. I usually do the full floor, then route out channels for the wire, then cover it with thinset and kerdi.

      • Tracy

        Ooohh, great idea! What do you use to route channels for warmwire in the top deck mud of the shower pan without breaking big chunks out? (I have routers, dremels, rotozip, chisels, flame-proof dog claws).It may be more work but it will save some height off the finished shower floor, which is nice. Plus my top deck mud is near perfect and I know it wouldn’t be so after trying to smooth thinset over 28 square feet of lumpy cables. BTW, with deck mud I noticed something interesting… when you beat the crap out of it enough with 2xs and a wood float, the pitch of the slapping sound gets higher. :idea:

        • Roger

          I use a rotozip with the big (3/8, I think???) tile bit in it. There are only two sizes of tile bits for the rotozip, the bigger one is what I use. Go fairly slowly and you won’t dig out chunks.

  • Eric

    Hi Roger,
    Great site and thanks for all the great info!
    I have an OSB subfloor that I am installing Suntouch warm wire over using modified thin set with a 3/16″ notched trowel and then Ditra. I will then use unmodified thin set to set my tile. Any issues so far?
    Couple of questions:
    – I am installing a pedestal tub in the bathroom that will not have heat wire beneath it. So, about 1/3 of the bath will have no heat wire. Should I use a slightly larger trowel in these areas to build up the thin set making the floor level? What size would you recommend?
    – Roughing in the drain to the proper height is very critical in being able to attach the trap adapter to the tub drain, so I am trying to estimate exactly how high my finished floor will be. Once the ditra has been properly smoothed out and cured what thickness can I plan for using the 3/16 or even a slightly larger trowel where the warm wire is not?
    thanks a ton!!

    • Roger

      Hi Eric,

      Osb is not ideal, but it’ll work. Whatever size trowel gives you a flat substrate is the properly sized trowel. Probably 1/2″ or so. Your last question – I have no idea. Get your ditra in, then rough in your drain. You can drill through everything you need to once the ditra is set.

  • greg

    Fantastic website! Glad I found it.

    I have an existing tile floor in our bathroom that is over the garage and its cold and *** in the morning. I am planning on ripping it out as part of a built in shower project to replace the existing tub / shower combo and add radiant floor heat. Mmaybe I am missing something.. but is it really that simple? wood subfloor –> mortar —> ditra heat decoupling sheet to wood subfloor –> install nuheat wire “in” ditra and then lay the tile over it? No concrete backer? That goes against everything I have ever learned.. but man that’s easy – if thats accurate :)

    • Roger

      Hi Greg,

      That is accurate. It is easy (well, easier than double ply, backer, etc.), but you’ll pay for it. Stuff ain’t cheap. :D But it’s worth every penny.

      • greg

        thanks for the reply.. what is your opinion of this. break up the old tile floor and scrape the morter down to the backer – i believe they used 3/4″ durrock – leave the durrock, install the heated floor and then tile over it with no ditra – or should I remove the durrock as well and start over? thanks again!

        • Roger

          Hi Greg,

          I would remove all of it. Do you have any idea what is beneath the backer?

          • greg

            Found out its 1/2 durock on 5/8 osb. How I found out.. In a twist of events – had a leaky toliet in the other bathroom – didnt realize it until I noticed a stain on the ceiling below.. one of those things where you see it everyday and it doesnt register until one day… bam.. when I took it a part, low and behold the subfloor was was rotted out! So I removed that tile and backer until I got to unrotted floor.. Then I decided this is a total gut because I needed to get to a floor joist to replace the subfloor. The part of the subfloor that was not rotted there was only minimal damage by prying up the tile and backer. So I would imagine the other bathroom is the same way. Here is my plan (whenever I get to it since the other bathroom just turned into a gut) – removing the tile and backer and then lightly sand / grind the osb to get it smooth again and then move forward with the ditra heat membrane (probably will use nuheat cables) on top of the osb. Sound like a decent plan? Now in the mean time, I am gutting the other bathroom and replacing the standard builders tub with a whirlpool or jet or air bubble tub – but guess what.. builder put the tile surround direct on greenboard.. It was all wet. luckily there was no damage! Ahh the joys of home ownership.

            thanks for your comments!

            • Roger

              Yes, that would be fine (sanding down the osb to get it flat).

  • Leighsa

    Hi Floor Elf – I am working with a contractor who intended to put the heating over the Ditra (I commented on part 1 – thanks for the reply). I checked with the floor heat manufacturer and Ditra, and both said that the heat should go below the Ditra, so he is doing that. The heating wire was laid out on the plywood sub-floor and the cracks were filled with spray foam, except one tiny crack between two pieces of plywood. I pointed it out twice but was told that it would be fine because there was a 2×4 below there. But then when he mixed up the SLC and poured it, I noticed that there was a low spot near this crack. He poured more SLC on this and told me that it would be fine – that it was setting soon. But when it did set, there is a major dip in this area, exposing the heat cables and the guides. And in a few other places, the cables are higher than the SLC, giving it a bit of a wave effect, not level. He said that the floor does not need to be level, but flat. To me, it appears to be neither. How can we rectify this situation? What can be done to level the floor now? I’m really annoyed that I paid $100 for SLC (plus labor) that didn’t even level the floor, but I’ll be much more annoyed if the floor heat doesn’t work or worse — that the 12×24″ marble tiles crack in the near future. What do you think, oh wise one? Can we fix this?

    • Roger

      Hi Leighsa,

      Yes, it can be fixed. Depending on the severity of the dips it can be fixed with thinset as the ditra is installed or with an additional layer of slc.

    • Rhonda

      I’m dealing with a similar situation – do you know if he put down a metal lath on the osb before pouring the SLC? If he did, did he put it on top of the heating cables?

  • Sarah

    Hi! I have a teacher system installed in my bathroom. I read through all the programming and have it all set up. When it’s on manual setting, and I put the temperature up if you degrees or so, he takes a really really really long time to heat up and I’m not even sure it’s really heating up- it may just be because of the shower heating up the room. How long is it supposed to take to heat up?

    • Roger

      Hi Sarah,

      I’m not sure what a ‘teacher system’ is? I’m assuming you mean a heating system? They will normally reach the thermostat temperature withing 30-45 minutes, sometimes an hour depending on your room conditions. These systems heat up the tile installation, not the ambient room temperature. More specifically, it will heat up the ambient room temperature but the system shuts off after reaching the thermostat temperature in the tile installation – where your probe is located. Once that probe reaches whatever temp you’ve put the thermostat on it will stop heating.

      Unlike a regular central heating system the temperature probe is located in the flooring itself, not on the wall measuring the air temperature. This is why you want to program your thermostat to kick on about 1/2 hour before you want the tile heated to that temp. People normally program it for an hour before they get up in the morning, for instance.

      • Gene

        Hi Sara. You must also consider the “mass” of your floor. If you have very thick tiles, or a thick layer of floor cement above and/or under your heating wires, the temperature both on the floor and in the room will take longer to raise (and fall).

  • Rod

    Hi Roger,

    What do you think of the Schluter Ditra Heat system? I was going to order a Suntouch system online, then I found that my local tile store stocks the whole Schluter line.

    • Roger

      Hi Rod,

      It’s one of the best products they’ve come out with in the last ten years! If you’re not concerned about a warranty you can use the suntouch system with the schluter mat, much better in my opinion and quite a bit cheaper as well.

      • Rod Brown

        Hi Roger,

        I really love your’e blog.

        I planed on a Suntouch system, just like you outlined above. I was going to order the Suntouch system from Amozon, but I recently found out that my local tile shop caries the whole Schluter line. Including the Ditra Heat system. Dollar wise both system are pretty close, even adding for shipping and the extra cost of Ditra for the Suntouch system. All things being equal I prefer to spend my dollars locally.
        What do you think about the Ditra Heat system? The “Ditra Heat” ditra looks different from the regular ditra. I don’t see how it makes a mechanical bond that you, so eloquently described in one of your rants. I know Schluter is a good company. Should I trust them? I trust you!
        I also had a problem posting from my mobile device. Probably operator error. :lol2:

        • Roger

          Hi Rod,

          Your first post came through. The little circles in the ditra heat mat are more like upside down cones, so they do lock the tile in. I would go with the ditra heat, at least the mat portion.

  • Kyle


    Great site! Great info!
    For a bathroom remodel, we are adding heated floor. Having the same installer put it down that did the original job 18 years ago – perfect job and a pain remove! Good solid concrete slab floor. Was going to use Wedi board and heat wire which we used in the kitchen a few years ago and has worked great – but I have now found CeraZorb synthetic cork and Ditra-Heat which should work great together with only about 1/2 inch added height.
    So my question is: CeraZorb recommends Polymer modified thinset under tile ( uses adhesive to concrete slab ). Ditra-Heat as you know with Ditra, wants unmodifed above and below. Any suggestions ???? Would you think either would work under the Ditra-Heat?
    The advantage I see to using these together is thermal break with higher R value, no SLC needed, goes down quickly, no clips etc. on wire, advantage of Ditra, custom layout for the wire and heat closer to tile.

    • Roger

      Hi Kyle,

      Unmodified will work just fine. The ditra heat provides mechanical fastening, something I’m certain cerazorb does not take into consideration, they expect the tile to be bonded directly to it.

  • Q

    I am going to set my suntouch mat with thinset and feather finish and skip the Ardex SLC. I don’t feel comfortable setting the mat and the ditra in one fell swoop. (worried it will skin over while I am messing around)
    I feel like the variation added by the mat wasn’t worth the added expense and difficulty I may have pouring the SLC.

    1. Is that a bad idea or OK?

    2. Should I burn in the thinset over the mat only, then feather up to it when I put down my thinset for the ditra?

    3. Feather out the thinset across the rest of my floor when I set my mat. (It is about 2 feet exposed with no mat.)
    3.Use feather finish to level the areas from the mat embedded in thinset out the extra 2 feet before using the thinset to set the Ditra?


    • Roger

      It’ll be fine. I would flatten the thinset out over the mat, let that cure, then flatten it all out where there is no mat. Once that cures install your ditra.

  • Paul

    Hi Roger,

    Great site, very detailed and concise. We are looking at installing the SunTouch Floor Warming Mats on our bathroom floor. The manufacturer’s installation instructions would have us put the mat on the hardibacker, unmodified thinset, then tile. So they skip the Ditra. Our plan was to follow this process until we found this site. What are the pitfalls of the manufacturer’s installation method?


    • Roger

      Hi Paul,

      There are no downfalls to it. However, with ditra you get the added benefits of that product which means more protection for your tile and a more solid installation. Their method is completely acceptable.

      • Paul

        Gotcha…..thanks Roger! Youdaman!

      • Ted Dewey

        Hi Roger,

        I just want to double check with you my plan to install a suntouch heated mat in a bathroom floor. My plan was to install heated floor mat under ditra and install tile on top.

        For the heated mat, I was going to install this over plywood with a 1/4″ notch trowel and smooth empty areas without wire and install ditra on top just like your post. For this step, I was going to use Versabond polymer modified thinset. 1.) Is setting the mat in 1/4″ square Versabond thinset under the ditra correct? Is 1/4″ too much?

        For on top of the ditra, I was going to waterproof the seams and around edge of tub and then use unmodified thin set and also install with 1/4″ square notch trowel for rectangular floor tiles 6″x 36″. I was going to use 1/8″ grout lines. 2.) Does this reasonable so far? Any changes or suggestions?

        Lastly, I was going to use unsanded grout. 3.) Does this make sense as well?

        Any help would be greatly appreciated. Thank you.

        • Roger

          Hi Ted,

          1/4″ is likely too small. You need to ensure you have full coverage under both ditra and tile. 1/4″ rarely does that. Tiles that size normally require AT LEAST 3/16″, more often a 3/8″ is needed.

          And yes, I realize this reply is likely too late, but the info will be here for others in the future. Sorry for the delay.

  • Scott

    Hi Roger,

    I just found your site as I was researching my project. Luckily, I am at the stage of having finished the demo and not too far into the rebuild. Here’s what I want to do and what I have:

    Floors: I removed ceramic tile from the bathroom floor and was able to leave a poured concrete floor basically intact by getting under the tiles with a pry bar and popping them off. What I have now is a flat and level concrete base after patching and leveling a couple spots. I want to put down a Suntouch heating mat and then limestone tiles. Based on your other posts, is it possible to attach the heating mat, go over that with modified thinset, go over that with ditra, then unmodified thinset, then the tile? Also, you mentioned a perimeter border. How much should this be for a heated floor? I plan on tiling the walls as well. Can those tiles sit over that border?

    Walls: Sledgehammered out ceramic tiles set on a 2 ” mortar base over wire mesh. Lots of debris! I am tiling 4 ft up all around the bath and the shower to the ceiling. I have started but not finished going around the shower with durock over plastic sheeting, but after reading your site I think I want to take those sheets down, remove the plastic, and go with a membrane on the surface to reduce the risk of cracks. Do I need ditra all around the room (over drywall or just the shower? I use the ditra I use on the floor? Modified or unmodified thinset?

    Any other pitfalls you see looming? I’m sure I don’t know what I don’t know . . .

    Thanks for all the help you give. I am a fairly experienced handyman but this is my first total gut job on a bath. I am willing to spend time and money to get it right.


    • Roger

      Hi Scott,

      Yes, you can do that with the mat and ditra. Technically it should be unmodified between the ditra and base. The perimeter joint should be a minimum of 1/4″, yes the wall tile can go over it. DO NOT grout that perimeter (but you already knew that).

      Ditra is for floors, walls get kerdi. You only need the kerdi in the wet areas. Unmodified thinset to bond the ditra to the walls.

      You can put ditra on all the wall, but no real need for it. Walls in dry areas of the bath can be bonded directly to the drywall with modified thinset. Be sure to leave the wall tile 1/16″ off the floor tile so that perimeter expansion can work, you can silicone that gap – not grout.

      • Scott


        Thanks for the advice. I pick up the tile tomorrow. I’ll let you know how it goes and send along a pic when I am done.


  • Don

    Given that DITRA has channels on the under side that lets air escape, why do you notch the under layer of thinset? Seems like this is more likely to nick the wire, and I’m not sure about the purpose.



    • Roger

      Hi Don,

      Same reason you use a notched trowel for anything – to get a consistent layer of thinset beneath whatever you’re setting. Foregoing that it would be nearly impossible to get consistent, proper coverage.

  • Dan

    Hi Roger… great info once again!
    After a month or so away from our bathroom reno we are now back at it and about to install our floor heating (Warmly Yours – wire in mesh so its similar I believe to what you are talking about here except there are no clips to secure it to the floor.) I had a couple of questions and maybe clarifications regarding our install procedure…. nutshell version at end of post, crack open a beer version starts here…
    • Warmly Yours suggests hot gluing the wire down (or staple or tape). I assume the tape method is the worst as it will not allow thinset to do its stuff. We will probably use hot glue or a stapler.
    • they also suggest either a one step or two step (includes a skim coat and is their preferred method) layer of modified thin set for securing the floor mesh to the floor. Is this the right type of thinset or should we be using unmodified throughout?
    • At this point we should probably do a floor leveling in our case though as it is a bit out of whack… so should we: a) staple/glue the assembly to the floor, then b) apply a self leveling coat to the top of the mesh and let it do its leveling stuff? If so you I think you mention that we can make the thinset a little runnier as opposed to using a true leveling compound? (modified or unmodified?) (also… just how runny should we make it?)
    • there will be areas that will not have floor heating (tub, toilet, vanities) so I suspect when we do the leveling coat we should also let the thinset run into those areas as well so the entire floor is level?
    • after the leveling thinset cures we would then do a layer of 3/8″ thin set (unmodified) to put the ditra on?
    • then of course would come our laying of the tile.

    Nutshell version (is this correct?) – Lay out and affix the wire mesh (hot glue and staples) -> Level floor with thinset (modified or unmodified?) applied with flat trowel -> after leveling layer has cured (how long?) install ditra over 3/8″ unmodified thinset using… 3/8″ notched trowel! -> instal tile (unmodified thinset)

    • Roger

      Hi Dan,

      Hot glue is the best option. Yes, modified thinset. Hot glue the mat first, then slc. Slc would be much better than watered down thinset, which will not have the compression stability it should, it will be weaker. Do not use watered down thinset, if you want to do it with thinset mix it correctly and trowel it. Yes, let the slc run into the non-heated areas. You should only need a 1/4″ trowel for the ditra.

      • Dan

        Thanks Roger… the floor is actually not very much out of level (maybe 1/4″ over 17 feet), but it is flat so can we do away with the slc? If so then what do you recommend…
        a) a skim coat of thinset put down over the mesh with a flat trowel so as to not wreck any wires, then another coat of thinset with the 1/4″ trowel for the ditra, or
        b) a thicker (1/4″ or 3/8″) layer of thinset directly over the mesh with ditra applied immediately to this layer?
        In either case do we wet/mist the plywood floor before applying the thinset to ensure a good bond?

        • Roger

          Either one works just fine. The first is easier. You need to skim-coat it with thinset and make sure you have it burned into the wood well with the flat side of the trowel. Clean it with a sponge about ten minutes before installing, make sure there is no water on the surface of the wood when you install.

          • Dan

            HI Roger… so far so good… we installed the modified thinset over the wire mesh assembly and over the floor that didn’t have it (where the toilet, vanities and tub will be installed). We’re not sure if we have it thick enough though as it looked like it has settled between the mesh in spots (or suffered the dreaded shrinkage we all fear!)… what should we do next…
            a) top up the area with more modified thinset?… if so should it be done before the first layer cures completely? or, after the 72 hours to let it cure
            b) let the are cure and continue with the unmodified thinset and ditra… and just be really careful with the metal trowels over the thinner mesh areas

            • Roger

              Just wait for it to cure then install your ditra. It’ll always dip between the wires. If you are pulling your trowel correctly you’ll never cut anything with it. :D

              • Dan

                thanks Roger… you’re the best! :dance:

  • Q

    If I prime then embed suntouch mat in SLC and let cure. Should I then use Ditra or is the SLC enough? No problem either way, I just want to know the best way to keep things from cracking or otherwise. I am guessing that adding the extra layers of thinset and Ditra will reduce the heat transfer. Maybe not.


    • Roger

      Hi Q,

      It is always best to have the ditra as well, but the slc is enough. Ditra will not reduce the heat transfer at all.

      • Gene

        My parakeet just exploded! How can Ditra not reduce heat transfer? It has to be, compared to SLC or thinset. So what do you mean in this case by decoupling? The only thing that is being decoupled is the thermal conductivity, no? Please explain a bit more why you used Ditra here. Thanks.

        • Steve_in_Denver


          I had the same question, obsessed about it, made a sample board with and without Ditra and did some pseudo scientific tests. You can read about it here if you want:

          After taking in all the input (including the ridicule) I came to a conclusion that was good enough for me:

          1. It absolutely does impede the heat transfer.
          2. It doesn’t really matter.
          3. There are other factors that dominate – the relatively low power of the heating element, and the relatively poor thermal interface from tile to air/surroundings.

          My two cents.

          Not the best analogy, but consider your outdoor water hose. Turn the valve until it starts flowing at 10% of maximum (this is analogous to the low power heating element). Put your hose in a bucket with a small hole in it (the water flowing through the hole represents the heat flow from the tile to the room).

          Now the question is this: If you used a 1/2″ hose (ditra) compared to a 3/4″ hose (no ditra) would it change how quickly your bucket filled up? The point is that both hoses have so much excess capacity that it really doesn’t make much of a difference even though the 3/4″ hose is capable of a higher flow than the 1/2″ hose.

          That’s what I came up with, and it’s good enough to satisfy me.
          (plus, my floor heats up just fine, so that goes a long way to convincing me that Ditra isn’t a big deal)

          I do think if you are trying to eke out maximum heat output (say for heating a room, not just the floor) or if you are concerned about energy costs/ efficiency, then I do think Ditra makes a difference…but insulate the underside of the floor and be done with it.

        • Roger

          The only thing being decoupled is the tile from any movement in the substrate. The ditra does not affect the heating capabilities of the heating element at all. Zero thermal blockage. Ditra was used as a substrate movement suppression membrane.

  • Chad Nickless

    You may want to wait a full 30 days before turning on the system. You can turn it on for a very short time, to test that it’s heating up properly, but to actually have it running…. 30 days. That’s is for a full cure. Straight from Suntouch’s website as well.

    • Roger

      Chad is absolutely correct! Every manufacturer will have a recommended wait time before firing up the heat. Doing it prematurely may compromise the final cure of the thinset and weaken the bond. Thanks Chad.

  • Steve


    I opted for the SLC method for my WarmWire…I used Ardex Liquid Backerboard (it specifically says that it doesn’t require lath). I mixed it exactly according to the instructions (including precise water measurement), and worked diligently to spread it, but it isn’t completely flat – about 5/16 difference between the high spot and the low spot (about 5 feet apart). Ignoring the low spot (which is under the vanity) my deviation is closer to 3/16″ over about 4feet.

    1. I plan on doing more SLC in the future – any suggestions on how to avoid this much difference. Just from looking at it I don’t think I could tell that it wasn’t even as I spread it…without some indexing reference.

    2. I could fix 90% of my problem if I could take 1/8 off of an area about 3-4 square feet. My sander is locked and loaded – is this a reasonable way to attack the problem?


    • Roger

      Hi Steve,

      1. You can always add more water to the mix than called for. Or use custom’s or laticrete’s slc, they work and level very well.

      2. Yes, absolutely that will work. Wear a mask. :)

  • Jim

    Roger, I have a customer who wants his heat mat installed on top of the ditra for better heating. I’ve never done it this way but can SLC be used on top of the ditra to embed the heat mat?

    • Roger

      Hi Jim,

      Yes, but you lose the effectiveness of the ditra as well as the warranty. Ditra has abolutely zero r-value, it doesn’t lessen the heat to the tile at all. The slc will actually hinder the heat more than the ditra, as does the thinset. Not going under the ditra actually hinders heating the tile more than having it above.

  • Nik


    First of all, thanks for your wonderful website. Here is my situation, I am installing warmwire in a roughly 8 x 8 basement remodel over a concrete slab that was poured in 1976. Here is the problem. There are two sections of vinyl on the slab that cover about 50% of the slab. They have asbestos (I sent samples to be tested, it is not the really bad asbestos, but still…). The vinyl is attached to the slab with no curling and the glue is throughout, not just on the edges. I do not want to mess with the vinyl. I talked to the warmwire folks and they suggested prep the floor, scratch up the vinyl, add redguard, then the wire. I plan to then attach the ditra and then tile. What do you think about this plan?


    • Roger

      Hi Nik,

      It sounds fine to me. I would give Custom building products (the makers of redgard) a call and see if they have any caveats with that, but it should work just fine.

  • David

    I plan to install the suntouch system. My question is about using a product like redgard from homers. I plan to put down the wire and encase it in self leveling cement. When should I put down the redgard- prior to the slc or after? Is this a feasible plan? My concern is simply that when I put down the tacking for the wire, I will be creating new holes in the backer board that would then need to covered.

    Thanks for the help,

    • Roger

      Hi David,

      It depends entirely on why you are using redgard, what purpose it serves. So, why are you installing redgard?

      • David

        Red gard will serve as a water proof membrane. It’s a bathroom (which I should have said up front) so I plan to do 2 coats on the shower walls and 1 on the bathroom floor

        • Roger

          In that case you want to install your wires, slc, then redgard. Redgard is right under the tile, so on top of the slc.

  • andy walker

    Hi Roger
    your comments and replies are extremely helpful, thanks. We have installed tile over concrete on a walkout basement application. The floor tends to get quite cold especially in the winter. We used the mat in floor heating from Lowes (Quiet Warmth) and the acrylic thin set that it called for with Ditra, then the porcelain tiles over that. Is it necessary or recommended to use epoxy grout for this due to the expansion and contraction when the in-floor heating is turned on? many thanks

    • Roger

      Hi Andy,

      It is not necessary at all, although you can use it if you want. Micro-movements will be compensated for in the ditra layer, not through the tile layer. Not sure what ‘acrylic’ thinset is??? Modified thinsets? I hope you don’t mean the pre-mixed ‘thinset’ in the bucket? Please tell me that’s not what you mean.

  • Nick

    If you weren’t going to use Ditra on a basement slab how would you do this without using self leveler? Would u just use mortar and flat trowel it slightly above the cable height, let it dry and tile over next day? Or would you install tile in your first pass right away? I’m not a big fan of having a 1/4 inch mortar bed as my substrate under tile, are you?

    • Roger

      I would use a medium bed mortar which can be built up to 3/4″ in some cases. I would likely do it the same day unless I had other projects in the house, in that case I would float it flat and set the next day.

  • Joel Rutledge

    Roger, I have purchased the mat style suntouch. How close to a wall CAN I put the blue heating wire? I have a section of floor by my tub leading to my shower (which you helped guide me to build!) that is only 29″ wide. Thw wire is 28″ loops

    • Roger

      Hey Joel,

      You can put them almost right against the wall if you need to. The reason they don’t mention nor recommend it is that nobody normally walks against the wall.

      • Joel Rutledge

        You rock as always. I had a handyman smooth the concrete with SLC but he didn’t use any primer before hand. The concrete was water absorptive so I assume it gripped ok? Now its smooth I plan on laying the suntouch mats then use modified thinset on top. I still have more levelling to do should use slp instead of thinset? Thx!

        • Roger

          Thinset will be fine.

          Can you please post any further questions as a response rather than a new comment. Thanks.

          • Joel Rutledge

            Hey Roger. How close can the braided supply wire be to the blue heating element of another mat? Where I need to run them it will be 1/2″ away…

            BTW, mobile users of your site will have to switch to nonmobile version at the bottom of the page, because the mobile version doesn’t let us reply…

            Thanks for the advice, wise elf. You’re saving my bacon again!

            • Roger

              1/2″ away from the braided portion of the wire is fine.

              Thanks for the heads-up on the mobile version, I’ll need to do something to remedy that. Appreciate it.

            • Joel

              Okeydokey I thought of some other questions… One is yes/no, the other is multiple choice…

              1) Do I need a control joint when using a floor warmer? It’s only a 12 x 12 room, in two three- to eight- foot wide legs of an “L”, but thought I’d ask, since the warmth probably affects expansion like sunlight does?

              2) I still need to level the floor some more after I put the suntouch mats down: if I use thinset to install the mats and fill them in, do I have to use thinset for the rest of the leveling in the room?
              2a) Or will SLC do okay over thinset?
              2b) Does it need to be primered first, even if on top of thinset?
              2c) Or should I just use SLC for all of it: installing the mats and leveling, to make things easier (but slightly more costly)?
              2d) If so, do I need primer on everything: the concrete, existing SLC and mats?

              Oops, I did it again: Joel is over-thinking things again… :roll:

              Thanks, Roger!

              • Roger

                1. No, but you do still need perimeter joints.

                2. If the height that you need to level is less than 1/4″ then just do it with the thinset as you install the ditra. If it is more then use slc over the wire (you can ‘level’ with thinset over the wire as I did above, then use the slc) and under the ditra. It always should be primed first, even over cured thinset. You do not need to prime the heating wires. It is always easiest to prime any substrate first, then put down the wires, then slc. This ensures your substrate is fully primed.

                • Joel

                  Oh, I should have clarified: I wasn’t planning on using the ditra. I already have height differences to compensate for and don’t want to add any additional height. Does that change the advice?

                  And yes, I need more than 1/4″ of height adjustment, and yes, I will need to do it in two pours of SLC in some places.

                  So let me see if I have this right:
                  1) Primer for SLC
                  2) Suntouch Mats
                  3) SLC over the mats and everything. Repeat SLC as needed.

                  If I use thinset on the mats, then I have to wait for it to cure before using the primer and SLC, so I think I would prefer to just use the SLC without thinset. My boss, I mean wife, is already upset with how long this project has taken me..

                  Anything wrong with this sequence, especially since I’m not using the Ditra? :eek: Thank you thank you!

                  • Roger

                    Nope, nothing wrong with it. That’s the correct sequence.

  • Lou


    First great site. You have done well.

    My project is installing Thermosoft heated floor on wood underlayment in a small, 10×4, bathroom. The wire-mat is down and I was about to install the Ditra but Schluter says to apply modified to the heated mat first. Then use unmodified to lay the ditra then unmodified for the tile. I was thinking of straight modified on the mat, then Ditra and unmodified under the tile. Or should scrap all that and use the SLC? Which I’d rather not considering I’d have to pull the heated mat up and prep the floor better for the SLC.

    • Roger

      Hey Lou,

      Just use the modified over the mats and under the ditra, then the unmodified over the ditra under the tile as you planned. It’ll be fine.

  • Glenn

    Kerdi on floor? Have you ever used Kerdi on a floor (to reduce height verses Dritra)? Have you ever used electric radiant heat under Kerdi? Reason doing a curbless shower an want heat under flat areas. Schutler site has a pdf of Kerdi on floor & shower but no mention of radiant heat.
    btw I like ur method of writing:-)

    • Roger

      Hey Glenn,

      Kerdi does not do the same thing on the floor as ditra. All it will do is make your floor waterproof and it may cause problems over wood because you have no separation layer between it and the tile, just a waterproofing layer. You need to transition to ditra on the bathroom floor. Radiant heat is placed under kerdi as well, yes I have done it several times. It is the proper method for heating with kerdi.

  • Brian G

    Hey Roger,

    I am looking to see if epoxy grout can be used with the Suntouch radiant heat system?


  • Evan M.


    Use mentioned using a 3/16″ square notch trowel for leveling the floor. First off, I can’t find a 3/16″ square notch plastic trowel (only metal), so I thought I’d buy the 3/8″ and run it the through the table saw….but, then I got to thinking. A 3/16″ square notch run against the floor will make a 3/32″ bed height, right? Wouldn’t that be shorter the the wire height and either make waves or voids between the floor and Ditra?

    Thanks! I can’t wait to be done with our construction work and start putting down our heated floor.

    • Roger

      Hey Evan,

      Only if you flatten it between the wires to the height the notches allow. The 3/16″ is to allow the trowel to straddle the wires as you are combing it as well as have the top of the notches at the correct height where there is not wire (don’t flatten those out). When you flatten it out you want to add enough thinset so it’s level with the height of the wires. When finished you should have a flat layer the height of the wire everywhere there is wire and notches the height of the wire everywhere there isn’t.

      • Evan M.

        I’m still trying to understand this one, I’ve read it several times and it keeps haunting me. I assume that after setting the Ditra and pushing it in that there would be no more notches in the thinset, that it’d be completely flattened out with no air “pockets” left. Is that wrong? If there should be no “pockets” left after pushing in the Ditra, then if I remember my Warmwire measurement correctly, there wouldn’t be enough thinset down to reach the height of the wire. Am I being too particular or should I go back and re-read the instructions a few times? I’m feeling dense on this one…

        On a similar note, would this technique work with a 3/8″ square notch trowel, or would that cause problems (like getting the Ditra flat?)?

        • Roger

          Hey Evan,

          No, you aren’t reading it wrong (I don’t think…) You simply use a trowel notched large enough to leave the height of the wire when flattened. If you do it all at once you need to use a larger trowel. If you fill between the wires with thinset one day, let it cure, then install the ditra the next you would just use the properly sized trowel for ditra installation over a flat substrate. You can use any sized trowel you want, flattening the ditra as you install it dictates how flat it is, not the amount of thinset beneath it.