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

  • Jeff Ward

    Hi Roger,

    I’m remodeling my master bathroom and am considering a heated floor. I’ve already purchased Ditra, not Ditra-Heat, so I would put the heating wires on the OSB subfloor. You mention that the next step you use is to put SLC over the wires before the thinset. I’m wondering why this step is needed if my floors are already level. Can I skip the SLC and just use thinset over the wires and subfloor?


    • Roger

      Hi Jeff,

      You can, but it’s a pain in the ass to get the ditra flat over the wires if you don’t have experience doing so. I just use thinset and ditra over the wires, but I’ve been doing it a while. I mentioned that because it’s the easiest, most efficient way of doing it. That’s why I have this site – to explain that to diy’ers. :)

  • Amit

    Do you really only have to wait 3-6 days for the installation to cure before turning the system on? I just installed a heated floor in my sunroom using a similar product, and everything I read says wait 28 days before turning the system on.

    • Roger

      Hi Amit,

      Different products have different requirements. If your product says 28 days it is falling back to the default cure time for cementitious products. While a full cure is ideal, it is often unrealistic. But it is always best to wait that length of time.

  • Chris

    Hi Roger,
    Getting ready to install my ditra-heat mat (finally!) and had a quick question. Once its down and the wire is in, what are your thoughts on doing a coat of thinset to fill in the mat and secure the wire, letting that dry and then doing another layer to install the tile? Seems like that would be a bit easier for a novice like myself. If I do that, should I use modified thinset as it will have access to air to dry and then unmodified when I install the tile?

    Also I think you mentioned you’ve used SunTouch wiring with the Ditra Heat – do you always use a spacing of 3 or can you get away with 2 if you need to in order to make the wire fit?

    Thanks again!

    • Roger

      Hi Chris,

      I pre-fill all the time. You want to use the same thinset you’re setting tile with. You can do 2, 3, or a combination of both with suntouch wire in the mat.

      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.

  • Chris

    Hi Roger,
    First of all, thank you for all the great information on your site, its been a ton of help! One quick question for you – I’m planning on installing Suntouch wire for floor heating. Do I need to put Hardiboard down over the subfloor (plywood/OSB/Particle Board mix) before the wire if I’m going to apply SLC over the wire? My thought was to do this – subfloor, wire, SLC, thinset/tile. Will that work? Also, for the SLC do you ever use the primer like they recommend?

    • Roger

      Hi Chris,

      No need for the backer at all, you’re plan will be fine. I ALWAYS use the primer over wood. Apply the primer first, then the wire, then the slc. That way you get full contact on the primer rather than trying to do it over the wire. And if there is particle board in that mix you need to replace it with plywood.

      • Chris

        Thanks Roger! There is actually a lot of particle board in the floor which is going to be a big pain to remove, so I think I’ll go with your original suggestion of subfloor, hardibacker, wire, SLC, thinset/tile.

      • Chris

        Hi Roger,
        OK, now that I’m thinking about it more and did a bit more research, it sounds like regardless of the method I do the particle board needs to go. Is that correct? From there I’m thinking these are my two options:
        1. Rip out old particle board and replace with plywood/OSB, wire, SLC, Tile.
        2. Rip out old particle board and replace with plywood/OSB, Ditra heating mat/wire, tile. The Ditra instructions say they can be placed right over ply/osb, any issues with that?

        Based on those two (and please correct me if there are any issues with either method), do you have a preference based on ease of install, cost or reliability?
        Thanks again!

        • Roger

          Hi Chris,

          That is correct. I prefer the ditra heat, it can go over the ply and it’s a hell of a lot quicker. It’s a little more expensive than using slc, but not much at all and well worth the difference for ease of install and time.

  • myriam

    I forgot, of course, before step 2: “Install the Cork All Inclusive Underlayment, built-in 6 mil vapor barrier”
    Thanks so much! Myriam

    • Roger

      I am unfamiliar with the particular product you have, but yes, that’s where it goes. You need to contact the manufacturer of the cork to ensure that it’s approved for use beneath tile.

  • myriam

    Hi Roger,
    I’m installing radiant heat in my bathroom floor and although I understood all your jokes being a novice I’m not sure I have the technical sequence right. I don’t have ditra, I just have a NuHeat mat. The NuHeat people say that you should install some cork or other membrane over the HardieBacker because the R factor for the HardieBacker is only 0.2, and you want to improve the upward heat transfer from the mat to the flooring surface. So now I bought a product called “Eco Cork Foam” at Home Depot, “Premium Plus All Inclusive Underlayment, built-in 6 mil vapor barrier”.
    Can you tell me if what I have makes sense, and if the sequence is right:
    1) Install the HardieBacker (I’ve already done that),
    2) Lay on latex-modified thinset (The NuHeat instructions say you must use modified);
    3) Roll out the NuHeat Mat;
    4) Then install SLC (wait 24 hours);
    5) More modified thinset (this step is necessary, right, you can’t just install tiles directy over SLC with no additional thinset?)
    6) Install tiles with “Urethane Grout”; I picked the “TruColor” brand because this is what they had at the tile store, they told me they never heard of your method of freezing portions of it while you work, but they would look up your blog.
    Thank you for the great blog. You’re making a huge difference out there!

    • Roger

      Hi myriam,

      You do have the order correct, and you do need to use thinset over the slc. However, you do not need to freeze urethane grout, it won’t do anything. That method is for epoxy grout – two different things.

  • Dave

    Thanks for the great posts. I’m looking to install this and one of the things I was considering was using the Ditra Uncoupling Membrane and running SunTouch WarmWire instead of the Ditra Wire. Here’s my reasoning and please let me know if my thinking is wrong….

    1. The cable length for my custom bathroom is available in SunTouch rather than Ditra which is either 10 sq ft too much or too little.
    2. I like the Uncoupling membrane a lot better than using clips and I think it would be easier to use and keep the floor even.
    3. Rather than using clips and then a thinset with Ditra underlayment, I would be able to save a step and thus keep the height lower and the tile closer to the heating coil.

    That’s my thinking…what do you think?

    • Roger

      Hi Dave,

      You’re just fine, that is actually how I do my heated floors now – ditra heat and suntouch wire.

  • Lisa

    Sorry if this is a repeat, but my head is swirling with all the google searching. I want to tile a bathroom and connected pantry. The current subfloor is several different materials (patched with rough plywood, some tongue and groove). Where the differing materials meet are slight gaps and varying heights (nothing more than an eighth inch). One guys advises me to just put heavier thinset in these spots and then screw down backerboard. Another tells me to use slc first and make it perfectly smooth before backerboard. Your post says use slc after?? My plan was to
    – use slc
    – screw down backerboard
    – thinset
    – install heated floor wires
    – install ditra mat
    – layer of thinset
    – tile

    Is this correct? Also, am only heating the bathroom portion and not the adjacent pantry, but the floor will be continuous through both rooms. How do I make the pantry floor the same level as the heated bathroom floor?

    • Roger

      Hi Lisa,

      You have a lot going on there, too much, in fact. :D Here’s how I would do it:

      Screw down backerboard (with thinset beneath it to fill any voids).
      Install heated floor wires
      Install slc to the top of the wires
      Then either tile or install ditra over that. The ditra is not necessary over slc, but it definitely doesn’t hurt. You NEVER want to screw anything through slc, it’ll crack it and compromise the slab (of slc).

      • Lisa

        Thank you!

    • Lisa

      Thank you!

  • Tom Curry

    Roger, I’ve read most of your site & am amazed at the information you have shared.

    One question. Going to lay several hundred feet of Ditra heat mat with some long runs, all over concrete. Have 75lb linoleum roller & wondering if that’s too much weight to roll the Ditra into the thinset.


    • Roger

      Hi Tom,

      Nope, that will be just fine.

      • Tom Curry

        Back in Vegas where we’re rehabbing a house & just noticed painters didn’t use spray shield so have anywhere from 4″ to 12″ overspray on the slab where we’re getting ready to lay Ditra heat mat next week. Paint is latex & using unmodified thinset. What can I do about the latex? It’s around the perimeter of all the areas geting the Ditra


        • Roger

          Hi Tom,

          Smack the painters first, then you’ll need to scarify the paint off the slab. A grinder with a scarifying wheel works best.

  • Brian G

    Hi Roger. Couple questions about a heated floor i am installing.

    I’m using the Suntouch system. Is it ok to use modified thinset? The wires are encased in a layer of SLC (Level Quick RS) and I’m tiling on that.

    Also my tiles are sized 24″x6″, can I use Versabond Thinset (MTSG50) or do I have to use the Versabond LFT for large tiles?

    Thanks. Brian-

    • Roger

      Hi Brian,

      Yes, you can use modified. And you can use either thinset with those tile.

  • Hank

    So I cut about a foot of wire off the mat wires that connect to the thermostat thinking they would still be color coated wire down the line, and to clean up the mess hanging out of the wall in order to finish remodeling. The ground wire is a no brainer but do the neutral and hot necessarily have to go to hot an neutral on the gain of the thermostat. If they do how will I be able to determine which is which. Any ideas thanks

    • Roger

      Hi Hank,

      I honestly have no idea, I’m not an electrician and I never cut the wires. I’d assumed they were color coded all the way down as well. I would contact suntouch and ask them.

  • Doug Tody

    My soon-to-be installation has Cerasorb, a heat mat, Ditra, and tile on a concrete substrate. What is not clear is whether to use modified or unmodified thinset between the Cerasorb and Ditra, encapsulating the heat mat. The best I can figure is to skim coat the heat mat with modified thinset
    and then set the Ditra on top of that with unmodified, after the skim coat dries. But in your article you managed to embed the heat mat and set the Ditra all in one go, which would minimize depth while making it easier to level the Ditra. What type of thinset was used for this, and could I do the same on top of Cerasorb? Thanks much for these articles, they are the most helpful resource I have found to help with this project.

    • Roger

      Hi Doug,

      It was unmodified thinset, which is required between ditra and concrete or cement board. Yes, you can do the same with the cerasorb.

  • Nichole Black

    Hi Roger,
    We are considering in floor heat in our bathroom. So far what we have read says do not put it under the cabinet. We have a raised cabinet of sorts and the tile needs to extend all the way underneath the cabinet. What is the best way to make sure we have a level tile surface throughout the entire bathroom area if we do not have heating cables that cover the entire surface to be tiled?

    • Steve in Denver


      I figured I’d weigh in on this because I just did the same thing recently. Take my [DIY] input with the appropriate amount of skepticism.

      1. If you use Ditra Heat you would just not run the wire in that area – the height of the Ditra (what you will apply the tile to) will be the same everywhere.
      2. If you use a mat or bare wires encased in thinset, you could just trowel the non heated area, too…you may also have to add a little extra thinset when you set the tiles in that area. It may not be perfect, but it’s under the cabinet anyway…probably not visible, eh?
      3. I did bare wires + SLC then ditra over that. The SLC did its thing and made everything the same height, wires or not. (more or less)

      The other thing I would consider (wish I had known) that your toes stick under the front of the vanity, especially when you are leaning in close to the mirror to pick that spinach out of your teeth. If I were doing it again I would run my wires at least 4 inches behind the front of the vanity, maybe more. It’s not going to hurt anything (my non-professional opinion), especially with a furniture style vanity.

      • Nichole Black

        Thanks Steve in Denver!

        • Roger

          Yeah, thanks Steve! :D

    • Roger

      Hi Nichole,

      The spaces where you aren’t using heat just take the trowel and put thinset there (with the ridged side) and let that cure, then fill in the stripes with more thinset. This should get you to the same height you have in the areas with heat cable.

  • Chuck

    Hi, My question is about leveling a floor without any heating wires, and it seems rather similar to some of the cases you have covered here.

    The floor is 1 inch yellow pine T&G with 1/2 ply on top, over 3×10 on 16 inch centers. I find with a staright edge that there is a general depression in the floor with a rise in the middle ( I looked below and nobody has put a jack under there etc, its just what happens after 80 years). The rise is about 3/8 inch from the lowest spot. I am interested in your advise regarding methods of filling the depressions, SLC, mortar or deck mud. I will probably plane down some of the hump to have it in line with the high areas along the walls, but feel that I should fill things. And what considerations should be made to the thin set that will holds the Ditra down if there is the leveling product as well as plywood (not OSB).

    Thank you, your advise has made a lot of sense to me.


    • Roger

      Hi Chuck,

      If you can plane that down to be less than about 3/16″ then you can just go over it with ditra and level it out with the thinset beneath it. If not, slc would be your best option. If going over both slc and ply I would use modified thinset.

  • mark

    I am confused about the cork use. I am sun touch wire over concrete. If I put down cork, I am assuming I must have to use SLC as I would not have any substrate for the thinset to attach to. The tile shop sells a plastic membane to go over the wire but I would defer to my previously inferred question about what would the thinset stick to?

    • Roger

      Hi Mark,

      The thinset will bond directly to the cork. Or you can pour slc over the cork and wires.

  • Christine "losing her mind"

    Ok…………I will forgo the swearing like a drunken sailor, or at least try!!! We have a basement, house built in 1940. Very short ceiling height at 6’10.5″ according to my husband the non vertically challenged human being. Basement floor is wonky to say the least and I want to put in the in floor heating but all those layers are gonna add up and he will soon have walk hunched over says he, So dramatic. Anyhoo, need to finish basement because I am tired of running a home for wayward spiders and would like it to actually look nice and not like some set from a horror movie. I was reading about the cork, blah blah, then backer board and then mortar and spinach leaves and tile and saran wrap and frankly seems crazy, any easy way ha ha haaaaaaaaaaaaaaa? Thanks for possibly saving my sanity, maybe. Christine :dance:

    • Roger

      Hi Christine,

      The easiest, and most low profile system for in-floor heat is schluter ditra-heat. Don’t freak out over the price! You can use the mat (the orange part) and use either suntouch or nuheat systems in it. You don’t need to use their heating wire or system (don’t tell ’em I told you that).

      Putting a thermal break (cork) beneath the mat helps it both warm up much more quickly and run more efficiently. Concrete will suck heat out of the system. With the mat only you’ll have about 1/4″ beneath your tile. With the cork it will be that plus the thickness of your cork.

      • Christine

        Hi Roger,
        Thanks for the quick response, where might I get cork and how the heck do I put it down. Glue, dog slobber-got lots of that. lol So I have to use leveler first then cork then the ditra heat either the wire or mat??? then tile. Can I do hardwood or would you suggest bamboo since its the dreaded basement.

        • Roger

          Any good flooring store should stock cork underlayment. It normally gets glued down, but method can vary by manufacturer, it depends on which you purchase. Yes, if you’re using leveler it goes down first. Over the cork you can use ditra heat mat. Heating wires snap into the heat mat, so once the mat is down you will have no more height beneath your tile, the mat is it. Yes, you can also do hardwood. Bamboo will be fine, but not necessary normally with a proper barrier or underlayment beneath it.

    • Steve In Denver

      I noticed that you said you floor was “wonky” and wanted to add that you will still want to make it flat before putting down the cork and Ditra heat. How flat you need it to be depends on your tile size, but it’s safe to say that “wonky” isn’t flat enough. :)

      You might find this helpful – and maybe a bit disheartening:

      You may need SLC or a floor patch to get your floor closer to flat, and in any case you should look into what is required for concrete floor prep before putting anything down. In my quite limited experience and rank amateur opinion, prep is where most of the work is done and has a significant impact on the end result.

      • Roger

        Steve is correct, the quality of any installation lies in the prep beneath it.

  • Mark

    Putting warm wire down over concrete floor. After reading all the info on the warm wire site we decided to put a layer of cork over the concrete for insulation with the warm wire on top of the cork. The next step would be self leveling mortar then the tile. But we have heard various versions on why we should NOT use self leveling mortar over the cork. Read your article about using the Ditra and was wondering if that is the way to go.. cork/wire/Ditra/ Tile? Or should we not use the cork at all? Any help would be appreciated. Thanks!

    • Roger

      Hi Mark,

      You’ve done it correctly. The cork will help tremendously as bare concrete will suck up a lot of the heat and your floor will take forever to heat up. You can use the ditra over the wires over cork. It works well.

  • David Holmes

    Hi Roger,
    Schluter have a decoupling membrane that takes the wire on top. Would there be issues using SunTouch WarmWire with that membrane? I’ wondering about this because I need a shorter cable than Schluter provides.

    • Signature Tile

      You have to use Schluters wire with their Ditraheat system. Other wires will not fit properly. For their system you will need to buy or rent a megohmmeter which not only tests the resistance like a regular voltage tester, but runs high voltage through the wire to test the cable jacket’s integrity.

      • Roger

        Chad, you do not need to use schluter’s wire. Suntouch fits just fine, I’ve done it several times now.

        • Signature Tile

          But you then have no warranty from either
          company. Schluter won’t warranty their wire, and the other wires are required to be under the mats for installation. At least that’s what a rep told me.

          • Roger

            That is true as far as I know. I try to make people aware that a LOT of the methods I advocate will void their manufacturer’s warranty. I think suntouch is talking about warrantying their wire in the new mats, though. Nothing tangible yet, though.

            • Signature Tile

              That would be nice! Let us know if you hear any progress, thanks Roger!

    • Roger

      Hi David,

      No issues at all. It works very well. It’s my preferred method of in-floor heat now. :D

  • Gary

    Hi Rodger,

    Any experience with, or advice on using the cabling mats recently developed?:


    • Roger

      Hi Gary,

      Never used the rpm mats. I now use ditra heat, I will NEVER install in-floor heat with anything else again. It is that easy.

    • Signature Tile

      I have used RPM many times, it’s a good system. Just like Ditraheat, the wires sits down inside the mats and is protected during install and makes repairs easier, with little chance of damaging a wire, in the future if needed.

      Price-wide they are pretty close, speed-wise they again are pretty close. Warranty on the wire is 20 years I believe with RPM (using Warmwire or Masterheat wire) and 5 years on the Ditraheat. Don’t ask me why they only do a 5 year warranty, I asked that same question during the training class when I first took it, and they did not really have a good answer. Just a new product for them, and they were expecting to be able to increase the time in the future.

      • Roger

        Thanks Chad!

  • Rod

    Hi Rodger,

    When installing large tiles say 20″X20″ over heating cables, is it necessary to increase the size of the grout joint for expantion?

    • Roger

      Hi Rod,

      Not necessarily, but you do need to ensure you have perimeter and expansion joints.

  • Bonnie

    Hi Roger,
    Thank you for this article. I have several questions for you.
    I have pulled up the old tile and thick cement bed underneath the tile in both of the bathrooms in my 50 year old house. (I will definitely need to bake cookies for the trash men for the next few weeks as I eek those bags of cement and tile out along with the regular trash.)
    I have pulled the toilets and will leave the original tubs in place. I plan on installing the subfloor as you recommended:
    – tile
    – mortar
    – Ditra
    – mortar
    – self leveling cement
    – heat wire
    – 1/2″ hardibacker w/ mesh tape seams
    – thinset
    – old plywood currently there.
    I have several questions.
    1. the plywood appears to be 1/2 inch. Floor joists are 16″ on center. Should I do a flex test or simply assume there is too much deflection and add structural support under the 1/2″ plywood by bridging diagonally to the floor joists?
    2. Should I seal the 1/2 plywood with a latex sealer so it does not absorb too much moisture from the thinset and compromise the bond?
    3. Should I use a particular bumper around the edges of the room and around the toilet flange when I pour the self leveling cement, and if so, what do you recommend?
    4. Suntouch warm wire recommend a dedicated GFI circuit. I am considering running both bathroom warm wires to the same thermostat, by placing a short conduit at the bottom of the wall between the two baths (back to back bathrooms). What I am wondering is, should I run two different wires, or can I run one long wire to cover both bathrooms? If I run two wires, is there room in the thermostat for both sets of wires. If I run two wires, will it overload the dedicated circuit?
    5. Do I need to have a flexible joint between the mortar and the tub?
    6. Do you recommend running a secondary thermostat in the floor in case the first thermostat fails in the future?
    7. The unmodified thinset, does that go above or below the Ditra?
    By the way, I love your humor interjected into what might otherwise be a dry explanation. Thanks for being so thorough in your explanations

    • Roger

      Hi Bonnie,

      1. You should add an additional layer of ply and forego the backer, just install the primer, wire, then slc over that second layer.
      2. No. If you use backer there is no bond required, it is only there to fill voids.
      3. Yes, regular sill-seal (pink or blue foam sold in the concrete department) works fine.
      4. That is a question for the heating manufacturer’s technical department. I’m not an electrician, I don’t even play one on the internet.
      5. What mortar?
      6. Yes, I always put two probes in there, one as a spare. They are the most common component to go out.
      7. If using slc over your wire then unmodified goes both under and over the ditra.

      • Rhonda

        Roger – :whistle: Why unmodified under AND over the DITRA if you use SLC?

    • Rhonda

      Did you use metal lath on the plywood? I’m trying to figure out whether it’s necessary when using DITRA and if so, how do you avoid damaging the heating wire with the lath? I used Levelquick over ply and didn’t use the metal lath and now am worried it will crack!

  • jeff

    if I got this right I first remove the vynil floor in the bathroom. Then lay down some thinset, put down backerboard and screw it into the floor. Then place down the radiant heat coils, then slc or thinset? Then ditra, then thinset and finally start tiling. 1) is that correct? 2) manufacturer says no sharp objects when laying the coils. Wont there be a problem after all the screws used in the backerboard the coils will find a sharp edge (screw head)

    • Roger

      Hi Jeff,

      The backer is not required, although you can use it if you want. The coils can go directly to the wooden floor, then coils, then ditra. The screws in the backer, if you use it, should be countersunk.