Either or. You can fill the waffles first, or do both steps at one time.

And one thing I did after putting the heating down, since I used wires that were arranged in mesh, was to put a *thin* coat of thinset over the wires themselves so as to make my eventual tilesetting MUCH easier to get flat.

If you're using a heating method that has any unevenness in it, you might want to do that too. I'm sure pros don't have to do it, but as a DIYer I found it an invaluable step that my local tile guy recommended to me. I imagine that getting the tiles level over the raw wires and mesh would have been even more demanding than it was...which was significant (large format tiles).

If I do not fill the waffles first and let that dry and do in ONE step...any tips on how to do this? Trying to envision this. Do I flat trowel the waffles to fill them, then lay the blanket on top of the smeared in waffles, then trowel for the tile? I am trying to figure how to lay the blanket as I go along versus having it all layed out ahead of time? Or do I lay the blanket on the Ditra, then smear in and fill the voids through the holes in the heating blanket, then notch trowel for tiles?

Flyleft..to confirm your step..are you saying you layed a thin layer over the entire floor, or just in certain areas to hold it down.

Before I answer, let me ask you-- Which system is this? The way you're talking, it sounds like Nuheat, but I want to be sure before I answer.

I was looking at Suntouch which I saw at Lowes or Warmly Yours, both have a mesh like grid with a wire running through it. The Warmly Yours website makes a good case on their website reasoning why their product is better? They all come in a roll, so I was trying to envision filling the waffles, laying the mesh, and installing the tile in one shot. I am a proficient DIYer that has always been able to manage anything hands on, but I work very slow and methodical. Is this too difficult? I envision all my marking lines going south too (between being covered by the filling waffle step and then topped with the mesh). Am I worrying too much and this is really not a concern?

Saxmaan, I put a thin layer over the entire floor so as to fill everything up to be even with the top of the wires. Imagine skimcoating the floor.

I then marked on that, btw.

What size tiles are you using?

Wanted to use 12"x12" porcelain floor tile

What brand of electric mat did you use...? So far, I have found Suntouch, Warmly Yours, and from Bill, Nuheat.

I would assume this skim coat layer is a bit rough or bumpy? I guess I just drag the trowel across the top of the waffles so some of the unrecessed waffle still shows (should it look like a checkerboard pattern of orange and thinset filled waffles. Or do you completely cover the Ditra mat with "x" inches of thinset? Does this pre-thinset drying step reduce the Ditra performance?

Here's a link to the Schluter site - seems to indicate that the heating mesh goes under the ditra, if you have a wood sub floor. I'm considering Ditra and electric heat too and am watching this thread closely.

http://www.schlutersystems.com/5570.aspx

Ah, 12x12 should be a bit easier, but still it would be helpful to have a flat surface between wires and tile, if that's what is next to each other.

IF you do wires-ditra-tile, then you don't have to do that step, because you just install over the ditra. It's regular enough. The wires are what you want to skimcoat, if you have the wires on top. So to repeat: if the process is wires-ditra-tile, then no skimcoat necessary.

We used a brand that fewer people have used but I've found the customer support pretty great and the product (so far) is sound (resistance stayed the same throughout installation--very important to keep checking it). I'll link it below.

I see that since we've built, the company has developed a heating mat for under wood floors--*just* too late. Man, heated wood floors, a soaking tub that has inline heating but no jets (in another thread)...we built a year too soon.

Here is a link that might be useful: ThermoTile

So correct me here...are we saying the layers go like this:

1) Plywood Subfloor
2) Modified Thinset
3) Electric Mat
4) Modified Thinset
5) Ditra
6) Unmodified Thinset
7) Tiles

What impact will the Ditra have on the heat getting through to the tiles? I thought I read the mat should go under the tile when reading on an electric mat site? Installing under the Ditra sounds easier though. If this is correct, I guess we are saying it is a one step process that would consist of laying down the mat on top of the plywood and then troweling over the mat and squishing the Ditra mat down of top. Would a bigger trowel be reguired? Ditra says to use a 1/4" V notched trowel.

Further confusion...after looking at the link above in the prior post, the picture shows latex pc mortar under the mat, then above the mat unmodified thinset, then the Ditra.
What is latex pc mortar. And how can you have these layers? Are we now saying the entire mat needs to be skimcoated in latex pc mortar. When dry, then V notch trowel unmodified thinset to lay the Ditra???

Here is a link that might be useful: Cross Section

My take on this. It is a picture and general guideline.

If your electric mat and Ditra is layed down in ONE step, latex modified thinset MUST be used. If layed down in TWO steps, you must set your electric mat onto the subfloor with latex modified thinset. When this layer is dried, you MUST set the Ditra in place using UN-MODIFIED thinset. Reasoning is that the Ditra will not allow the latex- modified thinset to properly dry/cure because it would be sandwiched between the Ditra and the first latex-modified thinset base you set your mat into and this will essentially seal it off from little to no air.

And if you are curious, with the ONE step method, the sandwich effect between the plywood and Ditra is not an issue because the plywood allows drying to take place. It is porous and has grains and cracks that allow this to occur. Reverse reasoning on the use of unmodified on the top of Ditra...the tile and Ditra makes an "airtight" sandwich.

And the electric mat MUST go under the Ditra, not on top. The tile should be as close to the Ditra as possible. Ditra will NOT impede the heating capability of the electric mat.

Maybe I can clarify a few things...or muddle the situation further.

Pete, "What is latex pc mortar", that's simply modified thinset. Thinset with an acrylic latex additive.

For everyone else...regarding heated mats over or under Ditra. Remember, Ditra is, for the most part, two things:
1) an uncoupling membrane, designed to absorb the differences between the tile and it's substrate, and
2) A waterproofing membrane.

As far as acting as an uncoupling membrane, thermal differentials "add stress" to the bond between tile and substrate. Ply and tile move differently, Ditra acts as the middleman. Putting heat under tile increases the stresses on the tile/substrate bond even more. Having Ditra between the heat and the tile allows for a better installation. Ditra has a negligible R-value, so it doesn't adversely affect the performance of the heating mats.

Waterproofing Membrane? If you want a wet room, you want the Ditra right under the tile to minimize wetting of the floor structure. Ditra also isolated the resistance mats from water. 'Nuff said.

The easiest way to tile over a mesh heating mat is to install the heating mat over the ply (staples or hot glue), then SLC over the mat. That'll give you a dead-flat surface. Then Ditra over the SLC and tile over the Ditra.

There are many credible DIYers out there who can trowel thinset both through and over mesh and get a fairly flat surface.

There are many "pro tilers" who can screw up the simplest tile installation, heating mat or not.

So to each their own. You want to try troweling thinset, have at it. If you want to try SLC, have at it.

What you do want is a void-free installation. No gaps in the thinset underneath the mat or tile.

As with most things, there are a few ways to do this. Mine is simply one more to add to the mix.

Mongo

Mongo..my floor is a bit unlevel, will SLC make another issue...will all of it will run to the low side of the room...and then I will wind up leveling the entire floor? I was not concerned with a bit of unlevelness because it was not extreme. If I dump SLC on the floor, will it run to the low point, or just fill up the mess to avoid the trowel work you talked about? Can I dump it into the electric mat mesh and use a squeegee to fill it in, or will it keep running to the low spot?

Here are your layers for both methods:

ONE STEP METHOD
1) Plywood Subfloor
2) Latex-Modified Thinset / Electric Mat
3) Ditra
4) Un-modified Thinset
5) Tile

TWO STEP METHOD
1) Plywood Subfloor
2) SLC or Latex-Modified Thinset to set/cover the Electric Mat (this dries)
3) Un-modifed Thinset
4) Ditra
5) Un-modified Thinset
6) Tile

And for those that have concrete floors...use un-modified thinset for everything.

I have never used SLC, but would like to try it some day. I always had success troweling and setting both the mat and the Ditra together. It can be a pain and you need to wrestle several things as well as walk around on the mesh that is trying to move and squirm too. But it works.

Very amateur tile setter, sort of an expert on electricity. I did my tile by using Hardibacker, then the heating mat. I definitely would put the mat on top of the Ditra unless one or more of the manufacturers involved suggests otherwise. I used Elektra mats, saved a bunch of money. Very satisfactory.

saxman,

If the floor is sloped and you don't want to level it, then don't use SLC.

One less thing to worry about.

If you want the heat on top of the Ditra, I suppose you can do so.

I trowel thinset on ditra using the flat side of the trowel, I first fill the recesses in teh area that I'm working.

I then flip the trowel and use the notched side to spread the thinset.

Unroll the mat in the thinset and embed it.

Here's your decision point...if you feel good, you can then set your tile. Realize it is more difficult to set tile in the "one step" process over the heating mats.

If you want to play it safe, then use the flat side of the trowel to level any thinset squeeze out that came through the mat. You should essentially end up with a thin coating of thinset over the mat.

Walk away and let it cure.

Come back another day and strike your grid lines and set your floor tile.

Mongo

Mongo thanks...but to clarify, I want the electric mat UNDER the Ditra, not on top. From this post, it appears the mat MUST be under the Ditra. That being said, I guess I lay the mat down on the wood subfloor, and do a skim coat on top with the flat trowel using the latex modified thinset. This will anchor down the mat. I should then let this dry. When try, I then use my V-notched trowel and set the Ditra mat using unmodified thinset. I can now set the tiles on top of the Ditra using unmodified thinset.

Good to see you want to do it the "correct" way, which is to take advantage of the waterproofing and the uncoupling characteristics of Ditra.

Attach the mat to the floor. Either staples or hot glue gun. Or whatever method the manufacturer advises. Obviously the mesh mat is what you anchor to the subfloor, NOT the resistance/heating wire. the more fastening points you use the less likely it will be for the mat to move around when you trowel on the thinset.

With the mat attached, trowel modified thinset over and through the mat to thoroughly fill the voids in the mesh. Use the flat side of the trowel. You can even use a grout float for this, the soft edge of the grout float will be less likely to damage the mat. Follow up the grout float with the hard edge of the trowel for a flat finish.

Get this as dead-flat as you can.

It's easiest to walk away and let the thinset cure.

Come back another day and use unmodified thinset and a properly sized "V" notched trowel to set the Ditra. For a waterproof surface, use strips of Kerdi or Kerdi-Band over the seams in the Ditra.

You can walk away and let this cure, or you can get right to tiling. Again, it might be easiest to let the thinset under the Ditra cure.

Strike your grid layout lines on the Ditra. You can use an ink line or a permanent sharpie marker. I usually strike lines every 2-3 tiles, depending on the size of the tiles. Roughly 3' square grids.

Use unmodified thinset to set the tile on the Ditra. I'll fill the waffles in the Ditra with the flat edge of the trowel, then comb more thinset using the notched edge of the trowel, then set the tile. Pull the first tile to see what type of coverage you're getting between the Ditra and the tile. Backbutter if required.

Work your way out of the room, grid-by-grid.

Be careful walking on the tile too early, as thinset can take a little longer to cure when using Ditra than you might have experienced in the past when working over cement board. You don't want to walk on an uncured floor and break the thinset's bond.

Mongo

OK - I think we've got it, thanks to Mongo..

Good thread.

Speaking as an electrician, I strongly suggest that electric radiant mats placed directly on plywood be only those mats that are specified BY THE MANUFACTURER as suitable for such installation. If the literature does not specify, ask the manufacturer directly.

Bus Driver...do you have a recommendation on the brand? It appears many are available with a wide range of pricing?

Found one at Costco also...Premier...? Is this a knock off of some other brand. I hate when there are too many choices, I guess everyone is jumping on the bandwagon??

Here is a link that might be useful: Premier

Mongo...and others...what brand mat is recommended...any reasons?

NuHeat, SunTouch, and warm yours are three mat heating systems that I've installed. I've also installed cable-and-clip systems, I do that fairly, often, but those won't be discussed.

The mats can offer differing amount of heat...10-15 BTUs per ft, with 15 being the max.

ST and WY are both open mesh mats. The mesh can be cut and flipped so you can effectively take a long rectangular mat and flip it to fit a non-rectangular floor.

NH does not have an open mesh, the wires are embedded in a fabric. NH does not want its mat being cut. You can use their stock sizes to fit your floor, use multiple whole mats to fit your floor, or have them make a custom sized mat for you.

All three are fairly easy to install.

Can't comment on pricing, you can find that on their websites.

They can all be installed over plywood subfloors. No fire worries.

All three companies have great customer service. They back there products up. No quibbles.

NH blanket can be easier to thinset to the floor, the fabric sticks better than the mesh. Nothing that would keep me ordering one over the other, but it is a difference worth mentioning.

Mongo

One must be careful to sort through all advice posted on forums, including this one. The claim is made in an earlier post that SunTouch may be applied directly to plywood. I cannot find any such suggestion on the SunTouch website. If I overlooked it, perhaps someone can cut and paste it here. All the illustrations show cement-based materials below the mats. And the words say it must be completely embedded in mortar. I have not personally seen any mats with differing instructions. Those who ask for my recommendations have not read my previous posts on this thread. Get SPECIFIC approval from the mat manufacturer before applying directly to plywood.

Here is a link that might be useful: Suntouch installation

So then you cannot install in under Ditra? This must be an installation using backboard then, with the mat on top of the backerboard?

Busdriver,

Look at page 6 of the SunTouch manual that your link leads to.

It shows instructions for installing over a framed floor, with the mat being installed over either backer board or plywood.

The graphic shows a plywood subfloor, then an additional layer of underlayment on top of the subfloor, with the underlayment being either ply or cement backer board. Also included is a note to use thinset between the cement backer board and the ply subfloor.

The mat can be installed directly on top of the underlayment, whether it be cement backer board or plywood.

All mats do require the mesh and wires to be "completely embedded in mortar."

To them, "embedding" means having the mat in a minimum 3/8ths inch thickness of thinset. That thickness can be between ply and tile, cement board and tile, etc.

If there is any latent confusion from reading the manuals, then calling the respective tech departments is certainly the way to go. It's what I did when I started working with these materials over 10 years ago.

Each mat has it's installation nuances, it makes sense to fully investigate each material prior to installing it.

You'll get the latest and best info through direct contact with their tech department.

Pete,

Regarding installing ST over or under Ditra, ST recommends installing it's mat on top of an isolation membrane unless the membrane manufacturer recommends otherwise. That makes sense, they want to minimize thermal resistance (R-value) between the mat and your feet.

Schluter recommends otherwise, recommending that the heating mat go below Ditra. Again, Ditra is not just an isolation membrane, it's also a waterproofing membrane that's often used in wet rooms.

I've spoken to Schluter, as an isolation membrane they told me heating mats can go above or below the Ditra. However, they prefer the heating mat go below the Ditra so Ditra can do what it's supposed to do...ease any stress on the tile bond due to thermal differentials.

In wet rooms the goal is to minimize deep wetting of the floor, you get that by having the Ditra immediately below the tile. the Ditra will also isolate the heating mat from any possibly moisture.

Realize Ditra is not Noble, and SunTouch is not NuHeat.

Every manufacturer has differing ideas depending on what their materials are and what they are primarily designed to do.

Again, if the installation manuals are not clear, their tech departments can clear up confusion. And yes, some techies are more knowledgeable than others.

Mongo

One must be careful to sort through all advice posted on forums, including this one. The claim is made in an earlier post that SunTouch may be applied directly to plywood. I cannot find any such suggestion on the SunTouch website.

I beg to differ, but when I did johnmari's bathroom, the only thing between her Suntouch system and the plywood underlayment was a thin layer of fresh thinset. I'm sure that if she'd had problems from it we would have heard about it by now, and that was almost 2 years ago.

I did look at page 6. Following the plywood illustration is the letters and numbers in parenthesis. I do not know what that reference means. Looks as if the extra layer of thin (possibly) plywood is the isolation layer, so I assume that the mat should never be placed directly on the subfloor. For my personal house, I would not place the mat on plywood- a personal choice. For the NC climate, I think a 10 watt per square foot mat is quite ample. Higher wattage gives quicker temperature rise, but also speeds thermal cycling. Other things being equal, the higher watt/density mats might increase the chance of cracks in the ceramic. If the mat has a UL listing and the instructions permit installation on plywood, it should be safe. Finally, long term installations of electrical equipment with no problems does not necessarily prove much. Many installations with code violations can work perfectly indefinitely from a technical perspective and yet present great hazards to personnel under certain conditions.

The "TCA" reference in the parenthesis refer to the Tile Council of America.

Those installations are approved by the TCA. In 2003 the TCA became the TCNA, of the Tile Council of North America.

The second layer of ply is not an "isolation layer". Why use more plywood to isolate something from plywood? The second layer of ply is underlayment. Intended to thicken the floor to help minimize deflection. You don't want to be tiling directly over 3/4" or thinner CDX. Not stiff enough.

It's a valid installation that won't burn your house down.

And it does not violate code.

I got called in as an arbiter for cracked grout in ceramic tile over a heating mat. It was due to there not being enough thinset within the mat's mesh. There were so many voids it was ridiculous. And it was over a concrete slab.

The installer lost.

Mongo

You don't want to be tiling directly over 3/4" or thinner CDX. Not stiff enough.

You don't want to be installing over a single layer at all, no matter HOW thick or thin, and for someone not VERY familiar with installing over plywood, it's not a good idea to install without either a membrane or cement board. Because of my last post, I felt I needed to add that disclaimer. I don't want to see a bunch of people DIYing their tile and have it fail because of misunderstanding something I said. :-)

I am getting confused. One post mentioned if I install on top of the plywood, there needs to be a layer of thinset under the mesh..is this the same thing as putting the mat on the plywood, and then troweling on thinset? Or do I need to put a layer on first and let that dry, and then attach the mesh with another layer of thinset? And I assume the post describing layers is OK, that I should use unmodified to attach the Ditra if latex modified was used first to put a layer over the mat (or else it will not dry correctly).

Mongo, what is a "wet room?" Is anything bathroom a wet room. Is Ditra overkill for a bathroom floor, it seems very controversal on this site when heating mats are being used. Am I safer just using backerboard, then installing the electric mat?

One post mentioned if I install on top of the plywood, there needs to be a layer of thinset under the mesh..is this the same thing as putting the mat on the plywood, and then troweling on thinset? Or do I need to put a layer on first and let that dry, and then attach the mesh with another layer of thinset?

The only system I know of that needs to have thinset under the system before you put it down over the plywood Nuheat. You need to trowel down fresh thinset, lay in the mat (which is basically the same mesh as cheesecloth with the wire running thru it), and then thinset over the top, so that the mat is basically sandwiched in the fresh thinset.

I'm not Mongo, but I'll answer anyway. A wet room is a bathroom where the entire floor is waterproofed so that no curb is needed for the shower. It's basically for handicapped showers, and the entire bathroom floor is more or less the shower pan.

Is anything bathroom a wet room.

Usually only the shower.

Is Ditra overkill for a bathroom floor

If you don't have a movement problem, or if you don't need to knoch the height down that extra 1/8" that the Ditra gives you in place of cement board, then 1/4" cement board should more than suffice for most ceramic jobs.

Yeah.

The guy who's not me? Same as what he said.

Regarding NuHeat versus the others, that's why you have to read the specific instructions for the specific product. The beauty of the internet is that you can read the installation manuals prior to buying any product.

I don't see any controversy with Ditra and heating mats. The directions that busdriver was looking for were right in the link that he posted.

If my opinion is worth anything, if Ditra is not required for the performance reasons that Bill mentioned, I prefer tiling over cement board instead of Ditra.

Mongo

if Ditra is not required for the performance reasons that Bill mentioned, I prefer tiling over cement board instead of Ditra.

Me too.

Again posting as an electrician, inspectors do inspect electrical and most codes follow the NEC, more or less. I am not aware of tile inspections which could result in failure to get a certificate of occupancy. Tile Council recommendations do not concern the inspector, but are no doubt important. In order to get a UL Listing or other acceptable listing, the manufacturer must submit both product and installation/use instructions for testing. The NEC requires that installations must comply with those instructions. And under-floor installations must be GFCI protected. The mats I have inspected look to be of superb electrical insulation quality. In the few such installations I have done, I walked directly on the mats only as necessary and only in sock feet. A piece of clean carpet padding laid on top temporarily should be good protection. I use a "Loud Mouth" alarm too. Never damaged a mat however.

Based upon these responses...I am definitely going with a cement board installation (with electric mat on top). Ditra seemed like a nice new high tech material, but I really do not have floor issues...just a basic bathroom with standard joists. I took the existing 1/2 plywood floor and added 5/8" exterior plywood to it. With the thinset layer and cement board, I am sure my installation will be beefy.

Thanks for the help...your first experience comments convinced me that Ditra is a NO GO for me.

I have installed ditra before but no mat.I am getting ready to install a 5'x 6'mat from nu-heat. The room is larger than the mat and has a t& g plywood floor.According to what I read is that the mat goes down on top of latex pc mortar (modified thinset).It also says do not put ditra down on plywood with unmodified thinset. The areas where there are no mat.So I am thinking That a skimcoat of modified thinset is needed over the whole room floor. Is that correct?