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Unread 02-26-2020, 12:45 PM   #1
John D. Bell
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correct mistake (electric floor heating, thinset)

Hello, all! I'm hoping you can help me correct a newbie mistake.

I am working in a small bathroom (approx. 8" x 6"), with a complete floor tearout to the studs. First down was plywood, construction adhered and screwed to the joists. I then installed electric floor heating tape ("EasyHeat WarmTiles" system), which covers about 2/3rds of the area of the floor (i.e., not under the vanity, tub, nor toilet). The unheated area was covered with cement board, and narrow cement board strips were put around the rest of the perimeter of the room (so the fill material could be screeded off more easily).

Here's the mistake - to fill the area with the heat tape, I used thinset mortar approximately 1/2" thick, all in one application, instead of self-leveling concrete or something else. It was a Cast-Iron B***CH to spread and level it correctly, as it was way too sticky for this purpose. But we "got 'er done". (Hearty thanks to my friend Kurt!) I had intended to put a pebble mosaic tile over it with more thinset.

10 days later, and the thinset has multiple very narrow cracks in it. Having done my research (too little, too late) I figure the thinset shrunk as it set, having been applied in too thick a layer all at once. (As well as being the less-than-best material in the first place.)

So. How bad a shape am I in? My friend found a "Waterproofing Crack Isolation Membrane" (actually a paint-on fiber-filled sealant) made by the same manufacturer as the thinset, which is supposed to be able to cure this kind of problem. Would this be good enough?
Or should I tear it out (with loss of the heating tape system)? If I do tear out, can I somehow clear everything off the subfloor and reuse it, or do I have to go back to the joists again?

Thanks in advance for all your advice. (Moderators, if this post belongs in an existing thread, please move it appropriately, or delete it and point me to the existing answers.)
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Unread 02-26-2020, 12:47 PM   #2
speed51133
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what make and model thinset mortar did you use? honestly, doesn't sound like a good scenario.
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Unread 02-26-2020, 12:59 PM   #3
John D. Bell
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Mike -

"TEC® White Sturdi Flex Thin Set Mortar" (You can find the datasheet at Menards.com - I'm not allowed to post links yet ).

The sealant is "TEC® HydraFlex™ Waterproofing Crack Isolation Membrane" .

Hope this helps. Yeah, I wasn't very optimistic about this, but I thought if there was any valid "shortcut", the people here would be able to figure it out.
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Unread 02-26-2020, 01:55 PM   #4
speed51133
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I think anything short of a tear out is a big risk. the cracking of the mortar there will only get worse and will want to telegraph into the tile. If the mortar was more like 1/4in thick I think you'd be ok. I really don't see a paint on membrane fixing this.

MAYBE something like Ditra would work??? It would raise your floor like 1/4in...

see what others say.
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Unread 02-26-2020, 02:55 PM   #5
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Welcome, John.

While I don't much care for the heating systems being "filled" with thinset mortar, that's the manufacturers' recommendation. If you had used some 1/4" CBU as your edging instead of 1/2" you might have avoided the problem, but there's no guarantee of that. While there are thinset mortars whose manufacturers advertise them for use at your 1/2" thickness, the TEC mortar you used is not among them. And I don't particularly like the idea of using the CBU edging at all, fearing that it could encourage cracking at the mortar/CBU junctions.

I don't expect your shrinkage cracking to develop any further, but there's no guarantee of that, either.

And while I always recommend that a membrane of some kind (crack isolation or uncoupling) be used over any floor heating system, I don't know that that's gonna work over what you've got, either. A sheet-type membrane is more likely to work, but certainly doesn't guarantee a good result.

Without knowing your floor structure, it's difficult to know whether the installation was even suitable to begin with, but that's a different problem. And I doubt that your bathroom floor was only "8" x 6"" to begin with.

My opinion; worth price charged.
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Unread 02-26-2020, 03:05 PM   #6
John D. Bell
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> Without knowing your floor structure, it's difficult to know whether the installation was even suitable to begin with, but that's a different problem.

3/4" plywood subflooring (only one seam, not in the heat tape area) with screws and construction adhesive to the 2 x 8 joists on 16" (inch) centers. We had to double some of the joists, as they had been previously hacked badly for the old drain plumbing, so 16" is the maximum spacing. The joists run across the narrow dimension of the room (see below).

> And I doubt that your bathroom floor was only "8" x 6"" to begin with.

Yeah, stupid typo on my part - think 8' x 6' (feet, not inches)

EDITED TO ADD:

> While I don't much care for the heating systems being "filled" with thinset mortar, that's the manufacturers' recommendation.

All the manual called for was "cement-based mortar". Details unspecified. I was the one who assumed that it had to be thinset.
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Unread 02-26-2020, 03:25 PM   #7
speed51133
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i know heating cable is not cheap....

are you familiar with Schluter Ditra? Can you raise the floor another .25in without a problem?

I think it would be your best bet short of a tear out.

https://www.youtube.com/watch?v=qa_l7U7zh0w


BTW, the instructions for your cables state the following:
Once the mats have been fitted to the area in which they are being installed, they can be affixed to the floor with the adhesive side
of the mesh. The mats are then covered with a uniform scratch coat so that it just covers the heating cables. For ease of tiling, the
scratch coat is applied evenly over the entire floor area. When the scratch coat is dry, the tiles can be set in the usual manner. This
method will result in a floor height increase of about 5/16” higher than without any floor warming system installed.


they go on to say:

The heating cable must be completely embedded in cementituous mortar to prevent overheating.
Self-leveling cement based compounds may be used that are appropriate for this product, but
consult with the manufacturer in advance.
1. Take care not to damage the heating cable. Use caution when setting heavy equipment or materials (such as pails of mortar) on
the mat.
2. The adhesive on the bottom side of the mesh should affix it securely to the sub-floor after being laid-out. Ensure there are no
sections that have popped-up, since this could interfere with the scratch coat process. If there are raised sections or pop-ups, they
can be smoothed out by pressing the mesh against the floor until it sticks properly. If this does not work, you may also staple or
hot-glue the mesh section to the sub-floor. DO NOT STAPLE THE HEATING CABLE! (Figures 4g, 4h).
3. Cover the complete mat with a uniform “scratch coat” of mortar that just covers the heating cables. The scratch coat should be
level and extend over the entire floor area, including areas not being warmed.
4. Once the scratch coat is dry, set the tiles in the usual manner.
*Self-leveling underlayments may lift the mesh off the floor during application. The mesh should be stapled/glued to the floor at
approximately 6” (15.24 cm) intervals to prevent the mat from floating above the finish level.
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Unread 02-26-2020, 03:58 PM   #8
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Personally, I would tear out and go back with Ditra heat and new cable. Looking at it from the perspective of a H.O. calling me to finish the job, and guarantee it.
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Unread 02-27-2020, 07:04 AM   #9
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The 1/2" mortar layer has likely shrunk down to less than 1/2", making it below the 1/2" CBU strips that were installed thickness guides, so you'd need to apply yet another layer of mortar to raise it. Then there is the concern of it cracking at the CBU to mortar joints as noted above. Adding a crack isolation layer will also add some degree of insulation (but admittedly not much) between the tile and the mortar with embedded heat wire, making the tile slower to reach its set temp.

You mentioned you installed 3/4" plywood for the subfloor, with 1 seam. Is the plywood tongue and groove? If not, did you install blocking under that seam? You also mentioned the 2X8 joists, some having been doubled, but I don't think I saw what the unsupported span of those joists are.
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