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Unread 06-30-2020, 11:41 AM   #1
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Thinset, cemennt board, or SLC for Suntouch heating mat?

Weekend warrior here. I have spent many hours reading the posts here, but I am still not clear on the best way to proceed with my project.

I have a small (5ft X 8.5ft) bathroom over a crawlspace. Toilet seal leaked because the previous homeowner did not raise the toilet flange when they tiled. I gutted the bathroom down to the studs, cut out all of the mushy wet OSB and replaced it with 3/4" plywood. Floor is level to about 3/16 in both directions. Joists are 2X10, 24" OC, 12 ft span. Deflecto says "L / 372". I am planning to use ceramic tile.

I want to put radiant floor heat in a 2.5ft X 6ft strip down the middle of the bathroom, with 12"X12" ceramic tile over that. I am confused about the best underlayment to use with the heating mat.

Suntouch says that you can glue the mat directly to the plywood, put down thinset and tile all in one step. I have read enough to think that this is a bad idea, especially for a weekend warrior with only a few small tile jobs under their belt.

I like the idea of putting down the mat, then pouring SLC over it. This is the method that I am leaning towards, but I am confused about whether or not I need lath, and whether the lath should be metal or plastic.

Specific questions:

1. Do I need lath under (or over?) the heating mat if I use the orange mats with the wire attached to the orange mesh?

2. Do I need lath on the areas where I am not putting heating mat?

3. Is metal lath OK, or is there more risk that it could short to the heating wires? Plastic mat seems like the safer bet, but I could not find plastic lath at my big box stores. Only 2.5 galvanized steel. Does anyone have a part # that I could order from HD or Lowes?

4. I still have a few inches of OSB around the perimeter of the floor. Joints are on joists where possible and I added 2X6 blocking to the butt joints where the ply meets the OSB. The floor feels solid. Do I need to paint redgaurd on the OSB before I prime for the SLC?

5. Since I have a small bathroom, should I pour SLC under the new tub as well, or build a dam where the tile will butt up against the tub? Seems easier to me to just pour SLC on the whole floor and give myself a perfectly flat area to set the tub. Just wasn't sure if there is any disadvantage to this.

6. Is there another method that is better for a beginner? Such as 1/4" cement board, gluing the mat to that, then covering the mat with thinset, letting it dry, then tiling.

Thanks for your help.
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Unread 06-30-2020, 01:19 PM   #2
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After more reading, now I am leaning towards the Suntouch Heatmatrix with the WarmWire.

So my floor would be:

Ceramic Tile
3/4" plywood / OSB

I am thinking that this will be easier to install than the SLC method. Is that true?

I am OK with the flatness of the floor. It isn't perfect, but the tile I picked out has tapered edges so I can accommodate a little bit of lippage.

As always, opinions and advice are greatly appreciated.
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Unread 06-30-2020, 02:07 PM   #3
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Welcome, David.

I would not recommend you tile over a single layer plywood subfloor over 24" joist centers regardless the tiling substrate you elect to use.

The use of SLC or thinset mortar to fill over the heating system will be dictated by the manufacturer of the heating system and the SLC or thinset mortar.

The use of SLC over the plywood will be dictated by the manufacturer of the SLC.

The tile industry requirement for substrate flatness will depend upon the size tiles you intend to use, which we don't know.

My opinion; worth price charged.

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Unread 06-30-2020, 04:01 PM   #4
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FWIW, most SLC products are MUCH easier to get flat when they are thicker, and some of them require a minimum thickness above the highest point when used over a plywood substrate. Many of them won't like the 24" OC joists, though.

Schluter's Ditra Heat achieves a light rating in the Robinson Floor Test over 24" OC joists, and then, all you need is the proper thinset.
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