First time poster on an excellent forum.
The project is a small 3.5m2 ensuite wet room. It's ground floor with concrete floor. I decided on a pre formed tray as I didn't want to worry about achieving the correct fall. The tray is fitted with the remainder of the floor levelled with outdoor ply. Room has been tanked floor to ceiling with a Mapei kit. I'm fitting a varme ufh kit which up until now has went fine. Fitted the thermostat on the bedroom wall just outside the wet room, the floor probe was recessed into the ply inside the supplied conduit and I've all the wiring up to the point of needing an electrician to finish it off.
My questions relate to the fixing of the cable mat and then tiling over the top with mosaic tile sheets. I've seen replies on other posts saying not to put the mat in the tray as it messes up the preformed falls but I wanted it heated for the room heating and to dry the floor after use. Should I avoid slc and just try a light bed of adhesive just to the height of the cable then when ready to tile just be careful with the bed to fix the tiles so that I don't damage the wiring?
Some suggest sticking the cable mat with it's double sided tape back then tile directly. I was worried that the area of the cable and it's backing are areas were tile adhesive is prevented from touching the floor and so would be a weak point. Would that be true?
Lastly the end of the cable that has been factory sealed is quite thick compared with the rest of the mat and I was concerned with having it completely covered. It ends on top of the preformed tray. If it was over the ply I would slightly recess it to make it level with the mat.
Any help appreciated.

Hi and welcome to the forum.

Have you considered a cable system instead of a mat? they take a bit longer but are more practical in my opinion. Fix them with a glue gun, it takes seconds to dry. The end of the cable is larger because its the return loop, I usually sink these into the thermal boards.
As for fixing to the tray, you wont feel any benefit as the hot water will negate the heat.
Are you using thermal boards? if not then you should consider them as they lower running costs and allow the installation to heat up very quickly.
Covering the cable with a flexible SLC is a good idea both in terms of protection and heating properties. Use something like Mapei fibrebond.

Thanks for the reply. I've already purchased the mat, I liked the idea of it being ready to roll out. I'm not using thermal boards as didn't want the floor raised up any further. Because of drainage the floor already has been raised with the former and ply. I thought the former and the ply would provide some insulation from the concrete floor.

The heated floor was not really to keep it warm whilst you shower it was with the idea that it will heat the room (radiator was removed) and to dry the floor.

The return loop is now over the top of the former and I didn't think it was a good idea to start cutting into this.

Would the slc not ruin my fall on the former?

Thanks for the reply. I've already purchased the mat, I liked the idea of it being ready to roll out. I'm not using thermal boards as didn't want the floor raised up any further. Because of drainage the floor already has been raised with the former and ply. I thought the former and the ply would provide some insulation from the concrete floor.

The heated floor was not really to keep it warm whilst you shower it was with the idea that it will heat the room (radiator was removed) and to dry the floor.

The return loop is now over the top of the former and I didn't think it was a good idea to start cutting into this.

Would the slc not ruin my fall on the former?

The SLC is there to protect the wires, putting it on a tray former kinda begs the question of 'why buy a former?'

As per Diamond - I'd have considered using a thermal board instead of ply on top of the concrete base. Fix with a fast set flexible adhesive onto your former base and screed over with the same to avoid slc slump into the outlet!
Which UFH system have you chosen as some are just tile warmers rather than primary source heat providers.

Diamond I wasn't going to use slc over the former I was just questioning the suggestion you made in your earlier post.
The reason I used the ply was that two sheets brought it to the exact depth needed to make it level with the former. I could still replace the ply with some thermal board but this area is quite small as there is a sink and toilet reducing the heated area. So don't really think it would be worth lifting the ply. It's a varme cable mat 2 metre square giving an output of 300 watt. A tile centre suggested going for a 100w per square metre mat based on the under floor but I went for the higher 150w.

I am with diamondtiling & timeless john on this.
We always say 100w/m2 systems on ply - chipboard etc, 150-160w/m2 systems on concrete or insulation boards.
If it was a loose cable system, the output per square metre is by changing the spacing betweem the cables as you install the system.
Wider on ply - chipboard etc and closer on concrete or insulation boards.