I know you are supposed to use self levelling, however I’ve put down the 50mm insulation and found I have little clearance to play with, 9mm would bring it level with the shower tray – I was looking for a flush finish. I’m thinking cable, SFL and adhesive will take more than 9mm so can I just tile straight onto cabling if I’m super careful?

Other option is all the insulation back out and hunt down some 40mm stuff. Im using marmox which seems to be online order only – what stuff do the diy sheds stock in that size that’s ok to use? Google is not helping much

You can. Just be sure to use flexible adhesive that’s designed to cope with the heat changes.

Plan to use mapei mapaker rapid set flex which says suitable for ufh,

Other than risk to damaging cables is there any other reason to use self levelling on top of insulation boards (which are by nature flat)

OP, not sure how much work involved with replacing the insulation but I wish I’d used self leveller when I did our bathroom floor.
Decided against it as it would have been waiting another day before laying the slates but…was a complete ball ache working the flexi adhesive around the cable with a small plastic spreader so as not to catch the cable sheathing with the usual metal spreader.
Would have saved loads of time working with a level surface and that was only a very small floor area.

Use floor leveller lol

I tile direct to UFH matting quite a lot, It’s not too bad if there’s not much messing around and you’re a confident tiler.

The problem isn’t with snagging the cables when spreading, it’s trying to lift a tile to adjust the bed depth, it tends to lift the mat in the unset adhesive. I use the smooth side of the trowel to apply adhesive over the wires and then used the notched side on top of that. If a large format tile you need to back butter the tile with the smooth side also. I have probably done over 500 installs like this and NEVER damaged a wire.

i always self level over it, stuff i use can be tiled over in a couple of hours although i usually plan it so i level it at the end of a day and tile next morning.
self levelling also means there is no wires sat in a void under the tiles which can cause the wire to overheat, not that there should be any voids but from the amount of jobs i’ve had to go and sort out a lot of tilers seem to struggle with this.
only downside of self levelling it is you need to make sure the UFH is stuck down properly or it can float up in the self levelling.

I’ll agree that levelling compound over is the ideal answer, but not always possible in the time frame offered, although I tend to carry Rapid Level 30 for this as it goes off in 40 mins so you can crack on.

Thanks for comments. It is only a small area, approx 2.5 metres pretty much flat due to use insulation boards and will be using 600×300 tiles. I have a plastic spreader and trowel so don’t mind taking my time to get it right. I as planning on trying out a mustang levelling system to pull up the tiles will be a bit of an experiment as not used it before but it looks like it should work well. Probably overkill for the size of the area I’m tiling

How thick would you estimate self levelling on top of the wiring would be? If tile adhesive is roughly 2.5mm for a 10mm trowel (I think) that gives me 6.5mm…

Pattern wise trying to decide how to lay, they are the same size but a contrasting colour to the walls laid in a grid. Would offset look weird? I’m thinking not all grout lines will line up perfect if I try to replicate the grid on the floor, as you I’ll have 2 tiles to every 1 length wise, so the grout lines will mean the later lines will be slightly offset

And as experienced tilers, do you lay out and cut all the tiles before adhesive, or as you go? I’ve been doing it as I go to get it as close as I can but means I’m only mixing small amounts of adhesive at a time (5kg) giving me a lot of cleaning of tools and buckets between batches.

And another question, I’ve left small expansion joints, obviously in wet areas you use silicone, for the rest should you use caulk? Our other bathroom was grouted (not by me) and after 4 years it has cracks down all the corners and where the tiles meet the ceiling. Looks a mess

i silicone all corners and edges where tiles meet anything else, allows movement and as you’ve found in your other bathroom grout cracks. silicone is available to match grout colours and in most cases is a pretty good match.
around the ceiling i grout but then get the decorater to run a bit of caulk around before painting.
it depends on the job but normally i do the cuts as i go but unless its a small area that i want to be able to grout quickly i use semi rapid setting adhesive to give more time and less washing out.
i try and line up grout joints with same size tiles and you quite often find the tiles aren’t exactly 600 x 300 so the 2 tiles with a spacer matches up with the 1 tile. even if it is slightly out your area wont have many joints and so the differance will be minimal and possibly not noticable once things have been fitted to the bathroom.
6.5mm should be enough, you only need to just cover the wire but unless your floor is very flat after levelling you may end up with more than 2.5mm adhesive and don’t forget you’ll be back buttering the tiles so that will add a little.

The levelling compound just needs to cover the wires so take the thickness of the wire and add another mm or so.

Ditra heat is your solution.

I’d echo Bigfoots comments – If a small cut heavy room, then I’d cut everything before laying but I use Rapidset. If using std adhesive, you have a couple of hours open time on your adhesive so you can cut as you go, which allows you to see the cut before and assess the correct cut for the next tile.

If tight for space, I’d just use the level compound to encase the wires, no more. I’d put in in a little at a time and use a flat tile run over the wires to just fill to the top of the wires, once it’s gone off, you can always decide to add another layer or not. The wires tend to float up in compound whatever, so your floor tends too be thicker than you’d like most of the time. You’ll end up using a thicker bed than you think with the adhesive, for 600×300 I start with a 7mm square notch trowel usually.

Def silicone all corners to allow for movement, except up to anything you want to paint!