I don't know why all of a sudden this is giving me a headache.

Our ensuite (about 4m x 1.4m) is 22mm Egger Protect boards over posijoists, I'd say pretty solid, with a fibreglass shower tray former flush level to the Egger boards.

We want low wattage electric UFH matting (100w), and then above that will be 12mm marble tiles (600x300 format). It's a small area (7sqm) so cost of materials isn't hugely important.

I can see 3 options here:

1) Tile straight onto the Egger Protect - UFH mat goes down, thin layer of self-leveling to smooth over the mat, then tile on top? Seems a bit prone to cracking especially at threshold to shower former, but equally I've tiled bathrooms years ago with nothing more than 6mm ply on more flexible floors and didn't get any cracking. Could run the UFH mat into the shower here as all flush.

ETA - could use somehting like the fancy BAL AF Max - apparently does a decent job of decoupling by itself? Would just put a bit of membrane on the join to shower former and tank that to the walls.

2) Decoupling mat - used this downstairs in lots of places, happy with it. My question is do I stick the mat down first, level over it, then decouple and tile, or do I decouple, then level, then mat, then tile?

3) 6mm insulated cement boards - in this case I would just screw them down, then UFH mat, level and tile. Will have to manage the transition to the shower tray, and won't be able to run the UFH mat into the shower.

Thoughts? Am I overthinking it and any of the above would work? I did go down a rabbit hole of googling and it seems pretty much every option and scenario is recommended/not recommended by someone.

thanks!

Edited 1 hour ago1 hr by SuperPav

No 3 is what I done. You could use 4mm jacko instead of 6mm to reduce the 'step' to the former when you come to lay mat/tile. Re #1 are there even any addy products approved onto chipboard? Re #2 ditra peel n stick can go on protect but principle of decoupling is somewhat lost in this case i.e. doesn't prevent vertical deflection which is what most people are trying to prevent.