What can i use as a filler/skim coat over my underfloor heating

If using the Devi system this should come on a mesh so there will be no need to cover with leveling compound as it is easy to cover with the Adhesive if you take a look at one of my images you will see that i am tiling over the mesh,Always do the walls first,and the floor centre but i always work for a whole tile in the doorway,Fast flex is a lot more exspensive than a normal one part flexable cement based adhesive i always use a cement based,I have never had problems.

From your question i would advise that you hire a professional to do your tiling

The best method to cover your underfloor heating is to use a 2 part latex self leveling compound and then use a flexable tile adhesive. The self leveler will give you much more easy to tile surface and protect the cables from your screeding trowel. In genral i start from the middle of the wall out and work out what leaves me the largest cut at each end. I tend to do my wall first so i dont get adhesive all over the new fllor tiles. I hope this helps

you can use a flexible levelling compound over the underfloor heating or you can use a Flexible adhesive but using Levelling compound makes it easier as this covers the heating mat or wire. I cant advise what products to use as I dont know what type of floor or if insulation boards have are used. But You will get good advice from Tilemagic on what to use. as for setting out the tiles its always best to lay tiles on floor first without adhesive to find the best layout. But as a rule of thumb if tiles are being laid square with the room is to find the centre line for both width and breadth of the room use a chalk line to mark the floor. start laying the tiles from the centre out towards the walls this way you get an even cut around the edges and same size cuts in the corners Providing room is square. Depending on how the tiles work out I cant say if you should lay the tiles centre of tile along centre line or edge of tile to centre line, You will have to see what gives the best cut at the edge. You should not start tiles against a wall probably wont be square

the norm in building is to use normal sand and cement scread, hopfully the underfloor heating is a plastic coated metalic pipe??? cement eats copper. scread is a 6 sand to 1 cement dry mix and is left for few days to set, the important thing with underfloor heating is to heat only 3 degrees per day and no more than about 41degrees anyway safely, what i do when seting out for tiles is to lay them on the floor and see whats going to be easiestto work to, for walls i use a 2x1 and transfer the length and width with spacers and mark on the wood the tile ends, simply move the wood over the wall to suit the cuts and mark the centre of where you want to start, use the same piece of wood as your straight edge to tile off, hope this helps