Bathroom underflooring.

Typical way - rip out floorboards, fit boards between joists, put pipes in thin layer of dry mix, 18mm wood on top, tiles.

What I'm wondering is a slightly different way.

The current floor is floorboards over joists, which are over dwarf walls of brick.

All at right angles of course.

I'm wondering about another way - rip out floorboards and joists, put a thick layer of rockwool down over a 100mm air gap between it and the ground, then a rough platform made of the floorboards and a tarp to hold up 100mm of concrete between the dwarf walls before it sets.

It seems to me that 10cm of decent concrete should be just fine to span the 1.6m or so between the dwarf walls.

Is this incorrect?

If it is possible, this would seem to have some advantages - mostly that you get decent thermal conduction between the pipes and the tiles, rather than a layer of rather good insulator between them, reducing the loop temperature.

Halfway there.

The first thi8ng is teh suspended concrete is possible, using block and beam, but of your soli is stable, the better way is this

first renmove EVERYTHING including the little walls. Smash them up to provide a hardcore base, and lay a concrete floor to about 6" below finished level Smooth that down reasonbly well. # Next use a DPM ..lay that UP the walls till its above the DPC.

Cover that with 60mm of dense polystyrene - blue or pink foam is the one for flooring.

Then lay down a piece of reinforcing mesh and lace your spiral water pipe to that. Tie wraps do the job. Its very cheap.

Pessurise the pipe to 4-5 bar t ocheck for leaks with a hired water pump, and leave the pressure ON while...

..you lay screed to fished floor level..at lest 75mm deep screed. Get this as even as you cann.

Then tile it.

If you have unstable soil replace joists and so on with a concrete block and beam floor.

Then finish off with DPM polystyrene and screed as above.