Concrete screed on wood floor

VillageIdiot

Apologies if this is a stupid quiestion.
I am making final arrangements for my 2 storey uber-extension. The upper floor will consist of plywood on timber joists. I would like to put a wet room on this floor. I was thinking of pouring a concrete screed directly onto the plywood, maybe 4 inches thick. Advantages:
1. Step up into wet room.
2. Slope to drainage trap at one end.
3. Can install underfloor heating.
4. Can incorporate traps/Ubends/pipes etc. into screed as much as possible rather than going through plywood at several points.
For finish, I am considering painting screed with resin paint. The partition around the wet room will be glass blocks and/or stud partition built up on the edge of the screed.
Is this madness? Will the concrete crack due to movement of the timber floor? Any thoughts would be appreciated.

sinnerboy

100mm concrete will over stress your timber floor strutcure

If you dont mind step , do a "step over " at the door . Use tilting fillets on the floor joists to make falls - like as with a flat roof .

Use Acrylicon -- looks like vinyl floorcovering , but it is a site wet applied system gives - jointless finish that can be "coved" to the wall , that is turn up the wall to provide a skirting . Carefully select a good floor drain . So long as you well wrap the drain pipes for sound insulation no big deal to run in floor ( dont butcher the joists , of course - think about pipe / joist directions )

Use an electrical mat form of underfloor heating ( Devi Mat is one brand name )

bertie1

The concrete will crack as the timber expands & contracts with the temperature difference.