The material doesn’t really matter there - that’s what the waterproofing membrane is for. My bathroom has a plywood subfloor with a 5 cm concrete slab cast on top, floor heating cables embedded inside it. Then there’s a waterproofing membrane painted over that, followed by the tiles. There’s even this rubber “funnel” over the floor drain that’s fully embedded in the waterproofing layer, making it virtually impossible for water to go anywhere but down the drain.
The material doesn’t really matter there - that’s what the waterproofing membrane is for. My bathroom has a plywood subfloor with a 5 cm concrete slab cast on top, floor heating cables embedded inside it. Then there’s a waterproofing membrane painted over that, followed by the tiles. There’s even this rubber “funnel” over the floor drain that’s fully embedded in the waterproofing layer, making it virtually impossible for water to go anywhere but down the drain.