We had a varnished wooden floor in the bathroom that was put in to this thirties house when they were done over in the 70s. This wooden floor was left over from when the house was two separate ones, and the wooden floor was the stairs landing before. This was obviously no good for the new bathroom we wanted to put in, so we scouted around, and found that a "ragréage de chappe liquide" was what would suit us, ie a liquid cement and sand mix which would flow over the wood, with polystyrene bubbles in it for flexibility. This flowed over the wood perfectly ; over this we laid quite large grouted square tiling. This was four years ago ; apart from the middle tile in the doorway, which has come loose - I have to relay it and put a triangular wooden end piece along the whole of the tile ends in the doorway - nothing else has moved or cracked, so it must be ok :)
I agree :-)
Wood is a material that is constantly changing according to ambient conditions. It is also subject to rot, fungal and insect attack. I would feel very uncomfortable about covering it with any impermeable layer, though I have seen places where it has been done. Apart from concerns about putting an inflexible covering on top of a material that moves, I would worry about not being able to inspect the wood for signs of deterioration that could result in sudden failure.
But perhaps I am a bit old-fashioned? What do others think?
I tend to use Wedi board much more these days as it's excellent insulation, light to carry and has a great tile on surface.
I also like fermacell compared to placo but not actually used that system.
Tiling on wooden floors is one thing but on a wooden floor with heating maybe too far without something like fermacell or Wedi as the movement (expansion, contraction) can break the tile bond. I have found they stay stuck for several years but begin to let go eventually with the grout being responsible for keeping them in place.