I am about to embark on tiling in a down-stairs shower room/toilet. I am getting conflicting advice from a couple of tradesmen (one plumber, one electrician) with regard to my preparation of the walls in the shower area. The walls are solid plastered block. The previous tiles were stuck directly onto the plaster. No problems. (They were a devil to get off. Took a lot of the surface skim of the plaster off which I have repaired). The question is with regard to how to prepare the walls for tiling. Can I tile directly onto the plastered walls as before or should I add some sort of waterproof membrane or backing board? I am rather should of space so I am not keen on the latter. Grateful for any advice anyone has to offer....