Old shower was silicone sealed to the wall, and behind it, there's a gap in the tiles and just like a square of wood. New shower says not to seal, so I guess I need to tile over the bare wood, but surely if the unit isn't sealed to the wall, there will be two gaping holes where the electricity supply and water supply come through? Or do these need sealing round? I just don't get how a shower unit can sit on a wall and not allow water down the back?