A number of apartments situated below penthouse terraces are having stained ceiling problems. I was told that the main problem in such cases is rainwater seeping in between tiles or apertures, leaving the screed in between wet or moist. Also, this kind of problem can be caused when new tiles are glued onto old ones, blocking the water drains of apertures. I know of at least one case where a membrane was applied before tile-laying and yet the problem occurred. Can an architect or an experienced tradesman explain how the job should be done properly and what to do before laying tiles?