What tradesman do I need for damp appearing near the ceiling?

I have recently moved into a top floor flat of a terraced house. The flat has a flat roof due to a roof terrace being constructed there. When I moved in no damp was detected by the surveyor, however over the past month, a large patch of damp has formed where the ceiling meets the wall. I was wondering what kind of tradesman would be needed to assess what to do in this situation? I don't know anything about this issue but I had assumed that there would be a waterproof layer between the roof and roof terrace, therefore preventing rain water getting in. I did get a tradesman in and was told that this problem could not be resolved as every time it rains the damp would return. Is this true?