I live in the top floor of an apartment (built in 1986). I notice damp coming in from outside in the corners of the walls touching the outside. The roof looks in perfect shape. What is the likely cause of the damp, (the problem started to happen about a year ago). Would such a problem normally require a roofing specialist to remedy it. What aspect of the roof would most likely need repairing, and how much effort would it normally take ?