We live in a 1950's semi detached house and have not had a damp problem until recently. I noticed a a damp mark on the ceiling and top of the wall in the corner of my bedroom. It is the front wall close to where my house adjoins my neighbour's house, it is not close to a window. I had some books on shelves on that wall, I have removed everything and allowed the wall to dry out, there is now nothing obstructing the air flow in that area, but it is clear there is still damp coming from somewhere to the top of the wall/ceiling edge. There is nothing in the loft that could be causing it, so I assume it must be the roof. We had the roof checked and repaired about a year ago. Today I thought I could hear a bird walking directly above my ceiling, so clearly it must have got in somehow. Where do I start, should I get the roofer back or do I need someone to check the brickwork first?
It is most likely that you have a hole in the roof.
Go into the loft and check the rafters for running water marks and staining.
Lay on the floor (if you have one) or get right into the area with a torch or preferably a bright lead lamp and take a look. Move any insulation for a good view.
Think about it, is the roof steep or shallow, can the wind blow the rain up the roof under the roof covering, water can travel long distances before showing itself, does it always happen or only when the wind blows from a particular direction?