Mysterious ceiling leak?

Hi,

I have a leaking ceiling and it has me totally baffled, maybe someone else could help me out. I have a drop ceiling in the first floor living room that leaks when it rains. I have removed the drop tiles and there are some old 1' by 1' tiles above the drop ceiling. In the leaking area most of these tiles are gone (removed by previous owner?) and above those are thin strips of wood closely spaced with plaster between (like the old plaster walls). The water seems to be coming from the plaster, the problem is that there are no leaks in any of the rooms on the second floor and all plumbing is in the basement and first floor so there are no pipes above the leaking area. We had a new roof installed this spring because of previous leaks in the upstairs bedrooms. Could this new leak still be related to the roofing problem? If so why aren't we seeing any leaks upstairs? Please help!