A simple tin roof is not likely to be much weight. What are the rough dimentions of the room, or the floor space? Also, are the walls opened up? Also, do you know which way the ceiling joists are running?
Sorry for so many questions, but don't want to address 20 different possible situations.
), if you manage to get a hold of a camera, be sure to get a couple of shots of the outside as well. Gives us a better idea of the overall structure.