I've been wondering if I can open this ceiling up for a tall room. I've been reading that the ceiling joists are often also rafter ties so I went up there today to take a look. I can see a few are tied to the knee wall and have an angle brace, but a majority seem like they just span the house, but aren't really tied to anything. I would like some opinions on this. The plan is to cut them out and replace them with steel rods and turnbuckles where needed to keep things tied together.
The roof is best described as a mansard with a flat top. I've tried to show that here:
This is the only point load I see. It actually doesn't rest on a joist, it rests on that 3/4 board that is spread across the joists so I can't imagine it provides a ton of weight bearing