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
Here is a drawing that kind of shows the roof shape 
Best Answer
Although I have done what you are asking several times, I had an engineer design and stamp plans, making a structural change this big is a big deal, actually the first one was a repair for a Bone head that just cut them out, the roof sagged walls bulged he could not sell and the bank shorted it, after repairing and turning this property I did several more on the side that had low ceilings.
All those places were similar, Recently helped a friend with a 2 story same issues but he did not have the $ for the engineering stamp. Good thing because after we repaired it it survived a huge snow storm that caused my barn roof to collapse. But again someone tried adding height with knowledge or experience and that’s why my friend hit a killer deal as I have in the past. So don’t change things without structural engineering review when they can cause massive problems including roof failure.