We recently bought an older home (built in 1911) and like any home this old it needed some TLC. We updated some light fixtures in the bedrooms and in the process we noticed in the largest bedroom that the ceiling is sagging in the middle. A look in the attic reveals the joists are 2" by 6" beams spanning a distance of 16 feet. I would think a span of 16 feet for a 2 x 6 would be quite a stretch. The ceiling and walls are plaster and lathe, what would be the best way to approach this issue?