Leveling floor due to sagging joists?
I am wondering about my options for leveling an attic floor in my 1.5 story house from 1874. My husband and I live in the foothills of Colorado at 7500 feet elevation. The foundation is in fine shape (thanks to being owned by Masons over the years) which includes a good-sized basement, but placing additional supports underneath the attic is not an option as it is situated above the main living areas of the house. We plan on replacing the current flooring and so have no problem ripping that out to access the joists.
Our home inspector guessed that the sagging has simply occurred over time due to the large distance the joists are spanning, which is ~25 feet. The joists are original 2x8's and in good shape. The sagging occurs gradually across the length of the room (28 feet) at no more than 2.5 inches at its maximum. However, it dips twice since the floor rises back up where the chimney is situated in the middle then dips again and rises again on the far side of the house.
Our goal is to gradually change the attic into both a full bathroom and another bedroom. I have had a general contractor take an initial look, and he did not seem to see anything that would prevent such a remodel. I'm assuming we should start with the floor and would like to do as much of the work ourselves as is sensible. The general contractor plans to use some combination of architect and/or engineer.
I'm I in way over my head here? Is leveling in this situation even possible? Or should I get used to the sloping floors and learn to live with them?
(I don't know if it matters, but someone noted with interest that the floor joists on the 1st floor run perpendicular to the joists on the 2nd floor. I have no idea if this is normal.)