I live in a house that was built sometime in the 20's. It's a two story home with a full basement and it is sagging in the middle. The footprint of the house is roughly 30'X 30' and the basement walls are stone and in excellent shape. The sagging is most apparent on the first floor, though it is noticeable upstairs as well. The structure seems to be very sound and it appears as though all of the framing is solid oak. At one point most of the lath and plaster walls were replaced with drywall. What I'd like to know is if there is a way to jack up the house to return it to plumb and level? And if I do jack it up will it crumble the drywall that was applied while the house was out of kilter? Or would it be best to leave it as it is and just replace and level the subfloor prior to doing any redecorating? Thank you.