We recently purchased our first home; a small in-town colonial built in the mid 1930's. The home has had a ouple of additions since first constructed but my problem is with the original structure. The floor joist have sagged in the center of the house. It appears that one section the joist have actually sagged because the nails that attach them to the stair header (which is a single 2x8) have bent downward considerably. A couple of other joist seem to have split some causing them to sag between the span. Obviously this has happened over the past 70 years and not rapidly so all of the floor boards (rough cut pine) have bent to conform to the sagging.

So finally the question, can I safely jack these joists and add lally columns and a steel stringer to carry the load?

Should I do this slowly over time or can it be done over a few days (some of the sagging is an inch or more)?

Should I just add additional support and live with what I have?

I really need advice soon, I want to make things right before they get worse.