I just got this house in December. I found that six of the floor joists had water damage. I am going to sister new 4-8-12 boards to the old one. Do I have to jack up the old ones first? Or, if I am adding new flooring also will the floor be legal with the new birds?
If your floor is sagging you should brace them up while adding the new joists. If possible just replace the bad ones. If not you should try using a 2x8 on either side of the existing joist like a sandwich and use screws long enough to go through the new wood, old wood and all but 1/2" of other new wood. So if your existing is a 2x and you add 2x’s on either side the screw length should be 4". Also an important step not to forego is to drill and countersink the holes prior to screwing. There is an actual pattern for sistering as well. At each end we screw a pattern of a square with four screws and one in the center and then we run screws about 3 inches apart on a zig zag pattern along the entire run.