I have a sagging ceiling on a 20' open run through the middle of my living room. It is also causing the roof to sag. The piers and foundation are fine. My question is would jacking the ceiling back up to level and build an 8' wide wall under the beam be able to fix this? I don't want to spend the time or money to repace the beam and would like to break up this huge open space anyway. Any help would be appreciated.
A wall perpendicular to the sagging beam would reduce the load by 1/2 and probably fix the problem. Instead of a full wall, you could install a beam instead and box (wood or drywall) it for a nice look. You just have to be sure your footings can support the weight load of the two sides of the posts.
And remember, if you start jacking the sag up, there is an equal and opposite action in the opposite direction. You will need support under the wall you create or support for a beam like cmv recommended.
OldDog is right . . . screw jack the new support beam slowly in multiple locations along the run to allow the framing time to adjust elsewhere in the house. Make sure the foundation floor under your new support posts is sufficient for the new weight load. There is a formula for determining the size and carrying capacity of support beams depending the on span . . . follow it in sizing the new beam and posts.
This is NOT a difficult fix but you have to be systematic and careful.
Realign the ceiling first. The roof may lift at the same time. Some people recommend a maximum of 1/4" screw jack per day. Others are more aggressive. Slower is probably better than faster because you want to give the framing time to adjust slowly.
Once you have the ceiling done, re-check the rook and look for progress there too!