I have a home I purchased roughly 18 months ago. The house is a traditional 2 story cape code (3BR 1BA) and is structurally very sound and extremely stable.
However, on the primary floor of the house, near where the flu/chimney rises from the basement, adjacent to the bathroom, the floor sags roughly 1/4" to 1/3" over a span of about 12'. From inspecting the floor joists below, trim work and hardwood floors, it appears it has been this way for a very long time.
My initial thought is that this could be corrected over an extended period of time by slowing applying pressure to the affected area from below. It's a relatively minor deviation in the floor, but it is noticeable.
Would it be advisable to true up the floor with a pair of threaded posts and a small beam under the affected area?
Are there any possible unwanted side effects or structural considerations that need to be made? (Ex. load shift, cracks in drywall, etc.)
Should an engineer be brought in to assess this situation?
Best Answer
Frankly, in an older home like yours seems to be, I would not try to correct so small a sag. But at the same time, so small a sag should be easy to fix.
Yes, the typical solution is just what you propose -- place one or two 6' jacks in the basement below the sag and span them with a 4x4, underlying 2 or more joists. And yes, cracked plaster is a risk. But if the current sag hasn't yet created a crack, it's likely that correcting your sag isn't going to move the wall far enough to cause cracking. Worst case, a plaster crack is actually pretty easy to repair; it's just a bit messy (patching, sanding, and repainting a wall).
In my opinion, correcting such a small sag is unlikely to introduce other problems (add stress to plumbing, electrical, shift door jambs, introduce trim gaps, etc). Consulting a structural engineer is definitely overkill. Most home renovation contractors should have experience with this repair.
"Getting to the Bottom of Sagging Floors"
"Fixing a Saggy Floor"