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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?

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    Why do you want to do this? What is in your basement (finished)? This could have easily happened during house's first few years while it was settling and jacking it up could cause other minor issues. And 1/4" over 12 feet isn't a sag, it is a difference but not a sag and I would be hard pressed to find almost anyone that would notice. I would just enjoy your home. – DMoore Jan 26 '15 at 19:41
  • If you don't follow the excellent advice above, yes, bring in an engineer before you start messing about with your home's support structure. Most of them will primarily advise following the above advice, at a guess; but if you are going to forge ahead, they can keep you from causing worse problems. – Ecnerwal Jan 26 '15 at 21:10
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    @DMoore Sounds like an answer to me. – bib Jan 26 '15 at 21:40
  • @bib - For a while in my area if there is a house not selling because it was sagging it was a given I would buy it. I had a very fast/good/cheap drywall crew - which is most the cost. 1/4" is nothing. I have seen houses at 6-8 inches over that span and when you walk you are disorientated. Now I have to look at my house and see if it is off by 1/4 inch over any part. But the good news is no real contractor would mess with this house... for fear that if the floor was 1/100th of an inch off job might not be done. – DMoore Jan 26 '15 at 22:44
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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"

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