Missing joist underneith sagging interior wall

[rowusanow]

Part of my kitchen has sunk, and prior to executing my plan of installing flooring, I am looking to flatten the floor to get rid of an peak (1.5" rise per 14 foot span) in the middle of the room where the old house (1920s) meets the new addition of the house (1960's addition). After inspection with a laser leveler and ruler, it is apparent that the foundation of the addition portion of the kitchen has settled and the original foundation of the house didn't (at least not at the same time). The interior wall of the kitchen is positioned parralel to and inbetween two joists (16" OC) and it seems that the subfloor (8" x 3/4" pine laid diagonally) spanning the joists is the only thing holding that wall up from underneith. Since we need to cut the subfloor at the wall to shave the joists in order to flatten the floor between the old part of the house and the addition, I am certain that I need to add or sister one or multiple joists directly below the wall before doing anything with the subfloor (and the joists I need to shave). Any suggestions? The wall is not skewed towards one of the joists, but rather smack in between both of them. I could flatten the floor by adding material (instead of shaving the joists), but that can cause stepped transitions between adjoining rooms which I am trying to avoid at all costs.

It looks as though the interior wall may not have been intentially load-bearing during construction some 85 years ago, but after years of sagging and adjustments and settling, it has earned additional responsibility other than holding up the plaster. Note that the floor at the wall tapers downward and has indeed locally sagged. To complicate things, the space between the joists that the wall is positioned between has plumbing and electrical placed right where I would idealy install additional support.

Sounds like fun? Any suggestions would be greatly appreciated.

[Piper]

I would support the ceiling by Framing a dummy wall over a joist capable of supporting it on each side of the wall. Now that you are not relying on the wall supported by the sub floor you can safely deal with the hump in the floor. Then I would run boards accross the bottom of the existing wall between the two floor joists where I could fit them above the plumbing and wiring if possible. Hope that helps.