The first floor of our 1860 home sags 2", according to a laser light shot from the side walls. I'm also seeing some corner cracking developing in the supporting bedroom wall on the second floor that lies directly over the center of the house, where the worst sag is measured, and the finished 3rd-floor attic room floor also sags a bit toward the center. It all appears to be related to the unsupported length of the joists.
I do a lot of work on my own homes and would like to give this a shot. The obvious solution is to jack it up.
Now, I've read where you have to check to see that the joists are flat against the foundation so you're not just jacking up the entire house or damaging your side walls while trying to correct the sag. The house is built on a stone foundation, and the joists appear to rest flat at all locations, judging from the view inside the basement, which has a poured concrete floor. So it would appear jacking it up is the way to go. At a minimum, it obviously needs to be permanently braced.
The trick for this Harry Homeowner is that the size of the joists varies. As noted in the subject line, the joists are 19' long, inside wall to inside wall. (The joist spans vary from 16 1/2"-to-20" OC.) But the other dimensions vary considerably - from 2 3/4"-to-3" wide, and 8 5/8"-to-9 1/2" deep. The latter figure is my biggest conundrum because if I run, say, a 4x4 across three or four joists, say, 8' in from each side, jacking each up with two adjustable Tiger jack posts, I'm not raising the floor above evenly because of the varied depth of the joists.
(The 8' mark for support is pure guess, as there isn't an ****** calculator for spans with (roughly) 3"x9" joists. I'm also thinking of killing two birds by replacing the temporary supports, once done, with supporting walls and turn this into a 3'-wide vertical storage space down the road.)
The joist dimensions don't vary a lot, however. If I follow this course, do I just measure and notch so the depth of each beam along the area where my 4x4 would cross it is uniform?
Another question: I don't know how thick the basement concrete floor is. All those original/remodel plans are long gone. So I'm not sure how much if any I should reinforce the floor under the jacks.
Someone has tried to fix this before by running a beam perpendicular to the joists across the half of the basement most affected, and supporting it with vertical posts. But it's totally inadequate: the beam is 1 1/2"x5 1/2", the supports are 4x4s and those are notched at the top to support the beam. The load in the center is such that the beam closest to the center of the house (and the stone wall, parallel to the joists, that supports the center) is floating. Zero support.
A structural engineer took a look and offered to fix the problem in a week - for $16,000. That's sounded pretty high and regardless is out of my hoped-for price range. In addition, everything I've read says to do this very slowly so the wood has a chance to "re-learn" a new shape. I'm willing to take the time. I did get something for what I spent for the engineer: He asserted that raising the first floor would correct the issues I've noted above that level.
That raises another concern I've read about: I don't know what type of wood the joists are cut from, but it is old wood. Am I running the risk that the wood won't return to at least near-level (not expecting perfection by any means) and that I could inadvertently end up jacking up the sides of my house instead? Should I be satisfied to brace the floor somehow and not expect any change to speak of? The goal, of course, is stability. Doubt I'll end up playing marbles up there.
So, yeah, it's quite the project. But the hardest part is choosing the right path. So, your advice and suggestions would be greatly appreciated.