If I'm understanding you correctly, you can probably do ok by removing the flooring, while leaving the joists in place and having them beefed up. Sounds like that approach would be easier, faster, and cheaper. If it were me, and not knowing exactly what you'll find, it might be prudent to do it sequentially, as follows: remove, say, 1/4 of the floor, beef those joists up, remove the next 1/4, etc.

The fireplace will be fine as it most likely is sitting on a foundation of its own (or ought to be anyway).

One question: is the problem weak, undersized or damaged joists, or is it weak, undersized or rotten sills (the things the joists are attached to along the perimeter of the building)? Or both? Often additions, especially kitchen additions, aren't up to the same quality of construction as the main building, being sometimes simply the roofed-over accumulation of several little bump-outs created over time.

Jacking it up may not provide enough convenient workspace for joist repair, which I think is easier done from on top, but jacking it up will need to be done to set in new sills.
Depending on the bilding's construction techniques this can be a big deal, or not.

No matter which way you do it, you want to install a vapor barrier on the soil and insulate under the floor. From up or down, each path has its own benefits and challenges.

One thing though, if you do any jacking you want someone with a lot of experience doing that.

Sometimes too, with kitchen additions, it's cheaper to tear it down and rebuild from scratch. (Of course salvaging as many items as possible.) Even I, who love old buildings, and always advocate saving them, recognize that in the case of service additions, sometimes they were very shoddily built right from the start. There's often no point in reproducing or preserving "temporary" or second-rate components for the sake of historical accuracy in most non-landmark buildings.
Molly~

Well, as I just posted in this new thread (click here to see), I finally got a look underneath today, and it doesn't have milled-lumber joists, it has the original hand-hewn LOGS! I have absolutely no clue how this will affect things.

Molly, I know all too well what you're talking about, but that's not really the case here. This wasn't a kitchen "addition", this is a 200 year old two-story house that was brought over from another part of the original family's property to be added to the house built in 1842. It appears that the two houses were connected and then sided and roofed together, which means that it was brought over when the main part of the house was built.

Unless my logic is way off, this now makes sense why the first floor looks so caved in, but the second is actually quite even and level for a house that age, because the logs in places are 4-5' apart, instaed of the much narrower spacing that we're used to seeing for standard milled-lumber joists.

I'm just totally taken aback by this. A little scared, and a lot totally fascinated!!

Ok, I misread your post on the other thread a bit. I was confused by the word logs, I incorrectly assumed you meant literal logs with bark still attached, but now I think you mean hand-hewn (with visible adze or tooling marks) timber framing members.

One of the reasons for the wide spacing is that large, often hand-hewn, beams are so massive they don't need to be spaced so closely together to be structurally sound. So don't assume the floor defects are only because the logs are too far apart. With framing members like that you are most likely talking about a timber-framed building that was fit together with mortices and tenons and all held together with wooden pegs or trenails. Very nice and very sturdy; and more important to you maybe, utterly dissemblable and moveable. Whack or drill out the pegs and the pieces can be taken apart, generally without damage.

I get the impression your building is in the Mid-Atlantic area, but there are some good books on timber-frame construction that though geared to the NE, would give you some clues.

A quick sketchy recall of some titles: Big House, Little House, Back House, Barn; The Impecunious House Restorer and one other with a lot of info, but mainly about Canadian timber frame buildings, but whose title I can not dredge up. I am away from home for several weeks, but would look those up for you when I get back if you haven't turned them up sooner.

Work on timber framed structure requires somebody who knows what they are doing. And vigilance when subs are working there so they don't compromise the structure while installing modern services.

My own house is framed with 14 X 14 oak pieces, and all timber framed (as are all six of my two-story barns). Remarkable structures, and all bearing the marks of careful handworking, with each piece marked with a Roman numeral indicating which rafter, joist, etc., it was in the assembly sequence. You can look for these marks with a raking flashlight beam.

Do you have any contemporary reports of the sequence and provenance of the structure's present form? I know I've been told a lot of total twaddle about by well-meaning, local, semi-experts about the which part of my house came first. It was only when I went back to contemporary public records that I finally winkled out the truth. Timber-frame buildings are relatively easy to disassemble and move, compared to modern structures. And making them was highly labor intensive so wasting an unwanted building was unthinkable.

Molly~

Actually Molly you were right the first time -- it's actual logs with the bark still on some of them, and the ends cut out to attach it to the sill.

> Do you have any contemporary reports of the sequence and
> provenance of the structure's present form? I know I've
> been told a lot of total twaddle about by well-meaning,
> local, semi-experts about the which part of my house
> came first. It was only when I went back to contemporary
> public records that I finally winkled out the truth.

There are records that document when the main house was built, but no record of the origins of this older section of the house - not uncommon, I've found, for things in this still very rural area. But the design of this part of the structure, plus the documented history and traditions of the land-owning families in the area strongly supports what they're saying about how this part was originally an older home on the family property that was brought over to allow for a larger home with less new construction.

Jacking, if there is a lot to raise, usually has to be done over a very long time - think months. You don't want the floor open that long even if it is cheaper and easier.

If your floor just sags a bit more than brand new construction, but is stable, you might just want to live with it. The floors of our main house don't sag at all, but in the addition they are far from flat. The nuisance of flattening them is not worth any benefit.