A am not a contractor, but I do have a cracked joist that my home inspector found and pointed out to me before we bought that I’ve currently got a screw-jack under that I’m going to have to replace, and have talked to a few people about replacing both that and taking out a supporting wall and replacing it with a beam in the ceiling. So since nobody else has answered yet:
First thing: Find one of the online calculators (The American Wood Council one is here, if you have height constraints you might have to use tables from the manufacturer for I-joists or gluelam joists) to figure out what size floor joists you need there. You talk about notched and otherwise compromised, so they could be cracking and splitting ‘cause of bozos doing shoddy changes to the house, but they may have been under-engineered to begin with so you may have to go bigger.
If it’s a crawlspace, you’ll lift the joists into place first with screw-jacks. If it’s a basement, you can get screw-jacks in the form of steel posts: They telescope, with pins at various setting points, and then have a screw extension in the end to get you your final lift. You’ll put these underneath your existing joists, jack the floor up to level or thereabouts, and then sister in your new joists. Once the new joists are in place, you can pull the old ones and then pull the temporary support posts. You should be able to do this a joist or two at a time, so you don’t have to jack the whole house up at once.
If it’s a dirt floored cellar, use pier blocks to get yourself a good solid base. If it’s a crawlspace, you could try landscaping bricks, but I’d try to find a piece of ½” thick 1’x1’ steel that you can use to set your screw jack on.
You’ll have to make arrangement for plumbing and electrical, which may mean cutting out pipes, putting in the new joists, and then re-running the pipes. There’s code for this (get the Code Check books), but holes through the middle of joists don’t weaken ‘em nearly as much as notches.
Finally, a lot of home improvement folks will say “don’t get the building inspectors involved”, but I’m not that way: Your local building inspection office can be a great resource. My city’s building permit department is happy to take a look at my plans and make suggestions. Yes, it costs a bit to get the permits and inspections and do everything legal, but the other way to think about it is: How much would it cost to get a knowledgeable contractor to check your plans before you start, and then come out and check your work after you’re done? I’ve heard that in some jurisdictions the building and planning departments are actively hostile to do-it-yourselfers, but my local office has been happy to work with me.
-- Dan Lyke, Petaluma California, http://www.flutterby.net/
