How large a developer or builder built the house?

You can get odd size lumber and it can be worthwhile if you need enough of it.

Any signs of planing to get a flat floor?

How does the final dimension compare to span tables?

And the reason for your query?

I'm replacing/sistering some of the joists, and I am wondering how to deal with the difference in size.

We are not told if the new are wider or more narrow than the old. But I would use wider if possible. Now tell us where the difference in width poses a problem.

Look in span tables and see how close you are to span limits.

Strength goes with the third power of joist height, so it has a large impact on deflection and load carrying.

Sorry... more info:

Span: No problem with the span. They are 14'ers at 12" on center.

New vs. Old: The new are wider than the old - new are standard 2"x12" (11 1/4") while old are around 10 7/8" on average.

The problem: The distance from sill/beam to sub-floor above is determined by the width of the current joists. So I can't just shove an extra 3/8's in there.

Note: the floors above are flat as is.

Every situation is different. The last one I did had wide outside sill plate to the rim joist with the other end of the joists notched and on a 2 x 4 ledger on the side of the wooden girder, circa 1945. A jack post was placed mid -span on the existing joist and adjusted to be very tight to temporarily lift any sag in the center of the joist. Nails protruding through the subfloor were ground flush -- FUN! The new joist was notched for 3/8 clearance on the outer sill plate for the width of the sill plate, cut to length of 2" less than the distance between the rim joist and the 2x 4 ledger, then notched for the ledger. Place the crown up on the new joist- this may mean that the new is jammed tight against the subfloor. Notching for the part over the sill plate so that the diagonal of the remaining portion is equal to the height over the sill plate will enable the joist to be placed in flat and then rotated to the vertical position. Work the new joist onto the sill plate and back far enough to permit lifting up next to the ledger at the other end -- easier said than done-- but doable. Spray the top edge of the new joist and the subfloor with silicone to help things slide easier. Deadblow hammer and come-along help with the movement of the joist. Another jackpost helps lift the end at the ledger. A 1/2" hole at an angle in the part over the rim permits inserting a 1/2" bolt to hook the come-along to help pull the joist onto the ledger-- you'll understand when you are doing it. Tighten the come along and jar the joist with the hammer to make ti move the couple of inches onto the ledger. When in place, jack both ends of the new joist up tight against the subfloor, shim tight both ends under the new joist-- I prefer sheetmetal shims.
In this case, the deep notch for the ledger concerned me, so I made joist hangers of 3/16" thickness in the shop (welding is a hobby I enjoy) to fit both old and new joists and support the joist for 2" beyond the notched area. Install hanger tightly and shim under the smaller joist so that the hanger is supporting both joists. Shim tightly at the sill plate. Then nail the joists together. Remove the center jackpost from under the original joist.
This process was quite slow for me working alone-- but the results were excellent.
Temporary removal of wiring and plumbing may be necessary. Just do it or have it done.

This post was edited by bus_driver on Mon, Mar 4, 13 at 7:49

If you are just sistering an already adequate design for additional stiffness you can simply notch out the excess width at the bearing points on each end.

Drilling about a 1 inch diameter hole and then ripping of the excess width to that point helps a little with splitting, but since the sistering is not required you can get away with a lot more.

If you really want to stiffen the joists, some 1/8 steel straps on the bottom edge with screws in tight tolerance holes would add a LOT of stiffness with out the hassle of sistering and bridging, plumbing lines, electric lines, ducts, etc. running perpendicular to the joists.

It takes about 3 inch screws in holes as close to the screw shank diameter as you can get through the steel, with a stagger pattern to hold the metal in place and make the wood and metal act as a single structural unit (a composite beam).
Screw spacing is usually right around 4-6 inches, screws around #10 to #12.

You can use the strain equations for welding up I-beams to figure out how much load the metal will have (the welders use it to determine the length and depth of required welds) to determine exact screw size and spacing.

Is the top of the foundation not a singe plane?

Someone may have planned joists to get a flat floor making up for height variation in a support wall.

This post was edited by brickeyee on Sun, Mar 3, 13 at 15:44

brickeyee, I've seen the "top-to-bottom-to-top" method advertised quite a lot lately, regarding metal straps, as in page 5 of the below link. Does that work as effectively as having the strap flat on the bottom as you describe? Thanks!

Here is a link that might be useful: Methods to stiffen joists

The thin top to bottom strap is not as stiff as a closer to full length bottom strap.