If I were to remove the floor and sister in new 2x10s do they need to sit on top on the load bearing walls?...or can they just butt up against them?
Because the purpose of sistering a joist is to strenghten the joist (i.e. make it harder to deflect under weight), you do not have to have the new joist sitting on top of the wall. And since it's all about strenghtening the existing joist, notching rules do not apply. That would only be the case if you were replacing the joists.
If you do notch the joists, then you have to account for the sag. If you notch the ends such that the ends are exactly the same 6" height of the original joists, the center of the new strait joists will ride above the existing sagging joist.
Last edited by HooKooDooKu; 02-03-2010 at 10:33 PM.
One other thought. Keep in mind that by sistering the existing joists, you are adding dead weight to the existing structure. If your walls were sized marginally as well, you might have to look into sistering the wall studs as well.