I'm going to be installing a 3/4 x 5 inch hardwood floor in one upstairs room in my house. It's a 100 year-old rowhouse, and my floor joists have sagged in the middle by about 1 and 1/2 inch (over a 14 foot span). They are real 2 by 10s with 16 inch spacing, so I think they should be fine for the load - it's just that they've sagged because of having been there 100 years.
I'd originally intended to level my floor before installing my new hardwood, so I was going to pull up my subfloor, sister 2 x 6s to the old joists (not to provide structural strength, of course, but to create a flat surface for the new subfloor), and then install 2 layers of 3/4 plywood (to bring it up to the level of the adjoining hallway floor (behind the camera in the picture).
My problem is that I have a doorway (the one seen in the picture) that is right in the middle of the sag. And that opens into a room that is only 1 and 1/2 inch above the joist (i.e. it has one less layer of subfloor than the hallway behind the camera). I was going to have 3/4 inch step down there anyway - but now, if I use the sisters to create a floor that is level with both sides of the house, my new floor would be 2 1/4 inch above the floor behind the door. Obviously that won't work.
I think my options are -
1.) Don't sister, and just go with the sag. If I do that, and lay my hardwood perpendicular to the joists (over 1 1/2 inch of plywood), will my hardwood lay tight and not move or squeek - sort of curving like barrel staves? The bend is significant near the walls (the joists drop about 1 1/2 inch in about 3 feet near the walls, and the flattens until it is about 3 feet from the opposite wall).
2.) Sister, but create a slant such that the whole thing slants up from the doorway to the hallway by about 1 and 1/2 inches (over about 11 feet).
3.) Sister, and do a transition down in the doorway, and another transition up to the hallway.
Consider adding a second "sistered" joist to the opposite side of the existing 2x10 joists. Now you have two "sisters" on each joist and have cut the 16" c/c span down to 12.5" + 3.5". You can easily get by with one layer of 3/4 plywood especially if it's glued to all the sistered joists with panel adhesive. Doesn't completely solve the problem but does decrease the issue by 3/4".