In helping to build a deck, we found one of the 2x12 joists had a sag in the middle of 1/2 inch. We used hangers and would like to find out how to recover the 1/2 inch. I have heard of sistering another joist and adding a post support at the half way point.
However, we would like to secure the screws at 16 on center. The above method would put the screws at this location 1and 1/2 inches off. If you sister a joist, can you still put your screws into the sagging joist? I realize this would result in part of the screw not be into the wood for 1/2 inch.
Are there other methods to correct the problem or do we just pry out the hangers and obtain another joist? The joist is 13 feet long.
If it isn't too late, I'd suggest just replacing the joist. If the decking isn't down yet, it should be fairly easy to remove.
If it is too late for some reason you can do one of two things...
1) Like you said, add another joist next to the warped one using a lot of nails to secure it. You could get away with screwing the decking into the warped joist I imagine.
2) You can essentially shim the low spot on the warped joist by attaching a ripped tapered shim from a piece of treated wood, reasonably matching the low spot on the joist. The shim would attach to the top of the joist.