Rotten End Joists
Are the joists dry rotted out? If so how far, and most important is why? The dry rot must be removed and treated with boric acid to kill any of the organism that is left. Then you can usually sister the joist. Pictures of your problem would help determine how much repair is needed.
If you think you can, or if you think you can't, you're probably right.
ASE certified master auto tech, Retired.