The diagram below shows the problem and also the atypical way the roof was framed originally (see this question) This garage door was already in place since long before we bought the house and separate garage about ten years ago, I've noticed no visible problems until now as I'm looking into flooring the attic. I dont know how old the cracks are in the wall plate under joists 2 and 3 (joist 3 wasn't cut at all) There are smaller cracks in other places and there are some metal reinforcing strips screwed into the plate crosswise.
One solution that came to mind was to add further joists on top of the upper plates over joists 1 and 2 and possibly 3, as in the diagram below.
Possibly related, the two rafters attached to joist 3 have a very crude collar tie between them, quite high up in the attic, well above head height standing on the third joist. There aren't any others except one, ten or twelve joists further along. The tie isn't even level.
Thanks for reading and especially for any guidance offered.
Edit to add photos
Taking these, i realised that the diagrams above are incorrect, the upper plates are not directly over the lower ones, they lie approx 4" lateral which I think might explain the upward bowing. The second photo maybe gives some clue as to the quality of the 10" by 2" joists. You can also see the cracking in the lower plate in the third photo.
The wall to wall span is 16 feet and the joists are every 16"