I recently completed (2 years ago) a custom build where the roof structure was built on site.
Unfortunately on one area of 3/12 slope they didnt brace the rafters correctly. half the purlins fall on to ceilings joists that are not structural. Note there is NO ceiling dry wall cracking, etc. probably because a lot of those ceilings joists dont have drywall directly on them due to drop ceilings, etc.
I need to replace the shingles due to a hail storm (yay north texas). I can visually see which rafters are properly support by looking across the roof (can't see from ground). The ones that are supported well are ~1/2" higher than the others. I did verify against a span table that the rafters should only span 13' MAX, and they are as much as 18' (assuming the supports do nothing).
Luckily I dont have large snow loads, but I need to decide if I want to fix this. The reason I dont just fix it properly (beam for purlins to fall on) is that the attic is VERY small (low slope roof) and I have spray foam on the roof decking. so I will likely have to remove drywall ceilings to touch up spray foam, get beams in, etc. I can just pull off the roof deck and brace. There are also a lot of AC ducts running near where this beam would be, so I would be concerned even if somebody could wiggle them selves into the spot what other damage would occur.
My assumption is if I can live with the 1/2" sag it probably won't get much/any worse. It looks bad when talking the roof, but it can't be seen from the ground. Probably more likely to raise a bunch of further concerns and pain in the ass if I have them rip the whole thing apart to fix. Also part of the area that is sagging is actually a vaulted ceiling so I would have to pull down all the ceiling drywall in there.
Just wanted some input from real world experience, the engineering tables say fix it.