I've got a detached garage which was built in the 1940s and I'm trying to figure out whether it's safe to add an attic ladder and use the attic space for storage. The ceiling joists are 2x6 [pine/fir], spaced 16" OC, and span 20 ft. The wide span makes me bit wary about putting heavy items up there or installing a ladder.
It should make you leery. Playing around with a wood beam calculator I had to trim loads down to 7 lbs per square foot to get any of the solutions to come up 2x6 for 20 foot span and 16" spacing. That's not even light storage (by my definition.) It's not even a residential attic with no storage by more official definitions (10 PSF) nor an attic with storage by those (30 PSF)
Either leave it alone or hire a civil/structural engineer to develop a plan where you'd either refit the roof structure (effectively turning a rafter and joist arrangement into trusses, if that's practical, by adding web members) or place a properly sized and supported beam down the middle to cut the span in half. In short, if you want storage up there, it will cost a bit more than a set of stairs.
Your concern is justified, but it really depends on the rafter construction. If they're set up like modern trusses, with W bracing and adequate gussets and fasteners at the connections, I'd be comfortable putting a modest amount of weight up there.
If not, you risk sagging and other damage, if not outright collapse.