I am refitting an old shop above my garage to be an ADU. It was built in the late 70s / early 80s, and the truss system is 2x4 trusses on 22" centers. The trusses are professionally built & engineered trusses, with metal plates at the joints, so it wasn't a "hand-rolled" truss situation.
The span across the truss from end to end is about 28', with no bearing wall or support at midspan. It's held snow load and been fine (no visible sagging etc.) for 40 years, but that span seems pretty wide to me-- I'm concerned that adding 5/8" drywall to the lid will be a lot of additional weight on a truss system that already seems a little marginal.
I know this is more of an engineering question, but I'm wondering if this setup would raise concerns with the experienced builders around here. I had a contractor friend take a look and he didn't mention any issue, but I had an engineer friend say it looked like a pretty big span for that size of truss, though it'd "probably be fine". Any consensus on this or way to check the load?