Strictly local codes don't apply to anything Federal, but you ignore the code as a professional engineer at your peril. The GSA has standards for structural,
I'm not aware of anything specific from the FAA, and have some basis for that. The airport commission may have some amendments but it feels unlikely.
I'm not clear you're asking about reducing snow load (thermal or occupancy effects, roof slope) or roof live load (square footage and slope) by the way. The two are distinct.
I think i had an Idaho project where they amended the minimum roof snow load above 20. Or where is Great Falls ?