I wasn’t quite sure if this was a code question at first but –
I have a new construction (spec build) going on now with a new framing contractor
& i just received his material list & I’ve noticed that hes using 2×6 joists throughout the entire house (excluding an attached garage).
Checking over the span tables it looks like hes referring to a 5psf dead load for his joist layout.
from what i figure, (accounting for doubled shingles) & what ive always done in the past was figure up my rafters, shingles x2, sheathing, underlayment, rock & insulation which would be +9psf & called it 10psf.

But looking over load calcs & span tables i’m starting to think maybe i’ve been wrong,
otherwise why is this 5psf data so prevalent.