Presi's are tedious as it is, that pitch makes it even more tedious. At that pitch it's easy to let a shingle slip, be very very mindful of your layout because also at that pitch it's going to be a very high-profile job. If you have a bad layout you're gonna see it in the next county for the rest of your life. Like they said above, we tend to price it like shakes and price it for the slope as well. But also like they said, the steeper the pitch, the easier it is on your back.
Oh since you're offering, what size floor joists do I need to span 18' for a loft I gotta start on in my shop? TJI or dimensional, nothing heavy going on it, just office equipment.