The steel roofing was super easy to put on. Much much much easier than shingles, and the same price per square foot. However if the roof wasn't square it could get tricky.
I put temporary jigs attached to the rafters to hold the bottom edge of the roofing. It made it really easy to get things positioned properly by myself.
The "purlins" are 2x4s, flush with the 2x6 rafters. There's also 1/2" hardware cloth and tar paper under most of the roofing. It doesn't usually need nearly that much support. I have some big pine trees nearby that I'm worried about branches falling from.
Thanks for the compliment on the framing. This was my first time cutting rafters although I did carpentry for about 3 years.