paul, metal roof allows you to claim 'slippery surface' as a deduct when running the numbers. is it BS, possibly.

yes, it could be shingled.
i agree, the straps look minimal.
but the images are tough to read.
not trying to debate it, as i have lots of respect for TD and simply stumbled on the barn when searching for images for thane.

here's what my software tells me.
12 pitch, cold roof, rough surface, unheated structure.
generic span of 28 ft with 10 ft between trusses.
15 pounds dead load, 50 total. (and a bunch of other stuff 'standard' that i only change when needed)

15960 point load @ peak.
total roof load climbs to 57 psf

same load using 'slippery' for roof surface:
9545 point load @ peak
total roof load drops to 34 psf.

so, it can change things quite a bit.
obviously, higher pitch, the better!

dropping the pitch to 8 we get:
rough: 57 psf
slippery: 43 psf
this doesn't take into account unbalanced loads

thane, have at it!
8 is better than 6.
i'm always weary of how flat hipped roofs can look...