Made a ton of progress on my garden shed building project over the last four days. I still need to pick up one board at Home Depot on the way home tonight as I'm short a final six foot piece, but otherwise the roof is ready for the final metal layer. I'm actually planning to put down roofing felt first underneath the metal, particularly since my mom had an entire unused roll left at her house. I'm hopeful that it will be enough to cover the whole thing up.
As far as the metal roofing goes I'm going to attempt to recycle some of the siding and roofing from our current metal garden shed after I disassemble it. I'm cautiously optimistic that this will work out, but I'm operating purely on guesswork and estimation at this point. I'll have a much clearer sense of things once the old garden shed is apart and I can fully measure and test the individual metal sheets.
On a funny hardware note I used almost $40 worth of deck screws to assemble the roof. A pound of 3" screws to attach the rafters to the top of the walls and 5 pounds worth of 1-5/8" screws to attach all the cross boards to the top of the rafters. My Craftsman portable Li-Ion Drill has been a serious trooper and even survived a longer than desirable fall!