Hi there, B³,
That looks very good. I'm quite envious, while I've been concentrating on my tin shed project, the workshop roof has developed a leak, right over the tailstock end of the lathe swarf tray! I can see where the water is coming in inside but I can't find any defect in the felt up on top.
Having looked at the price of roofing felt, I'm considering re-roofing with the same sort of box section steel sheets as you have used. The snag is, they really need to be fixed to timbers that run along the roof (purlins?) rather than rafters running down the slope - that would require a fairly substantial alteration to my roof structure.
Your photo seems to show timbers in both rafter AND purlin orientation. Please could you tell us a bit more about what's under your steel sheets? Did you happen to take any photos at that stage?
I envy you the size of your workshop - it makes my modest 12' x 8' look like a sentry box!