As for what to put metal roofing on I think skip sheathing is best. Solid sheathing won't allow it to breathe and could corrode from the back side.
We use approx 1" by 3, 4, or 6" and space them to suit.
Part of the that last roof was actually on 1/2" OSB and made the effort to still put skip sheathing on top of that so it could breathe.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-