fwiw I redid my shed rood with steel roofing sheets. I put roofing battens across the old felt roof and screwed the sheeting to the battens. It works great. It cost probably twice as much as new felt but will last a lot longer. the air gap between the sheets and the old roof act as an insulator. My garage roof is done in a similar way with plywood, then studding, then steel sheets. My mate did both his sheds in the same way, he was so pleased at how the first one came out.
Oh yeah, when my mate did his, he ordered the sheets cut to a specific length.
Mart.