Shed on the cheap.

I built a gigantic carport for my driveway and used heavy gauge sheet metal for the majority of the roof. Later I added an addition to the carport and decided to use heavy Vinyl (or maybe it was Asphalt).
Ondura 4-ft x 6.58-ft Corrugated Asphalt Roof Panel
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Either way it saved lots of money, is pretty heavy duty and has held up for many years so far. Also if you get something that is more transparent it will let some light into the rooms unless you plan to electrify it?? I bought the roofing at Lowes.