Galvanized steel roofing is the most common choice for a metal roof. Plain steel is coated with a layer of pure zinc to provide corrosion protection. When the metal is cut the zinc coating will protect the affected area from corrosion. When choosing a galvanized roofing system pay particular attention to the rating for the zinc that has been used in the galvanization process.  The rating ranges from g-30 up to g-90 and indicates how much zinc was used per section of steel.  A higher rating is better for corrosion protection.