The "G" rating is important. It describes the amount of galvanized or galvalume coating on the metal. A G-90 is normally the min (meets specific ASTM spec). It means .9 ounces of the coating per sq ft of the steel (total top & bottom). Galvalume (55% aluminum and 45% zinc) is the superior for a residential roof, but is lousy for ag. buildings due to adverse interactions with animal waste (and other reasons that I cannot recall). It is also typically more expensive than the galvanized roof. That's not to say that a galvanized roof is garbage. There are claims that the galvalume roof will last many times longer than a galvanized roof. A good galvanized roof should last 40+ years. For me, that time frame is plenty. Besides....the paint will likely begin to fail on either product before the steel integrity does.
I am right now researching and planning to purchase a metal roof for our garage/apartment project. We are opting to the painted galvanized roof that Menards offers. The coating is G100 (heavy duty coating) and carries a great warranty. I also expect that Menards will be around down the road....in the (hopefully) unlikely event. It is about $1400 cheaper than the comparable galvalume product and is an installation that I am already familiar with so I can handle it.
Thickness of the metal is another story. When they start talking about gauge of the steel, always refer to the actual metal thickness before the coatings are applied. .016" - .018" is normal and acceptable for residential applications.
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This is all info that I have recently found during research. If I am off the mark on anything, I encourage a tune-up from any of the forum members.
