I tried posting this awhile back, but I don't think it ever appeared. One reason for a metal roof vs. glass is that a metal roof in a lightning storm is safer. You've probably read that in an electrical storm the second safest place to be after your home is in a car. That's because the metal shell of the car creates a "skin effect" or more scientifically a Faraday cage, protecting you from the lightning. A glass roof does not provide that protection.