Fiberglass is a composite made by using a liquid ( polyester resin ) combined with glass fibers in a mat or woven form. A chemical reaction during the mixing process causes the product to harden and become a strong, light weight material in which the resin serves as the substrate and the glass fibers are the reinforcement. This is similar to pouring concrete over rebar ( steel rods)  in construction. The lifespan of this roofing system is unlimited. Pound for pound fiberglass is stronger that aluminum.

Fiberglass does not transmit heat like a metal/aluminum roof does and is also much quieter. The encased galvanized steel beams provide a roll cage effect in the unfortunate event of accident. 

( On a personal note - several years ago we had a pretty good storm complete with hail - after the storm I went to check for damage to the trailers on the lot - there was no damage to the Hawk trailer roofs, but on the hood of my husbands car and on the 2) aluminum roof trailers on the lot it was a different story - many many dings and dents. Now I certainly cannot say that hail will not ever damage a Hawk roof but it sure made an impression on me. )