We are proud to use fiberglass because it has stood the test of time. We have a lifetime warranty that our cover will not break from normal use. We often replace competitors' covers that are made from polycarbonate. They will initially hold the weight suggested, but after their covers have been through some hot summers and cold winters, wherever a hole was drilled, a small crack can develop. Once some weight is placed on the cover, that crack travels putting people and animals at risk. We do not have that problem using fiberglass. Our lifetime guarantee means lifetime.