Polyglass roofing sheets are placed over more traditional roofing materials to help reinforce the seal. Without a previous roof already in place, polyglass roofing materials may not work as well as a new roof. Polyglass is an excellent add on, but doesn't stand up well on its own.
If a roof suffers any damage underneath the polyglass layer it can be impossible to reach it without going in from underneath the roof, or peeling off layers of the roof to remove the polyglass. In Alaska and other places with very cold temperatures, polyglass roofing materials are used to cover existing roofs in need of quick, but temporary repair.
Some polyglass roofing is torch-down roofing, meaning it can be secured in place with a blow torch. Most roofers are more comfortable with nails, screws and other fasteners. Using a blowtorch requires the right kind of roofing--and the right kind of insulation so the heat from the torch doesn't set the structure on fire.