A couple years ago I built this for an outdoor kitchen at my place in the mountains.
Tinted polycarbonate, screwed together 2x6 rafters, 12" bays. My beams are each 5-2x6 bolted together lams which make a much stronger beam than a 6x8 solid (no worry about splitting grain). I googled the strength of polycarbonate roofing and it's virtually unbreakable (565 PSI). The fasteners would pull out before the plastic would break so it's all about your under pinning. Last winter I had 6 feet of snow on top with no problems