Kurt, You had me wondering so I double checked with a manufacturer and he said they were neoprene. When I asked how long they lasted, he suggested that "about the time you would check a 30 yr shingle roof, you should go up and check, as sometimes the screws back out. The washers should be fine." This is a link to a company's PDF for ordering and installation with lots of details about how to flash, for what it is worth, though there are no specifics on the composition of the 'GASKETED FASTENERS'. http://www.fabral.com/installation/gran ... lutuff.pdf