13/12 - 18/12 DuraVent DuraTech Galvalume Adjustable Roof Flashing 8DT-F18 8" DuraTech Galvalume Adjustable 13/12 - 18/12 Roof Flashing is used to weather-proof the penetration of chimney pipe through the roof. Storm Collar required. Select roof flashing based on pitch of roof. Made of galvalume. Diameter: 8" Pitch: 13/12 - 18/12 Galvalume Storm Collar required 10â€-24†diameter pipes feature an .027†430 stainless steel inner wall and an outer wall of .029†galvalume or .027†430 stainless steel. A Refractory insulation is encased between walls. Only stainless pipes use the stainless end rings. Regular galvalume pipe uses galvalume end rings.