SELKIRK CORP 104825 4-Inch  Adjustable Flashing 4", 4RV-AF, Adjustable, Flashing 0 to 6/12, Round, Type B, Gas Vent Used where the vent passes through a roof RV line of MetalBest round Type B gas vent pipe and fittings Outer material is 28 GA galvanized steel Inner liner is aluminum