Selkirk Metalbestos 6T-AF6 6-Inch Aluminum Adjustable Flashing Flashing is used to seal the opening where the chimney penetrates the roof and to divert rain and snow down the roof and away from the chimney system A series of adjustable flashings are available depending on your roof pitch Aluminum Opening is 8.1" for double wall pipe. Flashing is used to seal the opening where the chimney penetrates the roof and to divert rain and snow down the roof and away from the chimney system Aluminium A series of adjustable flashings are available depending on your roof pitch Opening is 8.1" for double wall pipe.