This Storm Collar should be fitted above the flashing (Roof Plate) to prevent water entering between the outter skin of the flue and the flashing.   PLEASE NOTE - We recommend a minimum of 1 metre vitreous flue pipe LINK is installed between the stove and stainless steel twin wall system.  Your vitreous flue pipe will either be 5" or 6" depending on which stove you have purchased.  Each stove product page has a link to which of the two stainless steel flue connectors you will require for your installation. All stainless steel twin wall flue systems are 6" (150mm) in diameter.