This flashing is to be used with twin wall insulated pipe and is used when a twin wall flue pipe passes through a flat, corrugated or plastix roof. resistance of 260°C and continuous temp. For example, if you had a 6" twin wall flue pipe which is 8" externally and your roof pitch is around 30 degrees, the best one to go for would be 4-3/4" - 10", or if you really want to be on the safe side you could also use the 5-1/2" - 11-1/2".
eBay