Flashings are used to seal the opening where the chimney penetrates the roof to divert rain and snow down from the roof and away from the chimney system. A series of adjustable flashings are available depending on your roof pitch. A flat and low pitch flashing is used to seal flat and low pitch roof (up to 2/12 pitch) openings. o determine your roof pitch, measure the number of inches from the end of a level ruler down to the roof.