Choose a flashing material that is at least as durable as the roof covering. For example, it would not make sense to flash a 100-year-old slate roof with thin aluminum. Once the aluminum begins to leak, the slates would have to be torn off to replace the flashing. Copper flashing would cost more initially, but because it could last as long as the roof, its life cycle costs would be lower.
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Flashing Shed Dormers
Shed dormers require a continuous strip of flashing along the top of the lower roof plane where it meets the steeper roof plane. Also, they require sidewall flashings. A critical juncture that often fails is where these two flashings meet. Use taller pieces of step flashing towards the top, so the pieces can be bent over the dormer roof to help keep out snow and wind-blown rain (Shed Dormer Flashing, below).