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Hemmed edges are used on most metal flashing systems. The hem or fold makes the flashing strong at the hem and helps control water flow. The hem can be turned up or down. Hems turned up are used at the high edge of flashings around roof penetrations, such as skylights, soil pipes and chimneys. Upturned hems are also used on the roof side of continuous sidewall flashings and on the vertical edge of open metal valleys. An upturned hem creates an air gap that resists the capillary migration of moisture from the metal to the roofing. They should never be hammered flat. Upturned hems also serve as hooks for attaching cleats. Downturned hems are used when flashings are lapped over the top of roofing or other materials, such as skylight glass.