Flashing is the term used for lengths of sheet metal, typically, that is used in roofing to deflect rainwater away from otherwise sensitive areas such as chimneys, roof edges, vent pipes, etc. Most flashing is made of sheet metal, but in areas that are concealed by roofing materials, plastic or roofing membranes may also be used. Metal flashing is usually made from aluminum but copper, steel, and lead or zinc alloys are sometimes used as well.
Essentially, the flashing bridges the gap between different construction materials and retains the waterproofing of the roof. For instance, where a small porch roof "dies into" a second-story gable end, flashing will be used that sits behind the gable siding and over the roofing on the porch. It thus directs water running off the siding over the joint in frame of the house and onto the waterproof roofing on its way to the gutter or off the roof.
When installing flashing, careful attention must be paid to how water flows (downhill mainly) so that water cannot enter the building's structure. As another example, consider a chimney. A section of flashing must be placed at the high end of the chimney which directs water to flashing that runs down the sides, which overlap a section of flashing on the downside of the chimney and the roofing shingles as they then allow water to flow down to the eaves. Flashing, or drip edge, is usually used at the end of the eaves, at the fascia and under the roofing, so that water is forced completely off the roof and to the ground or gutter, protecting any and all sensitive construction materials.
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