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Flashing is used to prevent moisture from entering under the roof covering wherever two or more planes of a roof meet or wherever the roof meets a vertical surface. It is also used along edges of roofs and other windows or doorways to direct moisture away from the house exterior and structural framing.

Roll roofing material is widely used for flashing, particularly along ridges and hips and at valleys. However, the most durable flashing materials are sheet aluminum, copper or galvanized steel. All are sold in rolls especially for the purpose.Chimney flashing is usually in two parts: the base (or step) flashing, which wraps completely around the base of the chimney and extends several inches under the roof covering, and the cap (or counter) flashing, which covers the top edges of the base flashing. Sometimes roofing felt extends up the sides of the chimney, taking the place of metal base flashing.

Flashing should be inspected at least once a year, as it is a prime location for roof leaks. Look for cracks and separations where the flashing meets the chimney, vent stack, dormer and abutment walls, and where roof planes meet at valleys. Sometimes damaged flashing is discolored. Very old flashing sometimes develops pinholes which are hard to see. So if possible, check the roof from below for leaks.

To maintain and repair flashing, coat all flashing seams periodically with asphalt roofing cement, especially at chimney and vent stack seams. Apply the cement using a small mason's trowel and smooth the contours of the cement so that it does not form hollows and ridges where water may collect and eventually lead to leaks and damage.

Where you find holes of 1-square-inch or more in the flashing, cut a patch from the same material as the flashing, 1-inch larger all around than the hole. Apply cement to the damaged flashing, press the patch in place, then cover the entire area with cement and smooth the surface.

Where flashing meets brickwork it is usually embedded in mortar. Separations here require immediate repair since the loose flashing actually collects water and funnels it down beneath the roof where it may spread and do considerable damage.

To repair, provided the flashing itself is sound, rake out the old mortar from the seam to a depth of about three-fourths-inch. Press the flashing back into place, wedging it if necessary with small stones, then fill the seam with fresh mortar, using a trowel. Smooth the seam carefully. Seal the flashing with asphalt roofing cement after the mortar has fully cured. If the flashing is corroded or damaged, you will have to replace it.

Metals in contact with certain acid-containing woods, such as redwood and red cedar can also corrode. When purchasing flashing, exterior wood shingles or siding, and fasteners to match, check with your building materials supplier for advice.