Pieces of metal or roll roofing used to prevent seepage of water into a building around any intersection or projection in a roof such as vent pipes, chimneys, adjoining walls, dormers and valleys. Galvanized metal flashing should be minimum 26-gauge. There are 4 main types of flashing used in residential roofing systems:

• Valley flashing: This flashing is used in open valleys of the roof. Most often leaks are found in the valley flashings due to flashing that is nailed to tightly to the decking or shingles that are not trimmed far enough off the flashing.
• Plumbing vent flashing: Plumbing vent flashing prevents rainwater from running into holes cut for pipes in the roof. This flashing is sold according to the size of the vent pipe and the roof angle. Roofing material is installed over the flashing.
• Lead flashing: When working with tile roofs, lead flashing is used. In the case of a plumbing vent flashing, the lead flashing is actually molded to the shape of the tile's surface. Then the top of the lead flashing is covered by the next tile to prevent water from seeping under the flashing.
• Step flashing: When a chimney or dormer wall intercepts the slope of the roof, step flashing is used. Step flashing is usually a metal piece that is bent in the middle, so that one end lays on the roof, and the other against the vertical wall of the dormer or chimney.
• Flashing is one of the most important elements of the roof because it seals the seams and joints of the roof--the locations where leaks are most likely to occur. Often, flashing is not maintained well, or installed correctly in the first place. Check for the following signs that your flashing needs maintenance or repair:
• Rusting of metal flashing
• Excess leaves and debris in valleys or seams of the roof (can lead to rusting and corroding of the metal)
• Prolonged exposure to the elements such as moisture, UV rays, climate changes--especially when asphalt compounds or caulking material is used. Look for cracks, loss of elasticity and delamination.

In many cases the flashing can be cleaned and then repaired, relaminated or repainted (even in the case of rust). In other cases, the flashing may need to be replaced.