Flashing materials can be found in many external areas of a house such as roofs, chimneys and porches. Any structure or protrusion that rises through or joins to the roof will have a flashing. Chimney, dormer window, skylight, vents and roof valleys will all have a flashing where they join, or disappear through, the roof.
Roof flashing is fitted primarily to maintain the watertight integrity of the roof. Without flashing fitted rain water would run down the chimney or the higher area of roof tiles and just disappear into the house interior, where the chimney rises through the roof. Similarly, on a house with a pitched roof that might contain a dormer window, where the two roofs join on both sides are called the valleys. Without a flashing built into these valleys, rain water running down both roofs would pour through into the house. The flashing ensures the joints remain watertight.