Flashing is a thin layer of waterproof material that is installed to prevent water from getting into places it doesnt belong). It is typically made of metal, such as lead, aluminum, copper, stainless steel, or zinc alloy, and is installed around discontinuities or objects that protrude from the roof of a building to deflect water away from seams or joints and in valleys where the runoff is concentrated). Flashing is essential to prevent long-term water damage to a homes siding and framing where the exterior deck meets the side of the house. It is also needed in a few areas on a roof that are especially vulnerable to leaks, including vents, valleys, and drip edges. Properly installed flashing is essential to its performance, and poor installation could lead to roof deterioration, water penetration, and more long-term issues. Flashing is usually layered with other building materials, and the upper edge of flashing is protected by house wrap or tarpaper, not installed over it.