Chimney flashing is a construction detail used to seal and protect the joints between a buildings roof and chimney from water penetration. The joints created by the intersection of the roof and chimney are among the most vulnerable areas of roofing systems. They constantly expand and contract in response to changes in humidity, temperature, and age. The greater the number of such projections, the greater the potential for serious leaks. Flashing is used at these intersections to keep rainwater from leaking into the building.