Flashing a Chimney by Jonathan Poore Flashing makes watertight joints at junctions between roof and walls, around chimneys, skylights, vent pipes, and in valleys and hips where two planes of a roof meet. Often, the flashing develops leaks before the roofing material does. This detail shows the most complex flashing problem: making a watertight joint between a chimney and a shingle roof. Chimney flashing is often damaged, badly installed, or missing altogether. Installing proper chimney flashing is within the capability of the competent non-specialist. Although special roofer's tools make the job go faster, the procedures described here can be done with ordinary carpentry tools. Layout. Study this series of drawings to determine what measurements you'll need. If you have an existing chimney flashing that was done