Time-Tested Approach to Chimney Flashing
Proven methods and durable materials keep out water for decades.
Synopsis: Flashing a chimney correctly involves two layers of metal: flashing and counterflashing. In this article, roofing contractor Dyami Plotke explains how he tackles each step in the flashing process. He almost always chooses copper for chimney-flashing jobs because of its appearance, endurance, receptiveness to soldering, and malleability. A series of three detailed drawings demonstrates Plotke’s steps. The first shows where to cut a groove in the masonry for the counterflashing, how to apply a peel-and-stick underlayment over the OSB sheathing patch and up the sides of the chimney, and how to install the apron. The second shows how to install the step flashing and the pan and includes Plotke’s methods for cutting and folding the copper at transition points to make the tightest fit. The third shows how to install the counterflashing. Before sealing the counterflashing to the brick, Plotke uses wedges of folded copper to hold the counterflashing in its groove, and copper pop rivets to connect the counterflashing on the up-slope edge to the counterflashing on the sides. On the up-slope side of large chimneys, he installs a copper cricket to shed water and snow.
By the time my roofing company was called to take a look at the leaking chimney featured here, the sheathing around the chimney was rotten, and the roof rafters beneath were showing signs of water damage.
When the asphalt shingles on this roof were replaced several years ago, the roofer put a new layer of architectural shingles on top of the existing three-tab shingles and reused the house’s original aluminium chimney flashing. He patched several small leaks in the flashing corners with roofing cement, but didn’t touch the flashing otherwise. Had the roofer done a better job with the chimney flashing, the customer would have been spared the headache and…