Time-Tested Approach to Chimney Flashing
Roofing contractor Dyami Plotke demonstrates how to prepare a chimney for flashing and how to bend, install, and seal metal flashing for a durable, weatherproof, and beautiful job.
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 expense of replacing the chimney flashing and patching the roof a few years later.
Copper flashing is worth the money
We almost always replace aluminium flashing with copper. Of course, copper costs more than aluminium, but it’s the superior material for several reasons. For starters, it looks good, it lasts almost forever, and it solders great. More important, though, it’s more malleable than aluminium. The flashing pieces can be formed by eye at the metal brake and can be adjusted easily by hand on the roof. When it’s time for new shingles, the flashings and counter flashings can be bent out of the way and repositioned without damage.
Flashing and counter flashing work together
Flashing a chimney correctly involves two layers of water-shedding metal: flashing and counterflashing. The front of the chimney has a single piece of flashing, the apron, as the first layer; the back has a similar piece called the pan. The step flashing, which is the first layer on the sides of the chimney, is made from L-shaped pieces of copper lapped so that they shed water running down the roof. The horizontal leg goes under the shingles, and the vertical goes up the sides of the chimney. Ideally, neither leg is fastened; nail or screw holes compromise the water tightness of the flashing.
Because there are no nails or screws to hold the vertical leg tight to the chimney, water running down the masonry can get past the step flashing and leak into the house. To prevent this, the flashing’s vertical legs are covered with counterflashing. The counterflashing directs falling rain and water running down the chimney over the first layer of flashing. This creates a finished assembly that looks good and, more important, is watertight.
As was the case on this job, original counter flashings are often installed in a stepped pattern following the mortar lines of the brick. We generally don’t install new flashing in mortar joints. Instead, we cut a 1 3⁄4-in.-deep groove about 6 in. above the roof deck all the way around the chimney with an angle grinder. Then we install a single piece of counter flashing into the groove.
From Fine Homebuilding #233