How to Flash a Chimney and Prevent Leaks in Your Metal Roof

Unless your chimney never had flashing to begin with, you’ll have to remove the old flashing before installing new pieces. You might be able to remove flashing with the back of a hammer. Otherwise, we recommend using a chisel or pry bar to remove the old pieces. Once that’s done, use a scraper and brush to clear away any tar, loose brick, or other debris.

Use a tape measure to find the dimensions of your chimney. Length and width are important, but height isn’t as much since flashing only goes a few inches high on a chimney. These figures will help you gauge the size needed for the flashing; most pieces should be a couple of inches longer than the chimney.

Cut a piece of apron flashing for the front of the chimney. Before installing the base flashing, attach a foam closure strip the same length as your base flashing to the roof. This helps the base flashing lay flat, which is the trickiest part of working on a corrugated metal roof. Use a power drill to screw the base flashing on top of the foam closure strip. Make sure the corners of the base flashing are folded around the chimney.

Cut two identical pieces of side flashing. They should be a few inches longer than the chimney because they must fold around the corners. Also called connection flashing, these pieces go along opposite sides of the chimney. Screw the side flashing into the roof. For extra stability, use a roofing sealant to adhere the side flashing to the apron flashing. 

Pan flashing goes at the back of the chimney, or the highest end. It’s the final piece of base flashing needed. Place a foam closure strip down first, then screw in the pan flashing. You’ll need to snip the edges so that it wraps around the corners of the chimney and lays flat against the side flashing. 

Cap flashing, also called counter flashing, comprises all of the upper components. Four pieces of cap flashing are needed to cover the base flashing. Screw the cap flashing into the base flashing and the chimney itself. To fully protect against rain, the top of the cap flashing should be folded into the chimney’s mortar joints. For the side pieces, use a masonry saw to cut a diagonal line into the chimney and then fold the top of the cap flashing pieces into that line. 

Seal all the joints with chimney-appropriate caulk to keep precipitation out, then use a finishing tool to smooth it out. Make sure that the caulk is waterproof and designed to adhere to metal. For aesthetic purposes, many people prefer using caulk that’s the same color as the flashing.