What you’ll need:
TOOLS
- Power drill
- Hammer
- Snips
- Masonry drill
- Chalk or pencil
- Scraper
- Tape measure
- Chisel (optional)
- Pry bar (optional)
SUPPLIES
- Sheet metal
- Caulk
- Galvanized roofing nails
- Foam closure strips
- Roof sealant (optional)
At the base of most chimneys, hidden in plain sight, is a thin metal border connected to the roof called chimney flashing. Flashing creates a waterproof seal between the chimney and the roof to prevent water pooling. This job isn’t for everyone—we recommend hiring a chimney repair pro to complete this job. But if you have roofing experience, here’s how to flash a chimney on a metal roof.
Preparing to Flash a Chimney on a Metal Roof
Before bringing all your tools and supplies up to the roof, there are a few preliminary steps to cross off your list:
- Assess the state of the chimney: Contact a local chimney inspector to check how weathered your chimney is and determine if any repairs besides new flashing are needed. They can spot issues right away, provide estimates on repairs, and help you decide if you can do the job yourself.
- Choose a flashing material: Homeowners with metal roofs tend to use the same material for flashing. Galvanized steel is the cheapest option at $0.50 per linear foot, followed by aluminum at $0.75 per linear foot, and copper at $2 to $3 per linear foot.
How to Flash a Chimney on a Metal Roof in 7 Steps
Photo: Jasmin Merdan / Moment / Getty Images
Working on your roof can be dangerous, so take precautions to complete this job safely before starting your project.
- Remove the Old FlashingUnless 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.
- Measure the ChimneyUse 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 and Attach the Apron FlashingCut 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 and Attach the Side FlashingCut 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.
- Cut and Attach the Pan FlashingPan 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.
- Cut and Attach the Cap FlashingCap 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 the Flashing With CaulkSeal 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.
What Are the Benefits of Flashing a Chimney?
Flashing is an integral part of chimney construction and maintenance because it provides multiple benefits:
- Waterproofing: Flashing prevents water from leaking inside.
- Durability: By guarding against water damage, flashing a chimney extends its life span.
- Energy efficiency: Cracks and leaks are detrimental to chimney insulation.
DIY vs. Hiring a Pro
Homeowners who are especially handy may be able to flash a chimney themselves. However, we strongly recommend that most people hire a professional roofer or a chimney repair specialist. Installing chimney flashing requires immense precision when cutting and assembling materials, as even slight errors can render flashing useless against the elements. Additionally, flashing a chimney means working at a dangerous height on a rooftop, and metal roofs are difficult to stand on.
Instead, hiring a professional ensures that the job is done safely. The downside, of course, is the added expense of labor. On average, replacing chimney flashing costs between $400 and $1,600. Prices vary based on the size of the chimney, the materials used, and the style of flashing. The biggest benefit of DIY flashing is that it costs much less since the only expenses are tools and supplies.
