How to Repair Flashing Around a Chimney

Replace damaged chimney flashing by removing old material, cleaning the area thoroughly, and installing new step flashing along the sides with counter flashing embedded into mortar joints.

1. Know What You're Fixing. Inspect the flashing from inside your attic and outside on the roof. Look for rust holes, separated joints, or loose sections. Measure the chimney perimeter to determine how much flashing material you need. Purchase galvanized steel, aluminum, or copper flashing that matches your existing material, along with roofing cement, caulk, and replacement fasteners.
2. Strip Away the Damage. Carefully pry up shingles around the damaged flashing using a flat pry bar. Remove old roofing cement and pull out damaged flashing pieces. For counter flashing embedded in mortar joints, use a chisel and hammer to carefully remove old material without damaging the brick or stone. Clean all surfaces thoroughly with a wire brush.
3. Layer the Side Flashing. Cut individual step flashing pieces that are 2 inches longer than your shingle exposure and 7-10 inches wide. Bend each piece at a 90-degree angle to match the roof slope and chimney wall. Install one piece under each shingle course, overlapping each piece by at least 2 inches. Secure with roofing nails driven through the flashing into the roof deck, not the chimney.
4. Wrap the Front and Back. Cut a wide piece of flashing for the front downhill side of the chimney, forming it to cover the full width plus 6 inches on each side. Install it under the shingles and over the step flashing. For the back uphill side, install the apron flashing under the shingles and ensure it extends at least 12 inches up the roof slope to prevent water backup.
5. Anchor the Counter Flashing. Cut counter flashing pieces that will extend 4 inches down over the step flashing and 1.5 inches into the mortar joints. Use a masonry bit to drill holes in the mortar joints, then insert the flashing and secure with masonry anchors. Seal the top edge with high-quality exterior caulk or roofing cement, creating a smooth transition that sheds water away from the joint.
6. Lock Out the Water. Apply roofing cement to all nail heads and overlapping flashing joints. Run a continuous bead of high-grade exterior caulk along the top edge where counter flashing meets the chimney. Check that all shingles lay flat against the flashing without gaps. Apply additional roofing cement under any lifted shingle edges to create a watertight seal.