How do you fix chimney flashing?
Short answer. A lasting chimney flashing repair replaces the layers rather than caulking them: a new apron across the front, new step flashing woven into each shingle course up the sides, a back pan or cricket on the uphill side, and counterflashing cut into a mortar joint and lapped over everything below. Sealant plays a small supporting role at the mortar groove and corners, never the main one.
The repair, in order
- Open it up. Unseal and remove the shingles around the chimney, pull the old flashing, and inspect the decking for rot. Replace soft wood now.
- Underlayment. Run self-adhering membrane onto the deck and a few inches up the chimney on all sides.
- Apron. Install the front apron over the shingles below the chimney, bent up the brick face.
- Step flashing. Weave a piece in with each shingle course up both sides, each piece over the shingle below and under the one above, nailed to the deck only.
- Back pan or cricket. On the uphill side, a back pan for a narrow chimney, or a framed cricket covered in membrane and metal or shingles for a chimney wider than 30 inches.
- Counterflashing. Grind a groove into a mortar joint, insert the bent top edge of the counterflashing, and seal the joint. Stepped pieces follow the brick courses up the sides.
- Reshingle and seal the shingles back down.
Common mistakes
- Counterflashing glued or nailed to the brick face with caulk along the top.
- One long bent strip up the side instead of individual step pieces.
- Nails through the step flashing low in the water path.
- Skipping the cricket on a wide chimney.
- Bare aluminum embedded in mortar, or copper mixed with galvanized steel.
When the problem is not the flashing
If water shows up in the firebox, look at the cap and crown. If the chimney breast is damp after driving rain, look at the brick and mortar. A hose test, one side at a time, confirms the source before you open anything up. See chimney leaks and crickets and roof flashing explained.