How do you fix a chimney leak?
Short answer. To fix a chimney leak, first find the source with a controlled water test, then repair that part: rebuild failed step and counterflashing, add a cricket if water dams behind the chimney, seal or rebuild a cracked crown, install a cap, or treat porous brick with a breathable water repellent. Caulking around the base without finding the source rarely holds for long.
Step 1: Find the source
- Let everything dry out first.
- With one person inside watching the attic or ceiling and one outside with a hose, wet the chimney in sections, starting low and moving up, about 10 to 15 minutes per section.
- Start with the downhill flashing, then the sides, then the uphill side, then the brick, then the crown and cap.
- When water shows inside, you have found the zone.
Step 2: Fix that part
- Flashing. The durable fix is new step flashing woven into each shingle course, with counterflashing cut into the mortar joints over it. Surface sealant over old flashing is temporary.
- Missing cricket. On chimneys more than 30 inches wide, the residential code calls for a cricket on the uphill side. Building one stops water from damming.
- Crown. Seal hairline cracks, or rebuild a broken crown with an overhang.
- Cap. Install one if rain is falling into the flue.
- Brick. Repoint failing mortar and apply a breathable siloxane water repellent. Avoid paint or non-breathable coatings.
- Chase cover. Replace a rusted flat cover on a framed chimney.
Step 3: Repair the damage
Once the leak is stopped, check the deck and framing around the chimney for rot, dry or replace wet insulation, and repair the drywall. If the leak followed a storm, photograph everything before repairs and call your insurer. See chimney leaks and crickets and roof flashing explained.