1. Flashing
The metal seal where the chimney meets the roof is the most common source. Flashing that is only caulked to the brick, or that was reused when the roof was replaced, fails first. Stains usually show on the ceiling or wall right next to the chimney. Repair from $250, replacement from $1,200.
2. The crown
Cracks in the concrete top let water soak into the brick and run down inside the chimney. You often see staining or flaking brick near the top. Crown repair from $500, rebuild from $1,200.
3. A missing or damaged cap
Without a cap, rain falls straight down the flue. Water shows up in the firebox or on the damper. Cap replacement from $350.
4. The chase cover
On sided chimneys, a rusted or flat chase cover pools water and leaks into the framing. Look for rust streaks on the siding of the chase. See chase cover replacement.
5. The brick and mortar
Porous brick and open mortar joints soak up water in every storm, especially on older chimneys. Tuckpointing and a breathable water repellent fix it. Brick replacement runs $25 to $50 per brick.
How to narrow it down
Note when it leaks: in every rain, only in heavy rain, or only during snow melt. Note where it shows up: the ceiling, the firebox or the attic. Then have the chimney checked from the roof, where all five sources can be seen. More on chimney leak repair.