What is chimney flashing? The metal that seals the chimney joint
Chimney flashing is the layered metal that keeps rain out where masonry meets the roof. Most chimney leaks start here, not in the bricks.
Chimney flashing is the set of metal pieces that seals the joint where a chimney passes through the roof. It is not one part. It is a layered system, and each layer has a different job.
The parts of chimney flashing
Think of it as a shirt tucked into trousers. The lower layer is tucked under the shingles. The upper layer hangs over it from the masonry. Water has to run over both.
- Apron flashing on the downhill face of the chimney.
- Step flashing up each side.
- A cricket (a small peaked ramp) behind a wide chimney to push water around it.
- Counter flashing set into the mortar joints, covering the top of the lower pieces.
Why the chimney is such a common leak
Brick and mortar move differently from the roof. The two expand and settle at different rates, so the joint is always working. A chimney is also a wall that sticks up into the weather, so rain hits it from the side as well as from above. That is a lot to ask of a few strips of metal.
How chimney flashing fails
- Counter flashing that has rusted or pulled out of crumbling mortar.
- Step pieces that were bent into one strip instead of overlapped.
- No cricket on a wide chimney, so water pools behind it.
- Thick tar or sealant along the joint, covering an earlier failure.
If you can see a wide black bead around the chimney base, assume the metal underneath has had a problem. Sealant is a cover, not a fix.
What a leak looks like
Stains show up on the ceiling or wall near the chimney, or around the fireplace itself. They often appear after wind-driven rain and not after gentle rain. That pattern points at the joint rather than the open roof. Rain can also come down the flue, which is a different cause, covered in water coming down the chimney.
What to check from the ground
Use binoculars from the yard. Look at all four sides of the chimney base. You want to see metal that is straight, tight to the brick and overlapped, not smeared with tar. Look for rust streaks and gaps along the top edge of the metal.
Inside, check the ceiling around the chimney and the attic framing beside it with a flashlight from the joists.
Why we do not show the repair
Redoing chimney flashing means cutting into mortar and working at the edge of the roof beside a tall masonry column. We do not publish those steps. Take photos, note the stained room and ask a roofer which layer failed and why. If a quote says only "reseal the chimney," ask for the layers to be named.
Frequently asked questions
What is chimney flashing?
It is the layered metal system that seals the joint where a chimney passes through the roof, made of apron, step, cricket and counter flashing pieces.
Why do chimneys leak so often?
Brick and roof move at different rates, and the chimney catches rain from the side. The joint is always under stress.
Is chimney flashing the same as counter flashing?
No. Counter flashing is only the upper layer set into the mortar. Chimney flashing is the whole system.
Is a bead of sealant a proper fix?
No. Sealant splits within a few seasons and often hides a failed metal layer underneath.
How can I check chimney flashing safely?
Use binoculars from the yard to look at all four sides, and check the ceiling and attic framing from inside. Do not go on the roof.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NRCA. National Roofing Contractors Association
- US OSHA. 1926.501 — Duty to have fall protection (OSHA)