How does chimney flashing work?
Chimney flashing is two layers. Step flashing tucks under each shingle course and turns up the brick; counter flashing is cut into a mortar joint and laps down over it, so water is shed outward twice with nothing relying on sealant. Our published rate to re-flash a chimney is $600, done with the roof.
Every part of a chimney flashing, and what it does
| Part | What it does | How it fails |
|---|---|---|
| Apron flashing, downhill face | Sheds water arriving from above out onto the shingles below the chimney. | Too short a lap onto the shingle, or nailed through the face where the fastener sits in the water path. |
| Step flashing, both sides | One L-shaped piece per shingle course, woven so each turns water out onto the shingle below. There is no single long piece. | Replaced by one continuous bent strip and a bead of sealant. It looks tidier, it sheds water for about two seasons, and it is the most common wrong detail on any roof. |
| Counter flashing, cut into the masonry | A second layer let into a mortar joint or a saw-cut reglet, lapping down over the step flashing so water cannot get behind it. | Surface-mounted and caulked to the brick instead of cut in. The sealant fails, and then the wall is the flashing. |
| Cricket or saddle, uphill face | A small framed ridge behind a wide chimney that splits the water and sends it around both sides. | Absent. Code generally requires one behind chimneys wider than 30 inches, and without it the uphill face collects water, ice and debris. |
Every part of a correct chimney flashing works by lapping. Nothing in it depends on sealant to keep water out. If the detail on your roof relies on a bead of caulk, it has a service life measured in seasons, not decades.
The chimney flashing problems that cause leaks
- Reused flashing under a new roof. The single biggest one. A tear-off happens, the old flashing is left in place, and a twenty-year-old detail is now under a brand-new roof with a brand-new warranty that does not cover it.
- Continuous flashing instead of step flashing. One long bent strip up the side of the chimney with the shingles cut to it. Water gets behind it at the first course that lifts.
- Counter flashing caulked to the face of the brick. Not cut into a mortar joint, so the only thing holding the top edge closed is sealant with a five-to-ten-year life.
- No cricket behind a wide chimney. Water and ice pond on the uphill side, and debris packs in behind it until the flashing is sitting in a wet sponge.
- Failed crown or open mortar joints. Not flashing at all — water entering the top of the chimney and coming out inside. A flashing repair will not touch it, which is why a leak near a chimney should always include the crown in the diagnosis.
- A chimney that is no longer used. An abandoned flue with an open top is a hole in the roof with a rain collector on it. Capping it is cheap and it is often the whole answer.
What re-flashing costs, and how long it takes
Our published rate to re-flash a chimney — strip the old detail, form new step flashing course by course, cut counter flashing into the mortar joints and rebuild the lap — is $600 per chimney, and it is priced as part of a roof replacement rather than as a standalone visit. Adding a cricket where one is missing is framing and is quoted with the roof.
On time: re-flashing a chimney is most of a day of one person hand-forming and cutting, and it is done inside the roof replacement rather than after it. The reason it belongs with the roof is mechanical, not commercial. Step flashing has to be woven into the shingle courses as they go on. Doing it to a finished roof means lifting and re-sealing the courses beside the chimney, which is slower, more expensive, and leaves a repair rather than a detail.
A masonry chimney over about fifteen years old nearly always needs the flashing remade when the roof is replaced, and our price book treats it as a discovered condition assessed at inspection with a published rate rather than a negotiation on the day. That is deliberate: a number you were told at the start is not an upsell, and a number invented on a roof is.
Flashing that looks fine from a ladder is the normal state of flashing that is about to leak, because the failure is behind it. A quote for a full roof that has no line for the chimney detail is a quote that has not decided what it is doing there, and it will be decided on the day at a price nobody agreed.
Ask instead: Is new chimney flashing in this price? Show me the line.Sealant around a chimney is a maintenance measure with a couple of seasons in it. It is a legitimate short-term stop-gap and it is not a flashing repair, and the difference should be stated rather than blurred.
Ask instead: Are you cutting new counter flashing into the mortar joint, or sealing over the existing?Chimney flashing questions
How long does it take to replace chimney flashing?
Most of a working day for one person on a typical masonry chimney, and it is done as part of the roof replacement rather than as a separate visit. The work is hand-forming a step flashing for every shingle course, cutting a reglet or raking out a mortar joint for the counter flashing, setting and sealing it into the masonry, and rebuilding the lap onto the shingles. A wide chimney that also needs a cricket framed adds time for the carpentry.
How should chimney flashing be installed?
In two independent layers. First, step flashing: one L-shaped piece per shingle course, each turned up the brick and lapped over the piece below, woven into the courses as the roof goes on so no fastener sits in the water path. Second, counter flashing: a separate metal let into a saw-cut reglet or a raked mortar joint, folded down over the top of the step flashing. Behind a chimney wider than about 30 inches, a cricket is framed to split water around it. Nothing in the detail should depend on sealant to be watertight.
How long does chimney flashing last?
Correctly installed aluminum or galvanized step and counter flashing generally lasts as long as the roof it was installed with, so twenty to thirty years, and copper lasts longer than the roof. What shortens it dramatically is being reused under a new roof, being surface-mounted and caulked rather than cut into the masonry, or sitting behind a chimney with no cricket where water and ice pond against it. The failure is almost never the metal itself.
What does chimney flashing cost to replace?
Our published price book rate is $600 per chimney, quoted as part of a roof replacement. It covers stripping the old detail, forming new step flashing course by course, cutting new counter flashing into the mortar joints and rebuilding the lap onto the shingles. Framing a cricket where one is missing is separate carpentry and is quoted from the chimney width. It is assessed at the free inspection and appears on the estimate as its own line rather than arriving as a surprise once the roof is open.
Is water coming in near the chimney always the flashing?
No, and assuming so is how a leak gets chased for a year. Three other candidates are common. The chimney crown can be cracked, letting water down the outside of the flue and into the house below the roof line. Mortar joints in the stack can be open, in which case the brick itself is absorbing water. And an unused flue with no cap is simply an open pipe. A leak near a chimney should be diagnosed with the crown, the joints and the cap in the picture, not just the metal.
Where to go next
Flashing is one detail among several that decide whether a roof leaks. These are the neighbours.
Roof flashing and penetrations
Where leaks actually come from. Every flashing part named, and what fails in each.
Read the standard →ThesisWhy a roof fails at its details, not its field
Roofs rarely fail in the middle of a plane. They fail at edges, valleys, walls and penetrations.
Read the standard →LeaksWho to call about a possible roof leak
Where roof leaks actually come from, what fixing one costs, and when it means a new roof.
Read the guide →EstimateHow to read a roofing estimate
What the price is made of, the seven places a cheap bid gets cheap, and how to compare two quotes line by line.
Read the standard →