How to Tell If Your Roof Flashing Has Failed

Ask any honest roofer where leaks really come from and the answer is the same: flashing, not shingles. The metal at every transition is the unsung hero of a dry house, and when it fails it fails quietly. Here is how to spot it before the water finds your ceiling. From Paragon Roofing BC, family owned since 2010.

HS
Harman Singh — Senior Roofing Specialist
2026-06-21 | Updated 2026
Quick answer
Tell if flashing has failed by checking the transitions where the roof meets walls, chimneys, skylights, and valleys, the spots where most leaks begin. Look for rust, lifted or loose metal, cracked sealant, crumbling chimney mortar, and split pipe boots. Inside, stains that cluster at a skylight's corners or beside the chimney point to flashing failure. Caught early, it is usually a targeted repair.
Number one
Flashing is the top leak source
Transitions
Where the roof meets everything else
Quiet failure
It leaks before it looks dramatic

Here is a thing most homeowners never learn until a stain appears. The big open field of shingles is rarely what leaks. The leaks happen at the seams, the transitions, the awkward joints where the roof meets a wall, a chimney, a skylight, a valley. And the thing protecting every one of those joints is flashing, the bent metal that bridges the gap and sends water back where it belongs. Flashing is the hardest working, least appreciated part of your roof. When it goes, the roof can look perfectly fine from the curb while water pours in behind it.

Know where to look

You cannot inspect flashing if you do not know where it lives. It is wherever the roof plane is interrupted. Around the base of the chimney. Up the sides of skylights. Down the valleys where two slopes meet. Along any wall that a roof butts into. At plumbing stacks and vents, where a collar or boot seals the pipe. Those are the addresses. Every one of them is a place where water is being actively redirected, and every one is a place where, given enough years, the redirection can fail.

The signs of failed flashing

Sign Where What it tells you
Rust or corrosion Any metal flashing Metal is degrading, integrity going
Lifted or loose metal Wall and chimney lines Wind or movement has broken the seal
Cracked or missing sealant Edges and laps The weak point sealant covered is now open
Cracked mortar at chimney Chimney counterflashing Water path opening behind the metal
Split or perished pipe boots Plumbing stacks Rubber collar has aged out, classic leak
Interior stains at a feature Below skylights, chimneys Flashing at that feature is letting go

The inside tell

You do not always have to be on the roof to catch failing flashing. The attic and ceiling will often tell you. A water stain that appears specifically below a skylight, hugging its corners, or in the ceiling beside the chimney, is flashing talking to you. Up in the attic, look for dark trails on the wood right around where a chimney or vent passes through the deck. Because flashing surrounds these features, its failure leaves a signature: leaks that cluster at the penetrations rather than out in the open field.

Why flashing fails, and what to do

Age and weather
Sealant dries and cracks, metal corrodes, mortar fails. Time is the usual culprit, and our wet climate speeds it up.
Bad original detailing
A lot of flashing fails because it was rushed or skimped at install. Corners cut at a transition leak for the life of the roof.
Movement
Buildings shift, chimneys settle, metal lifts. The seal that was tight at install works loose over the years.
What to do
Caught early, failed flashing is often a targeted repair. Left long, it rots the deck and framing behind it. Sooner is cheaper.

Flashing is where craft shows. Good flashing, properly detailed in the right gauge metal, quietly keeps a house dry for decades. Cheap or rushed flashing leaks no matter how good the shingles are. If you are seeing rust, lifted metal, cracked sealant, or stains clustering at your chimney and skylights, get the flashing looked at before the water reaches your structure. We detail and repair flashing across the Lower Mainland, and we do it in heavier gauge metal because it lasts. See our roof repair and leak detection pages.

Frequently Asked Questions

Why is flashing the most common cause of roof leaks?

Because it guards the hardest spots. The open field of shingles sheds water easily, but every transition, where the roof meets a wall, chimney, skylight, or valley, is a vulnerable joint, and flashing is the metal bridging those joints. Those are the places water is actively redirected, so they are the places most likely to fail over time. Get the flashing right and you have solved most of what makes roofs leak.

What does failed flashing look like?

Rust or corrosion on the metal, sections that have lifted or come loose along wall and chimney lines, cracked or missing sealant at the edges, crumbling mortar where chimney flashing tucks in, and split or perished rubber boots at plumbing stacks. Inside, it shows as stains that cluster right at a skylight's corners or beside the chimney rather than out in the open ceiling.

Can I see flashing problems from inside?

Often, yes. Stains that appear specifically below a skylight or beside a chimney are classic flashing tells, because the flashing surrounds those features. In the attic, dark trails on the wood right where a chimney or vent passes through the deck point to flashing failure at that penetration. Leaks that cluster at features rather than in the open field usually mean flashing, not shingles.

Is failed flashing a repair or a replacement?

Usually a repair if you catch it early. Re detailing or replacing flashing at a chimney, skylight, valley, or wall is often a targeted fix that does not require a new roof. Left too long, though, the water gets behind it and rots the deck and framing, which turns a small repair into a much bigger one. The sooner you address it, the smaller the job stays.

Why does new flashing sometimes fail too?

Often because it was rushed or skimped at installation. Flashing is detail intensive work, and corners cut at a transition will leak for the life of the roof no matter how good the shingles are. Thin metal, sloppy laps, and over reliance on sealant instead of proper metal detailing are common culprits. We flash in heavier gauge metal and detail it properly because that is what actually lasts.

What gauge of flashing should be used?

Heavier is better. Thin flashing metal corrodes and deforms sooner, especially in our wet climate, so we use a heavier gauge than the minimum as our standard call, because flashing is exactly the part of the roof you do not want to skimp on. Properly detailed flashing in solid metal is what keeps the transitions dry for the long life of the roof.

Stains at Your Chimney or Skylight? Check the Flashing.

We diagnose and repair flashing across the Lower Mainland, detailed properly in heavier gauge metal so it lasts. Catch it early, before the water reaches your framing. 20 year workmanship warranty on repairs.

Book Free Estimate Roof Repair Call us any time: 604‑358‑3436
HS
Harman Singh
Senior Roofing Specialist & Project Manager — Paragon Roofing BC
CertainTeed Master Roofer IKO Master Roofer BC Licensed Contractor

15+ years of Lower Mainland roofing experience. Harman leads project teams across Metro Vancouver and the Fraser Valley with a focus on institutional quality work for residential, strata, and commercial properties. Direct line: 604‑358‑3436.

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