Flashing, kerb height, debris damming above it — or condensation, which is not a leak at all. How to tell which one you have.
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Flashing, chimneys and skylights: the joints that leak
Flashing is the thin metal that makes two different materials behave like one surface. When a roof leaks, the flashing is the first suspect and usually the right one.
Every leak has a reason to be exactly where it is
If you remember one thing from this site, make it this: roofs leak at interruptions. A plain slope of shingles sheds water almost indefinitely. It is the chimney coming through it, the skylight set into it, the plumbing vent poking out of it, the valley where two planes meet and the wall that one edge runs into where the trouble lives. Every one of those is solved with flashing: shaped metal that laps under the covering above and over the covering below, so that water is always handed downhill.
Flashing fails in ways that are worth recognizing. Sometimes the metal itself corrodes or tears. Far more often the metal is fine and somebody tried to solve a flashing problem with sealant. A bead of roofing cement smeared around a chimney will hold for a season or two, then crack as the materials expand and contract at different rates. And then hold water against the joint rather than shedding it. A chimney that has been repeatedly caulked is a chimney whose flashing has been failing for years.
Plumbing vent boots are the quiet one. The rubber collar around the pipe perishes in ultraviolet light on a predictable schedule (roughly a decade, sooner on a hot southern exposure) and splits. It is a small, cheap component that causes a large number of the leaks in this section, and from the ground it looks like nothing at all. Skylights get blamed constantly, and the glass unit itself is usually innocent; it is the kerb and the flashing around it. Or in a cold climate condensation on the inside of the frame, which people reasonably mistake for a leak.
Chimneys deserve their own attention because they have several separate things that can let water in: the flashing where the masonry meets the roof, the crown on top, the cap over the flue, and the masonry joints themselves. They are also the one place on the roof where a wide chimney on a slope needs a cricket. A small ridged structure that splits the water and sends it around instead of letting it pond against the back.
This section is about recognizing which of these you are dealing with from the evidence available to you. And knowing what a competent repair looks like so you can tell it from a smear of tar.
Flashing guides
Flashing, crown, cap or the masonry itself. They leak at different times and in different weather, which is how you separate them.
A cricket splits water and debris around a wide chimney. How to tell whether yours has one, and whether it is failing.
One missing piece of bent metal at the bottom of a roof-to-wall junction can send years of runoff behind the siding.
The crown cracks on a predictable schedule, and it is masonry work, not a roofing repair. What it looks like before it fails completely.
A failing chase cover on a framed chimney chase rarely leaks on the roof. It shows up as a stain near the fireplace instead.
A valley can be built three different ways, and the choice decides what you can see from the ground years later.
Six different joints share this pillar. How to recognize the pattern and work out which one is failing.
Corrosion, physical damage, and undersized laps all fail differently. Why the material matters as much as the age.
Sequencing, materials, and the shortcuts that show up as a leak again within a year. What a legitimate job looks like.
Droplets on the glass are usually humidity. Where the water sits and when it appears separate the two.
Open cap, cracked crown, failed flashing or a cold flue. Each leaves different evidence you can read from the ground.
The main types, where they sit and how to spot trouble from the ground.
Flashing, crown, cap and masonry: what keeps a chimney dry and what a coating cannot fix.
The metal channel where two slopes meet, why it wears first and what to check from the ground.
What sealant can slow, what it cannot fix and how to spot an old patch from the ground.
The flashed collar around a vent pipe, how it fails and what to check from the ground.
The staircase of small metal pieces along the sides of a chimney, how it fails and what to check from the ground.
The upper metal layer that covers the joint, how it differs from base flashing and how it fails.
What sealant on the top of a chimney can close, and when it only hides a failing crown or cap.
The lower metal layer along a wall or chimney, how it differs from cap flashing and how it fails.
Why lead is used around chimneys, how it slumps and splits, and what to check from the ground.
How flashing should be layered and how to judge a roofer's work, without climbing up there.
The small round skylight that pipes daylight into a room, and how to tell a leak from condensation.
The upper metal cover over the lower flashing, how it differs from cap flashing and how it fails.
The row of small bent metal pieces where a roof meets a wall, and how to spot when one has failed.
The layered metal system around a chimney, what each part does and where leaks start.
The metal at porch roofs, additions and slope changes, and why leaks start there.
Step, base, counter, valley, drip edge and vent boot, and which joint each one seals.
How to find the failed joint from the ground and brief a roofer, without climbing up there.
The metal frame around a roof window, why the corners fail and what to check from the ground.
The layer order at the eaves and rakes, and how to judge the edge metal from the ground.
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.