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.
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.