Roof flashing: what it is, and why it is where roofs leak

What flashing actually is

Flashing is thin sheet metal — usually aluminum, galvanized steel or copper — shaped to bridge a junction. Wherever the roof plane meets a chimney, a wall, a skylight, a vent pipe or another roof plane, there is a gap that shingles cannot close on their own. Flashing closes it by directing water back onto the roof surface rather than letting it run into the join.

It is not glamorous and it is not expensive as a material. It is simply the part that decides whether the roof works.

The types you will hear named

Step flashing runs up a wall or chimney side in overlapping pieces, one per shingle course, each stepped over the last. It is the correct way to handle a roof-to-wall junction and it is fiddly, which is why it is sometimes replaced with a single bent strip and a lot of sealant.

Counter flashing sits above step flashing, set into the masonry, so water cannot get behind the step pieces. Valley flashing lines the channel where two roof planes meet. Drip edge runs along the eaves and rakes. And vent boots — the rubber collars around plumbing vents — do the same job at a much smaller scale.

Why it is the most common cause of a leak

On a roof that has been replaced once or twice, the flashing is frequently the oldest thing up there. Stripping and replacing it properly takes time, and reusing it saves an hour and a small amount of material. So it gets reused — and a year or three later the homeowner has a stain on a ceiling under a roof that is technically new.

That is why new flashing is a separate line on our quotes rather than something folded into a price per square. If a replacement quote does not mention flashing at all, it is worth asking directly whether it is being replaced or reused.

How it fails

Sealant-dependent repairs are the clearest warning sign. Sealant has a service life measured in a few years; correctly installed flashing does not depend on it. If a previous repair is a bead of caulk over a junction, it is a countdown rather than a fix.

Beyond that: corrosion on older galvanized pieces, movement where a chimney and a roof settle at different rates, counter flashing that was surface-mounted rather than set into the mortar, and step flashing replaced with a single continuous strip that cannot cope with the shingle courses.

What good work looks like

New flashing rather than resealed flashing. Step flashing in proper stepped pieces at walls and chimneys. Counter flashing let into the masonry. Valleys lined, not shingled over. And ice-and-water shield behind the details that matter, which in Maryland means the eaves and every penetration, because freeze-thaw drives water back uphill.

You should be shown photographs of the flashing before it is covered. On our jobs you are, because the deck and the details are photographed as a matter of course.