What flashing is, and why leaks often start there

When a ceiling stains, most people assume the shingles have failed. Sometimes they have. Often, though, a leak starts at a join: a place where the roof surface stops and something else begins, such as a wall, a chimney, a skylight, a pipe or another roof. The material that keeps water out at those joins is called flashing.

Shingles on an open slope shed water easily. Joins are harder. Water collects against them, runs along them and is driven into them by wind. Flashing turns that water back onto the roof surface, and when it fails, water goes where it should not.

What flashing is made of

Most flashing is thin sheet metal, usually aluminum, galvanized steel or copper. Some is lead, rubber or a flexible membrane. It is bent to fit the join and layered with the shingles so that water always runs over a lap rather than into one. The idea is the same as overlapping boards on a shed wall: each piece sheds water onto the piece below it.

Where it sits

A typical house has more flashing than people expect. It falls into three groups.

Where a roof meets a wall

• Step flashing, where a sloping roof runs along a side wall. Small bent pieces are woven in with each row of shingles.
• Apron flashing, also called headwall flashing, where the top of a roof plane meets a wall.
• Counter flashing, a second layer fixed to the wall or set into a chimney that covers the top of the step or apron flashing.
• Kickout flashing, a small angled piece where a roof edge ends against a wall. It throws water into the gutter instead of down the siding.

Around chimneys and openings

• Chimney flashing, which combines step, apron and counter flashing. A wide chimney often has a small peaked section called a cricket on its uphill side to split the water around it.
• Pipe boots, the rubber or metal collars that seal around plumbing vents.
• Skylight flashing, usually a kit made for that particular skylight.

Along valleys and edges

• Valley flashing, a metal or membrane lining where two planes drain toward each other.
• Drip edge, a metal strip along the eaves and rakes that keeps water off the fascia and the edge of the decking.

How it fails

Flashing rarely fails all at once. Usually it is one of a handful of slow problems.

Rubber pipe boots dry out and crack in the sun, and on many roofs they are among the first parts to go, often years before the shingles. Sealant used in place of proper flashing shrinks and pulls away. A nail driven through the wrong part of a flashing piece leaves a hole where water runs. Counter flashing set into mortar works loose as the mortar ages. And flashing is sometimes reused during a reroof, so a new roof ends up with old metal at its weakest points.

Missing kickout flashing is a small omission with expensive results. Without it, water from the roof edge can run down the wall behind the siding. The damage then shows up inside the wall rather than on a ceiling, sometimes years later.

Why the stain is not where the leak is

Water that gets past flashing rarely drops straight down. It runs along the decking, down a rafter or across the top of a ceiling before it finds a way through. A stain in the middle of a bedroom can start at a chimney ten feet away. That is why patching the roof directly above a stain often fails to stop a leak.

What you can check from the ground

You can learn a lot without going up, and you should never climb onto the roof to look. Standing in the yard or at an upstairs window, with binoculars for the details, check these things.

1. Look at the chimney where it meets the roof. Gaps, rust streaks or thick smears of sealant are worth noting.
2. Find the vent pipes. If the collar around the base looks cracked, or faded and gray rather than dark, it may be near the end of its life.
3. Look where roof edges end against walls. Stains or peeling paint on the wall below that point can mean a missing kickout.
4. After heavy rain, look in the attic with a flashlight, keeping away from any wet wiring or light fixtures, but only if you can get there safely through a proper hatch. Step only on joists or boards, never on the ceiling between them. Damp wood or daylight around a chimney or vent is a clear sign.

None of these tell you for certain what is wrong. They tell you what to ask a roofer or inspector about.

Flashing on a quote

When you compare quotes for a new roof, look for flashing as its own line. A good quote says whether the step flashing, counter flashing, pipe boots and drip edge will be replaced or reused. If a quote says nothing, ask. Reusing sound copper or heavy counter flashing can be reasonable. Reusing cracked pipe boots and bent step flashing on a new roof is not.

There is more on reading those lines in how to compare roofing quotes line by line, and each type of flashing is defined in the glossary.