What is flashing?
Updated August 2026
Flashing is thin sheet metal fitted at every roof joint and wall transition so water runs off the seam instead of into it. Roofers bend it from aluminum, galvanized steel, or copper and install it in overlapping pieces at valleys, chimneys, skylights, vent pipes, and anywhere a roof plane runs into a vertical wall. Shingles cover the open field of a roof; flashing handles the corners and edges that shingles alone cannot seal.
Where it is
Look at the places where two surfaces meet. Step flashing is the row of small bent squares tucked under each shingle course where the roof meets a sidewall or a chimney, and counter flashing is the strip mortared into the brick joints above it. Valley flashing is the wide metal channel where two roof planes drain together. Pipe flashing, called a boot, is the cone with a rubber collar around each plumbing vent. Along the eaves you will also find the drip edge. From the lawn, binoculars show rust streaks, lifted metal, and cracked tar; from the attic, a flashlight on the sheathing around the chimney chase shows old water tracks.
What it does
Flashing turns a joint into a drainable overlap. Water hitting a chimney runs down the brick, meets the step flashing, and gets carried back out onto the shingles below instead of soaking the sheathing. When the metal corrodes, the nails back out, or the sealant at the counter flashing dries and cracks (usually somewhere around year 5 to 10), that path breaks. The leak then surfaces feet away from its source: a stain on a bedroom ceiling, a damp patch below a skylight, dark streaks on attic rafters. Most roof leaks trace back to failed flashing or dried sealant rather than to the shingles, which is why a 12-year-old roof can leak with plenty of shingle life left.
Maintenance
Put eyes on it twice a year, spring and fall, and again after any windstorm that stripped shingles. Two items wear out on a schedule: the sealant beads at counter flashing and pipe boots, which want fresh polyurethane roof sealant every 5 to 10 years at $10 to $20 a tube, and the rubber collars on vent boots, which crack before the metal does. Galvanized steel starts rusting around year 20 to 25; aluminum and copper outlive the roof. Replace flashing whenever the roof is replaced, because reusing bent old metal to save $300 is the classic way a brand-new roof leaks. What failed flashing eventually costs gets measured in rotted sheathing and drywall, not in sheet metal.
What it costs to replace
A replacement flashing runs $300–1,000 at a chimney or a valley, with a reseal of sound metal at $150–400.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
How do I know if a roof leak is the flashing or the shingles?
Location tells you. A stain that shows up near a chimney, skylight, dormer wall, or plumbing vent is almost always flashing. A stain out in the middle of a roof plane is usually a torn shingle, a nail pop, or ice damming. Trace it in the attic on a dry day: follow the water track uphill along the rafter to its highest wet point, then measure that spot from the ridge and from the nearest wall so a roofer can find it from above.
Can I just caulk failed flashing?
Caulk buys time, not a fix. A bead of polyurethane roof sealant on a cracked counter-flashing joint holds for 1 to 3 years and costs under $20, which is a fair move in November when nobody is reflashing a chimney. What it cannot do is rescue metal that has rusted through, flashing that was never woven into the shingle courses, or a valley that was tarred instead of lined. Layered black tar on old flashing is a red flag on any inspection report.
What does reflashing a chimney cost?
Budget $300 to $1,000 for a standard chimney, depending on its size, the roof pitch, and whether the counter flashing has to be cut into fresh mortar joints with a grinder. A steep or three-story roof pushes past that. Valley reflashing lands in the same range, because shingles on both sides come off and go back on. A single pipe boot, by contrast, is a $150 to $300 service call and takes under an hour.
Does new flashing come with a new roof?
Ask, because it varies by bid. A good estimate lists new step flashing, new pipe boots, and new drip edge as separate line items. A cheap one says "reuse existing flashing," which shaves a few hundred dollars and puts metal that has already been bent, nailed, and sealed once under brand-new shingles. Reusing counter flashing set in sound mortar is defensible. Reusing step flashing is not, and it is the difference you are paying for.