What is flashing on a roof?
Short answer. Roof flashing is thin metal, usually galvanized steel or aluminum, installed wherever the roof surface is interrupted or meets something else, such as walls, chimneys, valleys, vents, skylights and edges. It directs water over the joint and back onto the shingles, because shingles alone cannot seal those seams.
Where to spot it on your house
- Roof edges: drip edge, the thin metal lip along the eaves and rakes.
- Where a lower roof meets a second-story wall: step flashing hidden under the siding, with a kickout at the bottom that turns water into the gutter.
- Around the chimney: an apron in front, steps along the sides, and counterflashing set into the mortar.
- Around pipes: pipe boots.
- In valleys: exposed metal on an open valley, or a lining hidden under the shingles on a closed one.
- Around skylights: a flashing kit much like a chimney's.
How long it lasts
Good metal flashing can outlast shingles, but sealant joints and rubber parts do not. Rust along the bottom of step flashing, joints that depend on caulk, and dried rubber collars are the usual failure points. All flashing should be replaced when the roof is replaced, not reused.
Signs of flashing trouble
- Stains on ceilings or walls near a chimney, skylight, or where a roof meets a wall.
- Rotted siding or trim below a roof to wall corner.
- Heavy tar or caulk smeared over the metal.
- Gaps or lifted metal visible with binoculars.
Go deeper with roof flashing explained and drip edge explained.