What is roof flashing and where does it matter most?
Flashing on a roof is the metal that keeps water out where the shingles stop.
Roof flashing is thin sheet metal (or a similar formed material) that carries water over the joints where shingles alone cannot. It goes around chimneys, skylights, vents and walls, and into valleys. Most roof leaks start at flashing, not in the middle of a shingle slope.
How flashing works
Flashing laps under the covering above and over the covering below. Water always moves downhill onto the next layer. It never has to cross a gap.
That is why a bead of sealant is not flashing. Sealant fills a gap. Flashing removes the gap from the water's path.
The common types
- Step flashing: small bent pieces woven with each shingle course where a roof meets a wall.
- Counterflashing: a second piece set into masonry or siding that covers the top edge of the first.
- Valley flashing: a channel of metal where two slopes meet.
- Drip edge: a lip along the eaves and rakes that guides water into the gutter.
- Vent boots: collars around plumbing pipes, often rubber, which split with age.
What failing flashing looks like from the ground
Use binoculars from the yard. Look for lifted or bent metal, rust streaks, gaps at a chimney, or a thick smear of old roofing cement. Inside, look for a stain on the ceiling below a wall or chimney, or dark marks on attic framing near a penetration.
A stain that appears only in wind-driven rain is a strong hint. It usually follows a joint, not the open roof.
Why we stop at looking
We do not give steps for resealing or replacing flashing, because that means working on the roof. Flashing sits at the edges and joints, which are the most exposed places to stand. Document what you see from the ground and have a roofer inspect it.
To tell flashing trouble from other causes, read how to tell it's the flashing and not the shingles. For wall joints in particular, see step and kickout flashing.
Frequently asked questions
What is flashing on a roof?
It is thin metal formed to carry water over joints such as chimneys, valleys, vents and roof-to-wall meetings. It laps under the roofing above and over the roofing below.
Is roofing cement the same as flashing?
No. Cement fills a gap and tends to crack as materials move. Flashing keeps water from reaching the gap at all.
How can I check flashing without going on the roof?
Use binoculars from the ground and look for lifted, bent or rusty metal. From inside, check ceilings and attic framing near chimneys, walls and vents.
Can I replace flashing myself?
We do not recommend it and do not give steps. It means working at the roof's edges and joints. A roofer should do it.
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.
- NRCA. National Roofing Contractors Association
- US OSHA. 1926.501 — Duty to have fall protection (OSHA)