Cap flashing: the upper layer that covers the joint
Cap flashing is the metal hood that keeps water from running behind the flashing below it. It is easy to overlook and common to fail.
Cap flashing is the upper layer of metal that covers the top edge of the flashing underneath it. It is also called counterflashing. Its only job is to stop water from running down the wall and in behind the lower piece.
Where cap flashing sits
You find it wherever a roof meets something vertical: a chimney, a brick wall, a parapet or a dormer. The lower flashing runs up the vertical surface. The cap flashing is set into a groove or mortar joint above it and hangs down over the top, like the brim of a hat.
That overlap is the whole design. Water running down the wall lands on the cap and drops onto the lower flashing, not behind it. This is why the two are always installed as a pair.
How cap flashing fails
The cap is exposed to weather on every side, so it wears in predictable ways. Rust is the most common. Another is movement: mortar crumbles, the metal loosens and the bottom edge lifts away from the flashing below.
- Rust or pinholes along the bottom edge.
- A visible gap between the cap and the lower flashing.
- Mortar missing or cracked along the groove where the cap is set.
- Sealant smeared across the top edge instead of a proper metal cap.
A sealant smear is the sign to take seriously. It usually means the cap has already failed once and been patched.
Is cap flashing the same as base flashing?
No, and mixing them up is how quotes get confusing. Base flashing is the lower layer, lapped under the shingles. Cap flashing is the upper layer that covers it. A roofer who says "the flashing is bad" may mean either. Ask which one, and ask to be shown.
What to check from the ground
Use binoculars from the yard. Look along the top edge of the metal where it meets the brick or wall. Rust streaks below the line, a dark gap or a thick bead of tar all deserve a photo.
Inside, check the ceiling and the attic framing near the wall with a flashlight, staying on the joists. A stain that appears after wind-driven rain, and not after gentle rain, often points at a gap high on the wall rather than at the shingles.
Why we do not show the repair
Resetting cap flashing means cutting into mortar or siding and working at the edge of the roof beside the wall. That is high, awkward work, and we do not publish the steps. Take your photos, note the stained room and ask a roofer which layer needs replacing.
If the answer is "just add sealant", ask why. A cap that has pulled free needs resetting or replacing, not another bead. For the pieces that sit below it, see what is roof flashing, and for the chimney sides in particular, step flashing around a chimney.
Frequently asked questions
What is cap flashing?
It is the upper layer of metal that covers the top edge of the flashing below it, so water running down a wall cannot get behind the lower piece. It is also called counterflashing.
What is the difference between cap flashing and base flashing?
Base flashing is the lower layer lapped under the shingles. Cap flashing is the upper layer set into the wall or mortar that hangs over it. They are installed as a pair.
How do I know if cap flashing has failed?
From the ground, look for rust, a gap between the cap and the flashing below, missing mortar or a thick smear of sealant along the top edge. Stains inside after wind-driven rain are another clue.
Can I fix cap flashing with sealant?
Sealant may slow a tiny gap for a season, but it does not replace a cap that has pulled loose or rusted. The usual repair is to reset or replace the metal.
Should I reset cap flashing myself?
We do not publish steps for it. It means cutting into mortar or siding and working at the roof edge. Document what you see from the ground and have a roofer 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)