What is roof flashing?
The metal that seals the joints — and the #1 place roofs leak.
By The T.A.P. Team · March 2024 · 5 min read
Roof flashing is the thin metal — usually galvanized steel or aluminum — that seals the joints and penetrations where a roof is most vulnerable: around chimneys, along walls, in the valleys where roof planes meet, and around vents and skylights. Shingles shed water off the open field; flashing waterproofs everything shingles can't. And because those joints take the most abuse, failed flashing is the single most common source of roof leaks. Here's what flashing is, the main types, and how to spot it failing.
- Thin metal that seals roof joints and penetrations
- Found at chimneys, walls, valleys, vents, skylights
- Shingles shed water; flashing seals the weak points
- Failed flashing is the #1 cause of roof leaks
- Often reusable on reroofs — but shouldn't always be
What flashing does and where it goes
Anywhere the roof plane is interrupted or meets something else, water wants to get in — and that's where flashing goes. You'll find it woven into the shingles around the base of a chimney, running up the side of a dormer or wall, lining the valleys that channel heavy runoff, and forming collars around plumbing and exhaust vents. It directs water back onto the shingle surface instead of letting it seep into the seam.
The main types
Step flashing is the series of L-shaped pieces layered up a wall or chimney side. Counter (or cap) flashing tucks into the chimney mortar and covers the top of the step flashing. Valley flashing lines the valleys. Drip edge runs along the eaves and rakes to guide water into the gutter. Vent boots and collars seal round penetrations. Each is a different part of the same job: waterproofing a joint shingles can't cover.
Why flashing fails
Flashing fails when its seal breaks down: caulk and roofing cement dry out and crack in our sun, the metal corrodes or loosens, chimney mortar erodes and lets counter-flashing pull away, or it was simply installed poorly the first time (surface-nailed and relying on caulk instead of properly integrated). Storms can also bend or lift it. Once the seal opens, water runs straight into the joint — see how a roof is layered.
Repair or reflash?
Minor issues — a lifted piece, dried sealant — can often be resealed or re-secured. Corroded, bent, or poorly installed flashing should be replaced (reflashed), especially around chimneys, where proper step-and-counter flashing beats a smear of tar every time. During a reroof, quality installers replace worn flashing rather than reusing it just to save a few dollars. If a flashing leak is active, our emergency repair team can help.
| Flashing type | Where it seals |
|---|---|
| Step flashing | Walls & chimney sides |
| Counter flashing | Into chimney mortar, over step flashing |
| Valley flashing | Where two roof planes meet |
| Drip edge | Eaves and rakes |
| Vent boots | Around pipes and vents |
You can DIY
- Visually check flashing from the ground
- Look for gaps, rust, or pulled-away metal
- Watch for stains near chimneys/walls
- Note dried or cracked sealant
Call a pro
- Reflashing a chimney or wall
- Replacing valley flashing
- Integrating flashing into new shingles
- Fixing an active flashing leak
The difference between a leak that stays fixed and one that comes back every season is almost always the flashing detail. Done right, flashing is integrated into the shingle courses and, at a chimney, uses step flashing with mortar-set counter flashing — not a thick smear of roofing cement that cracks within a year or two. When we reroof, we replace worn flashing rather than reusing it, because that joint is where the roof lives or dies.
Flashing leaks in North Texas
Our sun dries out caulk and roofing cement fast, so DFW homes with tar-patched or aging flashing tend to spring chimney and wall leaks — often the first thing we find on an inspection. We reflash and repair roofs across Frisco, Allen, and Plano. See our roofing team.
The short version
- Roof flashing is the thin metal that seals a roof's joints and penetrations.
- It's found at chimneys, walls, valleys, vents, and skylights.
- Shingles shed water off the field; flashing waterproofs the weak points.
- Failed flashing is the single most common cause of roof leaks.
- It fails when sealant dries out, metal corrodes, or it was installed poorly.
- Proper step-and-counter flashing beats a smear of roofing cement every time.
Roof flashing — frequently asked questions
Leak near a chimney or wall?
Get the flashing inspected and properly sealed. Serving Dallas–Fort Worth & North Texas for over 25 years.