What Roof Flashing Is
Flashing is thin, flat metal (or occasionally other waterproof material) installed at every joint, transition, and penetration on a roof where two surfaces meet or where the roof surface is interrupted. Its job is to direct water away from these vulnerable points and prevent it from entering the roof assembly. Most residential roof leaks originate at flashing failures, not in the shingle field.
Types of Roof Flashing and Where They Go
- Step flashing: Individual L-shaped pieces woven with shingles up a vertical wall (such as where a roof meets a chimney or dormer wall). The most common flashing type and the most common leak source.
- Counter-flashing: A second layer of flashing that covers step flashing, typically embedded in mortar joints on masonry chimneys. Prevents water from getting behind the step flashing.
- Valley flashing: Metal installed in the open valleys where two roof slopes meet. Takes the highest water concentration on the roof.
- Pipe boot / vent boot: A rubberized collar (sometimes with a metal base) around each plumbing vent pipe penetrating the roof deck.
- Drip edge: Metal installed at eaves and rakes to direct water off the edge of the roof and prevent wicking back under shingles.
- Skylight flashing: The metal kit around skylight frames that directs water around the skylight frame.
- Chimney saddle / cricket: A small peaked flashing structure behind wide chimneys to divert water around the chimney rather than letting it pool behind it.
How to Tell if Flashing Is Failing
- Visible gaps between flashing and the adjacent surface
- Rust or corrosion on metal flashing visible from the ground
- Cracked or missing sealant at flashing seams
- Interior staining or water marks near a chimney, skylight, or vent pipe
- A roofer finds separated counter-flashing during inspection
Related: Chimney Flashing Leaks
Frequently Asked Questions
What material is roof flashing made from?
Most residential flashing is galvanized steel or aluminum. Premium installs use copper flashing, which is significantly more expensive but much longer-lasting -- copper flashing can outlast multiple shingle replacements. Galvanized steel and aluminum are adequate for most applications but can corrode or separate over time. Lead was commonly used historically and is still used in some chimney applications.
How long does roof flashing last?
Galvanized steel flashing typically lasts 20-30 years before corrosion becomes a problem. Aluminum flashing is similar. Copper flashing can last 50+ years. The sealant that holds flashing in place often fails before the metal itself -- regular inspection and re-sealing can extend flashing life significantly.
Does flashing always need to be replaced during a roof replacement?
Most professional roofers recommend replacing step flashing during a re-roof since it is fully accessible only during a tear-off. Counter-flashing (embedded in masonry) is sometimes reused if it is in good condition. Pipe boot collars should always be replaced. Valley metal is typically replaced. At minimum, all flashing should be inspected and re-sealed during a replacement.
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This page is for general informational purposes only. Consult a licensed roofing contractor for professional flashing inspection and repair.