What Is Roof Flashing and Why Does It Matter?
Roof flashing is thin metal (usually aluminum, galvanized steel, or copper) installed at joints and transitions on your roof — around chimneys, skylights, vents, dormers, and where the roof meets a wall. It channels water away from seams that shingles alone can't protect. Without flashing, or with corroded flashing, these joints are the first places leaks develop. Flashing typically lasts 20–30 years before needing replacement, and re-flashing during a roof replacement adds $300–$800 to the project.
Cost Breakdown
| Service | Low | High | Notes |
|---|---|---|---|
| All flashing during re-roof | $300 | $800 | Included in most full replacement bids |
| Chimney flashing repair | $300 | $600 | Step + counter flashing |
| Pipe boot replacement | $75 | $200 | Per pipe |
| Valley flashing | $400 | $900 | Per valley |
What Flashing Does
Shingles shed water across flat, continuous surfaces. But every time the roof plane is interrupted — by a chimney, skylight, wall, valley, or vent pipe — water can pool, seep under shingles, or wick sideways through capillary action. Flashing creates a waterproof bridge at these intersections, directing water back onto the shingle surface and off the roof.
Types of Roof Flashing
- Step flashing: L-shaped pieces woven into each shingle course where a roof meets a sidewall or chimney. Each piece overlaps the one below, creating a staircase of waterproof barriers.
- Counter flashing: Metal embedded into mortar joints of a chimney or masonry wall, overlapping step flashing below.
- Valley flashing: Continuous metal sheet running down where two roof planes meet.
- Drip edge: L-shaped strip along eaves and rakes directing water into the gutter.
- Pipe boot flashing: Pre-formed rubber or metal cone sealing around plumbing vent pipes. Rubber degrades after 12–15 years.
- Kickout flashing: Small diverter at the bottom of roof-to-wall junctions directing water into the gutter.
Common Flashing Problems
Flashing failures cause an estimated 90% of roof leaks. Common issues include sealant used as a substitute for proper step flashing, corroded galvanized steel, cracked pipe boot collars, and missing kickout flashing.
Cost to Replace
During a full re-roof, replacing all flashing adds $300–$800. Standalone chimney flashing costs $300–$600, pipe boots $75–$200 each, valley flashing $400–$900 per valley.
Related Questions
How long does roof flashing last?
Aluminum and galvanized steel flashing lasts 20–30 years. Copper lasts 50+ years. Rubber pipe boots crack after 12–15 years of UV exposure.
Should flashing be replaced during a re-roof?
Yes. Reusing old flashing is a false economy — the labor to install new flashing during a re-roof is minimal.
Can I repair flashing myself?
Pipe boot replacement and small sealant gaps are DIY-friendly. Step flashing and chimney counter flashing require removing shingles and metal bending — best left to professionals.