Roof Flashing Explained
Most roof leaks come from failed flashing, not shingles. Chimney, wall, valley, pipe, and skylight flashing — when to replace vs. reuse, and why details matter on a quote.
Most roof leaks don't come from the shingles themselves — they come from failed flashing, the thin metal that seals roof transitions at chimneys, walls, valleys, vents, and skylights. Reusing existing flashing on a re-roof is a common corner-cut that's invisible from the street but predicts leaks within a few years. This guide explains where flashing lives on a roof, when it should be replaced versus reused, and why a quote that says 'flashing as needed' is a quote leaving room to reuse.
What flashing does
Flashing is the thin metal that seals the places where the roof meets something else — a chimney, a wall, a vent pipe, a skylight, or another roof slope (a valley). At every transition, water can find its way under the shingle field if there's no metal sealing the joint. Flashing sheds water away from the transition and onto the shingle field where it can run off normally. Without flashing, transitions leak; with old or reused flashing, transitions develop leaks within 3–5 years of a re-roof.
Where flashing is used
Common flashing locations on a typical California home: chimney (step flashing along the sides, counter-flashing across the top, saddle on the upslope side), wall-to-roof transitions (step flashing under the wall siding), valleys (open metal valley flashing or closed shingle-woven valley with metal beneath), pipe vents (rubber-gasketed metal boots), skylight curbs (manufacturer-specific flashing kits), and ridges/drip-edge perimeter (continuous metal). Each location has different failure modes and different replacement decisions during a re-roof.
Chimney flashing
Chimney flashing is the highest-leak-risk location on most roofs, and it's where reuse is most damaging. Chimneys cycle thermally — they get hot when the fireplace is used, cool when it isn't — and the constant expansion/contraction degrades the flashing seal faster than other locations. A re-roof that reuses chimney flashing is reinstalling shingles around 20+ year old metal that's near end of life. New chimney flashing is $400–$1,200 of work depending on chimney size and complexity. Reputable contractors replace it as a matter of course.
Wall flashing
Where a roof slope meets a vertical wall (common at second-story additions, dormers, attached garages), step flashing carries water down the slope while sealing against the wall. The flashing pieces sit under each course of shingles and behind the wall siding — meaning they're partially hidden once installed. Reusing step flashing is technically possible but rarely the right call: it requires careful removal without damaging existing siding, and reused pieces typically have nail holes from the original install that compromise their water-shedding. New step flashing during a re-roof is the standard approach.
Valley flashing
A valley is where two roof slopes meet in a V shape — a high-water-flow channel. Two main approaches: open valley flashing (continuous metal in the V, shingles cut back to expose the metal) and closed valley (shingles woven across the V with metal underneath). Open valleys are easier to inspect and clean, last longer, and are common on premium installations. Closed valleys are more common on lower-cost work but harder to maintain. Either way, valley flashing should be replaced during a re-roof, not reused.
Pipe flashing
Plumbing vent pipes that penetrate the roof are sealed with rubber-gasketed metal boots (sometimes called pipe flashing or pipe boots). The rubber gasket is the failure point — it degrades from UV exposure over 10–15 years, eventually cracking and leaking around the pipe. Pipe boots should be replaced during every re-roof regardless of apparent condition. Cost is small ($50–$100 per boot installed) but the leak risk from a failed boot is real.
Skylight flashing
Skylight flashing depends on the skylight type. Curb-mounted skylights sit on a wood curb that gets its own step flashing on the sides and saddle flashing upslope and downslope. Deck-mounted skylights install flush with the deck and use manufacturer-specific flashing kits. Either way, skylight flashing should be replaced during a re-roof — skylights are second only to chimneys in leak frequency. Reusing skylight flashing is a corner-cut that predicts leaks.
Replace vs reuse
The default for a quality re-roof is replace all flashing. Reuse is only appropriate when the existing flashing is recent (under 5 years old), in clearly good condition, and made of long-lasting material like copper. For most California re-roofs on homes 15+ years old, the existing flashing is at or past end of life regardless of whether it visually looks intact. A contractor who proposes to 'reuse flashing where possible' is a contractor who plans to reuse most of it — the language is permissive.
Why flashing details affect quote quality
Flashing scope is one of the single best predictors of overall roofing quote quality. A quote that specifies flashing locations and replacement decisions in detail is a quote written by a contractor who's actually planned the job. A quote that says 'flashing as needed' or 'flashing replaced where required' is written by a contractor leaving themselves room to reuse. The same correlation tracks across other details: contractors who treat flashing as a real line item usually treat ventilation, underlayment, and warranty terms with the same care.
Questions to ask about flashing
Ask: which specific flashing pieces will be replaced versus reused? (Get answers by location: chimney, walls, valleys, pipes, skylights.) What material — galvanized steel, aluminum, or copper? Is the existing flashing being inspected before the reuse decision is made, or is the decision based on the quote? Is your scope priced assuming replacement, with reuse as a credit, or assuming reuse with replacement as an upgrade? Reasonable answer: replace at all locations, galvanized or aluminum (specified by location), priced into the quote not as an add-on.
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Frequently asked questions
Should chimney flashing always be replaced during a re-roof?
Almost always, yes. Chimney flashing typically degrades faster than shingles because of constant thermal cycling and water exposure. Reusing it is a $400–$1,200 corner-cut that predicts leaks within 3–5 years. A complete quote names chimney flashing as a replacement line item.
What's the difference between step flashing and continuous flashing?
Step flashing is L-shaped pieces tucked under each shingle course where the roof meets a wall — it's the correct approach for shingle roofs. Continuous flashing is a single long piece, which works for tile and metal but fails on shingles. A contractor proposing continuous flashing on a shingle wall transition is using the wrong method.
Is copper flashing worth the cost?
Copper lasts longer than aluminum or galvanized steel — often 50+ years versus 20–30 — but costs 4–6x more. It's worth it on high-end homes, coastal homes with salt-air corrosion, and on chimney flashing where access for future repair is expensive. Galvanized is usually fine elsewhere.
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HomeGoSmart is not a contractor and does not provide legal, financial, or construction advice. Homeowners should verify license, insurance, references, permits, and written contract terms before hiring.