Roof anatomy
Roof flashing: the small details that let water in
Chimneys, valleys, walls, vents and skylights all rely on flashing. Understanding the details explains why two leaks that look identical can cost very differently.
The short answer
Flashing is the metalwork that keeps water out where the roof meets something else. Most leaks that are not caused by storm damage start at a flashing detail, which is why two similar-looking stains can be priced very differently.
What flashing is, and where it sits
Shingles shed water down the slope. Wherever that slope meets a wall, a chimney, a valley, a pipe or an edge, something has to bridge the two and direct water away. That something is flashing: usually sheet metal, sometimes a moulded rubber or plastic component.
| Detail | What it does |
|---|---|
| Step flashing | Individual L-shaped pieces stepping up a side wall, woven into the shingle courses |
| Counterflashing | The metal let into the wall or chimney above the step flashing, lapping over it |
| Apron / headwall flashing | The piece at the junction where a slope meets a vertical wall above it |
| Valley metal | A channel in the internal corner where two slopes meet |
| Drip edge | The metal at the eave that makes water drip clear of the fascia |
| Pipe boot / collar | A moulded seal around a plumbing or vent penetration |
| Skylight kit | A proprietary flashing set around the curb |
Why flashing fails
- Thermal movement: metal expands and contracts, and fixings work loose over years.
- Corrosion, particularly where dissimilar metals touch or where debris holds moisture.
- Sealant used instead of a mechanical lap. Sealant has a service life, and when it fails the detail is exposed.
- Poor original installation: step flashing laid over the shingles rather than woven in, or no counterflashing at all.
- Blocked valleys holding water against the metal.
- Siding or render changes that conceal or damage the flashing behind them.
The common thread is that these are small, hidden details. A five-dollar piece of metal installed wrongly can cost more to correct than a large area of shingles, because correcting it means opening the surrounding work.
Signs a flashing detail is failing
- Staining on a ceiling near a chimney, wall or skylight rather than in the middle of a room.
- Rust streaks on the roof surface below metal.
- Visible gaps between metal and masonry, or sealant that has cracked and lifted.
- Damp patches on a chimney breast, especially after wind-driven rain.
- Water in a valley that does not drain after the rain stops.
- Lifted or missing pieces around a pipe after a storm.
What repairing flashing involves
A flashing repair usually means removing enough of the surrounding covering to reach the detail, replacing or reforming the metal, then reinstating the covering so the laps work again. That is why the price is driven by the length of the detail and how much has to be opened up — not by the size of the visible stain.
Our repair cost calculator has a flashing category priced per linear foot, with the access factor shown separately. It is a planning band from a provisional model, not a quote.
What to ask
- Which detail do you believe is failing, and how did you confirm it?
- Will you replace the metal, or seal over the existing detail?
- How much covering has to be removed, and will it be reinstated with matching material?
- Is the counterflashing being replaced as well as the step flashing?
- What warranty applies to this detail specifically?
What to ask a roofing professional
- Which flashing detail is failing, and how was it identified?
- Will the repair be mechanical or sealant-based?
- How much of the surrounding covering will be opened?
Questions this page answers
Can flashing be repaired from inside the attic?
Rarely. Flashing works by lapping water away on the exterior; sealing it from inside does not restore the lap and usually traps moisture in the assembly.
Does flashing need maintenance?
It benefits from it. Clearing valleys and gutters, checking sealant at terminations, and clearing debris around chimneys prevents the conditions that make flashing fail early.
Sources
Accessed 2026-11-10. Listed as general references for the organisations behind the guidance on this page; the page itself is our own explanation.
- Asphalt Roofing Manufacturers Association — residential roofing resources — ARMASupports: Reference material on shingle systems, underlayment and maintenance.
- National Roofing Contractors Association — consumer and contractor resources — NRCASupports: Professional context for scoping and comparing roofing work.
Occasional, and only when there is something worth sending. The checklist and worksheet resources are free on this site whether you subscribe or not.
This page is general guidance, not an inspection of your roof and not a quotation. Roof Repair Roof does not perform roofing work or dispatch contractors.