When a roof leaks, the culprit is usually not the broad field of shingles. It is the flashing, the thin metal that seals the tricky spots where the roof meets something else. In fact, most roof leaks, commonly cited as 80 to 90 percent, start at flashing and penetrations rather than in the open shingles. Here is what flashing is, where it lives on your roof, why it fails, and what it costs to fix.
What flashing is
Flashing is thin metal, usually aluminum, galvanized steel, or copper, installed at the joints and transitions of a roof to guide water away from the seams where it would otherwise get in. Anywhere the roof surface is interrupted or meets a wall, flashing does the sealing that shingles alone cannot.
Where it goes
You will find flashing at all the vulnerable transitions:
- Step flashing: L-shaped pieces layered with the shingles where the roof meets a sidewall or dormer.
- Counter flashing: set into a chimney and lapped over the base flashing to complete the seal.
- Valley flashing: lining the channel where two roof slopes meet and shed a lot of water.
- Apron or base flashing: at the base of walls and penetrations.
- Drip edge: along the eaves and rakes, directing water off the edge into the gutter.
- Kickout flashing: the small diverter at the bottom of a roof-to-wall joint that kicks water into the gutter instead of behind the siding.
Kickout flashing is the one builders forget
That little diverter where a roof edge meets a wall is often left off, and when it is missing, water runs down behind the siding and rots the wall from the inside, out of sight, for years. If you have a roof-to-wall junction above a gutter, make sure it is there.
Why flashing leaks
Flashing fails for a handful of reasons:
- Poor installation, the most common cause, such as pieces layered the wrong way or left out entirely.
- Caulk used as a substitute for real metal flashing, which dries, cracks, and fails in a few years.
- Rust and corrosion on older metal.
- Wind and storm damage that lifts or bends it.
- Simple age.
Signs your flashing is failing
Watch for water stains on ceilings or walls near a chimney, skylight, or wall line, visible rust, gaps or open seams, and loose, lifted, or missing metal. Because flashing hides at the transitions, its leaks often show up as a stain some distance away, which is why tracing them matters. See how to find a roof leak.
What a repair costs
Flashing repairs are usually modest:
- A minor reseal or spot repair: about 150 to 300 dollars.
- A typical localized repair around a skylight, vent, or short valley: about 200 to 600 dollars.
- Chimney flashing: roughly 300 to 700 dollars to repair, and 500 to 1,500 dollars or more to replace, higher on tall masonry chimneys.
Always replace flashing with a new roof
One of the most common corner-cuts is installing new shingles over old, reused flashing. It creates weak points right where leaks start and undercuts the value of your new roof. Insist that flashing is replaced, not reused.
DIY or pro?
Resealing a small joint is within reach for a handy homeowner, but proper step, counter, and kickout flashing is woven into the shingles and, at a chimney, into the masonry. Getting the layering wrong causes hidden leaks, so anything beyond a simple reseal is a job for a pro. If a leak has already started, our guides on the common causes of a roof leak and roof leak repair cost will help.
“Shingles get the attention, but flashing does the hard work. When a roof leaks, the metal at the transitions is the first place to look.”
The other top leak spot
Flashing is not the only usual suspect. The rubber boots around plumbing vents fail just as often. See our guide to roof pipe boot leaks.
With Hommy you can post a job once and get flashing repairs and inspections from vetted local roofers, each with real ratings from other homeowners, so the leak gets sealed at the source.
