Most people think roof leaks start with shingles. They don't. The majority of leaks we trace in Seattle come back to flashing. Flashing is the thin metal (aluminum, galvanized steel, or copper) that seals every transition point on your roof. Chimney meets roof. Skylight meets roof. Wall meets roof. Anywhere water would pour straight into a gap between surfaces, flashing keeps it out.
## Types of flashing and where they live
There are more types than most homeowners realize.
**Step flashing** runs along the joint where a roof slope meets a vertical wall. It's a series of small L-shaped pieces woven into each course of shingles, each one overlapping the one below it so water steps down and away from the wall.
**Counter flashing** is the piece that tucks into the mortar joints on a chimney or into a reglet cut in the wall. It overlaps the top edge of the step flashing and keeps water from getting behind it.
**Valley flashing** is the metal channel where two roof planes meet. Every rainstorm funnels water into the valleys, so this piece handles more volume than anything else on the roof.
**Drip edge** runs along the eaves and rakes. It directs water into the gutter instead of letting it curl back under the shingles and rot the fascia.
**Kick-out flashing** is a small but critical piece at the bottom of a roof-to-wall transition. It diverts water away from the siding and into the gutter. This one is missing from most pre-2000 Seattle homes, and its absence causes some of the worst hidden damage we see.
**Pipe boot flashings** (vent collars) are the rubber-and-metal cones around plumbing vent pipes. Technically a flashing, but they fail in their own special way.
## Why flashing fails
Flashing doesn't fail all at once. It's a slow process, and the causes are predictable.
Sealant breaks down first. Thermal cycling expands and contracts the metal, and the caulk cracks. In the PNW, constant moisture cycling breaks sealant down over 8 to 12 years. Nails back out too. A nail that lifts even a sixteenth of an inch creates a channel for water. Galvanized steel rusts through after 15 to 20 years, especially in our wet climate.
Improper installation is more common than you'd think. Flashing pieces need to overlap in the right direction so water sheds downhill. If a step flashing piece is lapped the wrong way, water runs behind it instead of over it. We see this on DIY jobs and, honestly, on some professional jobs too. Moss and fir needle buildup make things worse by trapping water against the flashing junction.
## What failure looks like from inside
If you see water stains on a wall near a chimney, that's almost certainly a flashing problem. Stains around skylights, same story. Discoloration along an exterior wall where a lower roof meets the siding usually means the kick-out flashing is missing or failing. For a full breakdown of how to trace a leak back to its source, our guide to finding and fixing roof leaks walks through it step by step.
## The number one flashing failure in Seattle
Pipe boot flashings. We fix more of these than everything else combined. The rubber gasket around the vent pipe dries out and cracks from UV exposure in about 8 to 12 years. Once it cracks, rainwater runs straight down the pipe and into the house. The leak often shows up on a ceiling nowhere near the pipe because the water travels along framing before it drips.
The good news: a single failed pipe boot is a $150 to $300 fix. Quick, straightforward, and it stops the leak immediately. The bad news: a lot of homeowners don't know to look for it, so the water runs for months before anyone notices. By then, you might be dealing with rotted sheathing or mold in the attic. If you're not sure what's going on up there, a roof inspection will catch pipe boot problems before they turn into bigger ones.
## Repair costs and what to expect
A pipe boot swap is on the low end, $150 to $300. Resealing step flashing along a wall is similar if the metal is still solid. A full chimney reflash (pulling the counter flashing, replacing step flashing on both sides, resealing everything) runs $800 to $1,500 depending on chimney size.
Valley metal replacement is usually done during a roof replacement because you have to pull up the shingles on both sides to get to it. If your valleys are corroded but the rest of the roof still has life, a roofer can sometimes patch or overlay, but full replacement is the right long-term fix.
If you're weighing the cost of a flashing repair against other issues on your roof, our repair vs. replace guide breaks down when it makes sense to fix individual problems versus starting over.
## Kick-out flashing deserves its own mention
At the bottom of every roof-to-wall transition, water running down the step flashing needs somewhere to go. Without a kick-out diverter, it pours behind the siding. The damage happens inside the wall where you can't see it: rotted sheathing, mold, ruined insulation. Most Seattle homes built before 2000 don't have one. It wasn't required by code, and builders skipped it. If your house has a roof that terminates into a sidewall above a lower section (common on split-levels and bump-outs), check for it. Adding one is cheap insurance.
## When to call a pro
Small sealant touch-ups are reasonable DIY territory if you're comfortable on a roof. Anything beyond that, especially step flashing, counter flashing, or valley work, is best left to a roof repair crew. Flashing done wrong leaks worse than flashing left alone, and the consequences of a bad install show up months later when the damage is already done. Use our roof inspection checklist to assess what you're dealing with, and if the flashing is the problem, reach out. We'll get it sorted.
