What Is Roof Flashing, and Why Does It Matter?

A roofer mentions flashing during an estimate, or a stain appears on the wall beside your chimney, and you start wondering what roof flashing is and why it keeps coming up. Flashing is the small metal detail that keeps rain out of the places where your roof changes direction or meets something else. It rarely gets attention until something goes wrong, and in the Pacific Northwest it works hard for most of the year. This guide covers what it is, where it sits, what it is made of, and how to tell when it needs a look.

by DIRECT PLACEMENT | October 27, 2026 | Roofing

What Roof Flashing Actually Is

Roof flashing is a thin strip of corrosion-resistant metal installed wherever your roof meets something else. That can be a wall, a chimney, a vent pipe, a skylight, or another section of roof. Most flashing is galvanized steel, and some is aluminum or copper.

Shingles shed water well across an open slope. Joints are where the trouble starts. A seam between a roof and a brick chimney leaves a gap, and a gap gives water a way in. Flashing covers that gap and sends water down and off the roof, away from the seams beneath the shingles.

The design relies on overlap. On the high side of a joint, the flashing tucks underneath the shingles, so water running down the slope flows over the metal. On the low side, the flashing lies on top of the shingles and stays visible. Water travels from shingle to metal to the next shingle down, and keeps moving toward the gutter.

Most homeowners rarely see it because it is small and sits at the edges of the roof, so roof flashing is among the first details a roofer checks on an older home.

Where Flashing Does the Most Work (and Fails Most Often)

With close to 37 inches of rain falling on the Portland metro in a typical year, valley and roof-to-wall flashing spend months at a time channeling water off the roof. Most of that rain arrives between fall and spring in Vancouver, Camas, Washougal and Portland, which makes these two spots the best place to start during a fall inspection.

Roof Valleys

A valley is the line where two roof slopes meet. It collects the runoff from both slopes and funnels it into one narrow path, so it carries more water than any other part of the roof. When valley flashing fails, water reaches the wood decking underneath, and rot follows.

Roof-to-Wall Intersections

Wherever a sloped roof runs into a vertical wall, such as a dormer or a second-story wall, rain streams down the wall and meets the roof along its full length. If the flashing there lets go, water slips behind the siding and into the wall cavity. Interior stains and mold are the usual result.

Chimneys

Brick and mortar expand and contract at a different rate than a roof, and the joints around a chimney can open over time. Water also pools on the high side of the chimney. Leaks here often show up as stains on the ceiling near the fireplace.

Vent Pipes

Every plumbing vent is a hole through the roof. A flashing collar seals the space around the pipe, and the collar can crack or lift with age. A small gap is enough to let water drip steadily into the attic.

Skylights

A skylight frame rises above the roof surface, so water runs into it from above and from both sides. Flashing wraps all four sides of the frame. Failure there tends to appear as a ring of staining around the skylight opening.

Roof Perimeter and Drip Edge

Drip edge is the metal strip along the eaves and the sloped edges of the roof. It guides water off the edge and into the gutter. Without it, water can curl back under the shingles and soak the fascia board and the edge of the roof deck.

The Main Types of Roof Flashing

Step flashing is a series of small, bent metal pieces, one woven into each course of shingles where a roof meets a wall or the side of a chimney. Each piece overlaps the one below it, like a staircase. Sizing matters here. Professional guidance calls for pieces at least 10 inches long and roughly two inches wider than the shingle's exposed face, which works out to about 10 by 7 inches for a shingle with a 5-inch exposure.

Valley flashing lines the channel where two slopes meet. In an open valley, a strip of metal runs the length of the valley and the shingles stop short of the center, so the metal stays visible. In a closed valley, shingles from both slopes cover the center line, and the waterproofing layer sits beneath them.

Base and counter flashing form a two-part system at the chimney. Base flashing wraps the bottom of the chimney and weaves into the shingles. Counter flashing is set into the masonry above and folds down over the base pieces. The overlap lets the chimney and the roof shift slightly with the seasons while the joint stays sealed.

Drip edge runs along the perimeter of the roof. It is the simplest of the four, a strip of bent metal that gives water a clean path off the roof and into the gutter. It also protects the fascia and the roof deck edge from moisture that would otherwise run behind them.

What Roof Flashing Is Made Of

Three metals cover most roof flashing. Galvanized steel is the most common. Its zinc coating resists corrosion, it costs less than the other two, and it suits most asphalt shingle roofs. Aluminum is lightweight and easy to shape, and it needs a protective coating where it touches masonry, since fresh mortar and concrete can corrode bare aluminum. Copper is the premium choice. It lasts the longest and develops a soft green patina as it ages. Properly installed, galvanized steel and aluminum flashing typically last 20 to 30 years, while copper can last 80 to 100 years. Those ranges give you a yardstick if your home still has its original flashing and the roof is approaching the 20-year mark. Thickness counts too. Most building codes require flashing to be corrosion-resistant metal at least 0.019 inches thick, about the gauge of a 26-gauge galvanized sheet. Material choice depends on your roof, your budget and how long you plan to stay in the home, and a roofer can match the metal to the shingles and the surrounding masonry.

Signs Your Flashing Needs Attention

Flashing trouble shows up in a handful of ways, inside the house and on the roof:

• Water stains on a ceiling or wall, or in the attic, near a chimney, wall junction, vent or skylight

• Daylight visible around roof penetrations when you look up from inside the attic

• Loose, bent or visibly lifted metal at chimneys, valleys and walls

• Rust streaks, or a build-up of shingle granules, near the flashing

• Moss or algae along flashing edges, which is common on Pacific Northwest roofs and holds moisture against the metal

A walk around the house with a seasonal roof inspection checklist makes these signs easy to spot from the ground and from the attic.

Caught early, a damaged section is usually a roof flashing repair that leaves the rest of the roof in place. Flashing repairs typically run in the low hundreds of dollars, with a national average of $200 to $500, so a prompt look saves money. The work happens at height, and disturbing the shingles around flashing the wrong way can put a shingle warranty at risk, so this job belongs to a roofer.

Get Your Flashing Checked Before the Rain Sets In

Months of steady rain will find any weak point in your flashing, and a fall inspection lets you fix that point first. Journeyman Exteriors handles leak diagnostics and storm damage repair alongside full re-roofs for homeowners in Vancouver, Camas and Washougal, WA and across the Portland, OR metro. The team brings more than 30 years of combined experience and installs roofing systems from Malarkey, CertainTeed, IKO and Owens Corning, so a flashing concern gets a crew that knows how the whole roof fits together. To schedule your roof inspection, use the Request a Quote form on the contact page.