What is roof flashing?

Flashing is a thin, weatherproof material — usually metal like galvanized steel, aluminum, or copper — installed at the points on a roof where water is most likely to find a way in. Those are typically places where the roof surface meets something else: a chimney, a vent pipe, a skylight, a wall, or where two roof slopes come together to form a valley.

Shingles and tile are good at shedding water on a flat, continuous slope. They are not designed to seal around an object poking through the roof or a joint where two surfaces meet at an angle. That is the job flashing exists to do — it bridges those gaps and directs water safely down and off the roof instead of letting it seep underneath the roofing material.

Why flashing matters more than its size suggests

Flashing typically makes up a small fraction of a roof’s total surface area, but it is disproportionately responsible for leaks. Roofing material covers broad, uninterrupted surfaces reasonably well on its own. It is the transitions and penetrations — the joints, seams, and edges — where water pressure concentrates and where a small gap or a piece of corroded metal can let water through. Industry experience consistently points to flashing failures, not worn-out shingles or tile, as one of the leading causes of roof leaks.

Common types of roof flashing

Chimney flashing

Chimneys require some of the most involved flashing work on a roof. A properly flashed chimney typically combines base flashing at the bottom, step flashing running up the sides, and a cap or counter-flashing mortared into the brick to lap over everything else. Each piece protects a different part of the joint, and a failure in any one of them can let water in.

Valley flashing

A roof valley is where two sloped sections of roof meet, forming a channel that collects and funnels a large volume of water during rain. Valley flashing lines that channel and directs water down and off the roof rather than letting it work its way under the shingles or tile on either side. Given how much water passes through a valley during a storm, this is one of the highest-stakes flashing applications on the roof.

Step flashing

Step flashing is used along the sides of chimneys, dormers, and other vertical walls that meet a sloped roof. Individual L-shaped pieces are layered with each course of shingles as they go up the roof, creating a stepped, overlapping seal that sheds water down and away from the wall.

Vent and pipe flashing

Roofs almost always have plumbing vent pipes, exhaust vents, or other small penetrations passing through them. Vent flashing — often called a boot — wraps around the base of the pipe and seals against the surrounding roofing material. The rubber gasket component of many vent boots is also one of the more common points of failure over time, since rubber degrades with age and UV exposure.

Drip edge

Installed along the eaves and rakes of the roof, drip edge flashing directs water away from the fascia board and prevents it from wicking back underneath the roofing material at the roof’s edges — a detail that protects both the roof deck and the fascia and gutters below it.

Wall and headwall flashing

Where a roof meets a vertical wall — such as where a lower roof section runs into a second-story wall — wall flashing extends up and behind the exterior wall covering, directing water down and out onto the roof surface below rather than behind the siding or stucco.

Why flashing fails

Flashing does not typically fail all at once. Metal flashing can corrode over time, especially at seams or fastener points. Sealant and caulking used alongside flashing dries out and cracks under UV exposure and thermal cycling. Flashing can also become loose or bent from wind, impact, or simply age, and rubber components like vent boots deteriorate faster than the metal around them. In many cases, flashing failures develop slowly and go unnoticed until water damage shows up inside the home — a stained ceiling, a damp patch in the attic, or a musty smell that indicates moisture has been getting in for a while.

Signs your flashing may be failing

• Water stains on ceilings or walls, particularly near a chimney or where an interior wall lines up with the roofline
• Visible rust, corrosion, or lifted edges on metal flashing during a roof inspection
• Cracked or missing sealant around vent pipes, skylights, or chimney bases
• Daylight visible through gaps around roof penetrations from inside the attic
• Granule buildup or debris collecting specifically around flashing points, which can be a sign shingles nearby are being disturbed

Why flashing needs a professional eye

Flashing failures are often subtle and easy to miss during a casual glance at the roof from the ground. They also require the right materials and installation technique for the specific application — chimney flashing, for example, needs to be properly integrated with the masonry, not just caulked on top of it. A roof inspection that specifically checks flashing at chimneys, valleys, vents, and wall intersections is one of the most valuable things a homeowner can have done, whether as routine maintenance or after a storm.