Roof Flashing Guide: Types, Installation & Repair

Flashing is the unsung hero of your roofing system. These thin metal pieces installed at every joint, transition, and penetration are what actually keep water out of your home. Poor flashing is the number one cause of roof leaks, making it the most critical detail in any roofing installation.

What Is Roof Flashing?

Roof flashing is thin, shaped metal (or sometimes rubber or composite material) installed at every point where the roof surface meets another surface, changes direction, or is penetrated by a pipe, chimney, skylight, or other structure. Its purpose is to redirect water away from these vulnerable joints and onto the shingles, where it can flow safely into the gutters.

Without flashing, water would seep into gaps between the roof and walls, around chimneys, down vent pipes, and at every valley and edge. Even the best shingles cannot waterproof these transitions alone.

Types of Roof Flashing

Step Flashing

L-shaped pieces installed in a stair-step pattern where the roof meets a vertical wall (dormers, chimneys, sidewalls). Each piece overlaps the one below, creating a waterfall effect that directs water away from the wall. Step flashing is the most common type on residential roofs.

Counter Flashing

Metal embedded into a mortar joint in masonry (chimneys, brick walls) that folds down over step flashing. It provides a secondary barrier against water running down the wall surface and getting behind the step flashing.

Valley Flashing

Wide metal installed in the valley where two roof planes meet. Valleys concentrate water flow, making them high-risk leak areas. Valley flashing can be open (metal visible) or closed (shingles woven over the metal). Open valleys with W-shaped metal provide the best water management.

Drip Edge

Metal installed along the eaves and rakes (gable edges) that directs water into the gutters instead of allowing it to run back under the shingles or down the fascia. Building codes now require drip edge on all new installations.

Vent Pipe Boot

A rubber or lead collar that fits around plumbing vent pipes protruding through the roof. The base slides under shingles above and over shingles below, while the collar seals around the pipe. These are one of the most common failure points.

Skylight Flashing

A specialized flashing kit (usually provided by the skylight manufacturer) that integrates step flashing, head flashing, and sill flashing around all four sides of a skylight.

Kickout Flashing

Installed where a roof-to-wall intersection ends at an eave. It kicks water away from the wall and into the gutter. Missing kickout flashing is a common cause of water damage to exterior walls and the structure beneath.

Flashing Materials

Where Flashing Is Needed

Every roof requires flashing at these locations:

• All eave edges (drip edge)
• All rake edges (drip edge)
• Valleys where roof planes meet
• Around chimneys (step, counter, and cricket flashing)
• Around skylights
• Around plumbing vent pipes
• Where the roof meets any vertical wall
• At roof-to-wall transitions (kickout flashing)
• Around HVAC units, exhaust vents, and other penetrations

Installation Basics

Proper flashing installation follows key principles:

1. Overlap in the direction of water flow. Upper pieces always overlap lower pieces so water flows over, not behind, the flashing.
2. Use compatible metals. Dissimilar metals cause galvanic corrosion. Do not mix copper and aluminum or galvanized steel and copper without a barrier.
3. Sealant is a supplement, not a substitute. Flashing should shed water by design. Sealant provides additional protection but should not be the primary waterproofing method.
4. Allow for thermal movement. Metal expands and contracts with temperature. Fasten at one end and allow the other to slide.

Flashing installation is one of the strongest arguments for hiring a professional roofer. Improper flashing is the single most common cause of roof leaks.

Common Flashing Failure Points

• Rubber vent boots: Sun exposure degrades rubber, causing cracking within 10-15 years. These should be checked annually and replaced when cracked.
• Chimney flashing: Mortar joints crack over time, allowing counter flashing to separate. Step flashing can rust or shift.
• Sealant failure: Roofing sealant dries and cracks within 5-10 years. Joints relying solely on sealant will eventually leak.
• Missing kickout flashing: One of the most overlooked details. Causes water damage to siding and the wall structure.

Check flashing during your regular roof inspections to catch failures early.

Repair vs Replace

Repair when flashing is intact but sealant has failed or a small section has shifted. Resealing and repositioning is straightforward.

Replace when flashing is rusted through, cracked, or missing. Also replace all flashing during a full roof replacement - reusing old flashing with new shingles is false economy. New shingles will outlast old flashing, creating leak points partway through the roof's life.

Frequently Asked Questions