Roof flashing is a metal barrier that directs water away from vulnerable roof areas like chimneys, walls, and valleys. It’s made from steel, aluminum, or copper and prevents leaks and rot. Flashing should be replaced during new roof installs and inspected yearly to avoid failure. Damaged flashing causes leaks, mold, and structural damage.
What is Roof Flashing?
Definition and Function
Roof flashing is a thin sheet of metal installed at roof transitions, edges, or penetrations to direct water away from joints and seams. It’s used around chimneys, walls, skylights, and valleys—areas where shingles or roofing membranes alone can’t fully seal.
Made from corrosion-resistant metals like aluminum, steel, or copper, flashing protects against leaks by creating a water-tight barrier in vulnerable areas. Without flashing, rainwater can easily infiltrate the roof deck, leading to rot, mold, insulation damage, and structural weakening.
Flashing is not one-size-fits-all. There are various types, each with a unique role depending on the roof’s layout and architecture.
Common Types of Roof Flashing
Apron Flashing
Apron flashing is an L-shaped strip used where vertical walls meet sloped roofing surfaces. It is most commonly found at the base of chimneys or dormers. The vertical leg runs up the wall while the horizontal leg overlaps the roofing material to guide water away.
Step Flashing
Step flashing consists of individual metal pieces layered with each course of shingles, forming a staircase-like pattern. It’s used where a sloped roof meets a vertical wall. Each flashing piece overlaps the shingle below and the one above, directing water away at each step.
Counter Flashing
Counter flashing is embedded into a masonry wall and overlaps step flashing to create a sealed waterproof system. This method is often used around brick chimneys or walls to prevent moisture from getting behind the step flashing.
Valley Flashing
Valley flashing is installed in the valleys where two roof slopes meet. These channels handle the highest water flow and require a durable flashing piece that can effectively direct runoff toward gutters without pooling.
What Materials Are Used for Flashing?
Steel
Galvanized steel is strong, affordable, and commonly used in residential roofing. It typically lasts 15–25 years when properly maintained. The zinc coating helps prevent rust, but over time, sealants and fasteners may degrade.
Aluminum
Aluminum flashing is lightweight, flexible, and rust-resistant, lasting approximately 20–35 years. It’s ideal for detailed roof shapes and is often painted or coated to extend its life.
Copper
Copper flashing is a premium option with a lifespan of over 70 years. It naturally resists corrosion and develops a protective green patina. It’s commonly used in upscale or historic homes due to its appearance and durability.
Cost and Durability Comparison
| Material | Lifespan | Corrosion Resistance | Relative Cost |
| Steel | 15–25 years | Moderate | Low |
| Aluminum | 20–35 years | High | Medium |
| Copper | 70+ years | Excellent | High |
Should Roof Flashing Be Replaced During a New Roof Installation?
When to Replace vs. Reuse
Flashing should be replaced with a new roof unless it’s made from copper or in excellent condition. Even if it appears intact, old flashing may have hidden weaknesses like cracks, corrosion, or unsealed edges.
Risks of Keeping Old Flashing
- Leaks around chimneys or sidewalls
- Wood rot beneath roof decking
- Mold growth in attic insulation
- Voided warranties due to incomplete replacement
Replacing flashing ensures long-term durability and leak protection, especially around penetrations and valleys.
For professional replacement, see roof replacement or residential roof repair services.
Does Flashing Require Maintenance?
Annual Inspection Importance
Roof flashing should be inspected at least once a year—preferably in spring or fall—to catch early signs of wear or damage. Inspections focus on:
- Dried or missing caulk
- Rust or corrosion
- Lifted flashing edges
- Loose nails or cracked joints
In high-moisture regions, semi-annual checks are advisable.
What Can Go Wrong if Flashing Fails
- Interior water stains
- Fascia and decking deterioration
- Shingle damage from water backup
- Energy efficiency loss from air leaks
Even a minor flashing failure can escalate into major structural issues. Regular maintenance prevents costly roof repairs.
For inspection or repairs, consider roof inspections or emergency roof leak repair.
FAQs About Roof Flashing
What causes flashing to fail?
Age, corrosion, UV exposure, improper installation, and severe weather are common culprits. Over time, even quality metal flashing can lose its seal or detach from roofing surfaces.
How long does flashing last?
- Galvanized steel: 15–25 years
- Aluminum: 20–35 years
- Copper: 70+ years
Proper maintenance can help flashing reach the upper end of its lifespan.
Is DIY flashing replacement safe?
No flashing must be integrated under shingles and sealed with specific fasteners and caulk. Improper installation leads to leaks and invalidates warranties. Hire a qualified contractor for secure flashing.
Explore options with metal roof installation or asphalt shingle roof installation.
For deeper technical guidance, visit the Oatey official guide to roof flashings.
