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What Is Roof Flashing? (2024 Guide)

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Default Author Icon Written by Angela Bunt Updated 04/01/2024

Flashing creates a watertight seal around roof penetrations and transitions, preventing leaks and protecting the underlying structure. Installing it incorrectly allows water to seep under your shingles and cause serious problems such as water damage, mold, and weakened structural integrity. Our guide explains what roof flashing is and how it works, as well as average costs and how to hire a professional.

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What Is Roof Flashing?

Roof flashing is a thin strip of sheet metal or plastic installed around the edges of roof features such as chimneys, vents, and flue pipes. Roof flashing redirects water away from various other parts of the roof:

  • Roof valleys: Valley flashing is used at the low points of a roof where two slopes join.
  • Roof protrusions: Windows, skylights, chimneys, vents, and any other protruding features need flashing where they penetrate the roof.
  • Side walls and front walls: These walls require step and headwall flashing where they intersect the roofline.
  • Roof edges: Flashing is often used along a roof’s edges—especially at the eaves, where a drip edge helps protect the fascia and guide water into gutters.

Roof flashing is usually installed over the underlayment and under the shingles on shingled roofs. On metal roofs, it’s installed on top of the panels. When flashing isn’t installed correctly, serious structural issues and water damage can occur. The longer a problem goes unnoticed, the higher the cost of roof repairs will be.

Roof Flashing Materials

Most roof flashing is a flat, thin piece of metal. The best material for your home depends on factors such as climate and roof pitch. According to Ken Byler, owner of Arizona-based Easy Roof Solutions, copper and aluminum flashing are generally considered the most durable and longest-lasting options.

Here’s a look at the different metals used for flashing and their strengths and weaknesses:

Copper

Aluminum

Steel

Lead

Copper flashing is durable but one of the most expensive flashing materials. It’s highly resistant to corrosion, so it’s especially popular in regions with harsh weather. Although it doesn’t rust and has a long life span, copper develops a green patina over time. Copper’s malleability makes it easy to solder and shape to fit any roof’s contours.

Similar to copper, aluminum is durable and able to withstand harsh weather. Aluminum is lightweight and low-cost, but it can corrode if left unpainted and exposed to salty air, wood, concrete, mortar, and other masonry materials. The aluminum alloy found in flashing can also pit and oxidize over time.

Galvanized steel is the most common roof flashing material. It’s the cheapest but also the least durable type of metal flashing. Though its zinc coating holds up fine under normal conditions, it can’t typically withstand harsh weather. Over time, the galvanized coating wears away and leaves the underlying steel vulnerable to corrosion. Contact with masonry materials (such as brick, concrete, or cement) or pressure-treated wood will speed up its deterioration.

A longer-lasting option is stainless steel, which is highly resistant to corrosion and often used in coastal environments. Stainless steel, though, is considerably more expensive.

Lead is one of the oldest roof flashing materials. It’s also one of the longest-lasting, with a life span of over 200 years. Lead is soft and highly malleable, making it easy to bend and mold to the roof. However, lead must be managed and disposed of as hazardous waste under the Resource Conservation and Recovery Act (RCRA) subtitle C requirements. Inhaling or ingesting lead can cause serious health problems, so it’s rarely used today.

Not all roof flashing is made of metal. Some products use thermoplastics like polyvinyl chloride (PVC), thermoplastic polyolefin (TPO), or ethylene propylene diene monomer (EPDM) rubber. Plastic flashing is generally not as durable as metal flashing, and specific formulations may vary between manufacturers.

The best material for a specific application will depend on factors such as your roof type, budget, aesthetic preferences, and regional climate. For instance, homes in coastal areas require materials highly resistant to salt corrosion.


How Does Roof Flashing Work?

When rainwater or melted snow flows down the roof, it can pool in areas where the roof intersects with a vertical surface or changes slope. Contractors install roof flashing to redirect water away from these areas into gutters and drainage systems. Here’s how roof flashing works in roof installation:

  • Identifying vulnerable areas: A roofing professional will identify areas where roof flashing is necessary. These are areas vulnerable to roof leaks such as skylights, vents, and chimneys.
  • Material selection: Your chosen flashing material depends on factors such as climate, budget, personal preferences, and local roofing codes and regulations.
  • Cutting and shaping: The roofing contractor will cut the flashing to make sure that it fits around each roof penetration.
  • Secure installation: The roofer installs flashing using nails or screws and shapes it to the roof’s contours.
  • Watertight sealing: The contractor uses roof sealant or cement to create a watertight seal between the flashing and the roof surface.
  • Strategic overlapping: Multiple flashing layers are usually overlapped to better direct water away from the roof and toward the gutters or downspouts.
  • Final inspection: The contractor inspects the installation to check that the roof flashing is securely attached, watertight, and sloped for drainage.

Flashing works in tandem with roof insulation to seal your home from the elements. Good insulation can minimize condensation, which reduces the risk of moisture reaching vulnerable roof areas where flashing is installed. Meanwhile, proper flashing is necessary to keep water from reaching and ruining your insulation.

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The type of roof flashing used depends on its location and intended purpose. Here are the most common types of roof flashing:

Base Flashing

Base flashing is a waterproof layer installed over underlayment and underneath roof shingles. It prevents water damage where vertical walls intersect the roof deck.

Step Flashing

Step flashing is installed using L-shaped pieces. Each piece goes under the shingles and bends up against the vertical surface. This flashing type is partially visible, so some homeowners choose a material and color that matches the home’s exterior.

Counter Flashing

Placed above or opposite base flashing, counter flashing—also called cap flashing—is commonly layered on top of step flashing between the roof’s surface and chimney seams. While you can add counter flashing to an existing chimney, it’s best to handle this installation during the chimney’s construction.

Chimney Flashing

Chimney flashing typically consists of two components: base flashing and counter flashing. It’s found at the base of a home’s chimney and prevents water from seeping in between the chimney and roof.

Drip Edge Flashing

Drip edge flashing is installed along the edges of a roof and acts as a barrier between shingles and the roof’s edge. The purpose of drip edges is to redirect water away from the roof’s eaves and into the gutters, preventing water from seeping into the roof decking or running down the sides.

Headwall Flashing

Contractors use headwall flashing, or apron flashing, at the point where the roof’s upper end meets a vertical headwall. It’s made from an L-shaped metal and is custom fit to the section of headwall.

Valley Flashing

Valley flashing directs water away from the intersection of two slopes. Water and melted ice easily collect here, but valley flashing helps guide it toward the gutters.

Gutter Apron Flashing

While very similar to drip edge flashing, gutter apron flashing is installed underneath the shingles and over the gutter’s back edge. It creates a barrier to prevent water from getting behind the gutter and into the roof system.

For a quick tutorial on drip edge and step flashing, check out this video with general contractor Tom Silva. You’ll see what these two types of flashing look like, plus a visual demonstration of how to install them.


How Much Does Roof Flashing Cost?

Roof flashing repair costs an average of $200–$500, while replacement can range from $300–$1,500.* The price depends on the flashing material, the roof’s size and configuration, cleanup costs, and the extent of damage.

Labor costs typically fall in the range of $45 to $75 per hour but can be significantly higher for a large or complex job.

*Article cost data sourced from Angi and Fixr.


How To Hire a Professional

You may be tempted to install roof flashing yourself, but this job requires specialized knowledge and tools. Byler says it should always be completed by a licensed professional. Here’s what to look for in a roofing expert:

Ask friends and family for referrals and search online for local roofing contractors.
Check ratings and reviews on sites such as Yelp, Trustpilot, and the Better Business Bureau (BBB). Keep an eye out for complaints and negative reviews.
Ask for examples of previous work.
Verify licensing and insurance. You can check with your state’s licensing board online.
Request quotes from multiple contractors and compare prices.
Get a cost breakdown and request a project timeline.

Our Conclusion

Roof flashing is vital to protect your roof from water damage. Without it, water can seep into the roof deck and inside your home, resulting in the need for emergency roof leak repair or even a full roof replacement Hiring a professional contractor to properly install roof flashing can help to prevent these issues and save you money down the road.

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FAQ About Roof Flashing

What is the best type of roof flashing?

The best type of roof flashing depends on your budget, the required durability, and its visibility. Durable, corrosion-resistant materials like copper and stainless steel offer the longest life span, but aluminum and galvanized steel are budget-friendly options with reasonable durability.

What happens if you don't have roof flashing?

If you don’t have roof flashing, water can leak between joists and seams into your roof deck. This causes structural damage along with mold and mildew growth. Repairing water damage is expensive and time-consuming.

Can I install roof flashing myself?

While it’s technically possible to install some types of roof flashing yourself, we don’t recommend it unless you’ve worked as a professional roofer. Installing roof flashing requires specialized knowledge, tools, and experience to ensure a watertight fit. A professional roofing contractor will help you choose the best flashing and materials to get the job done right.

Does flashing go over or under shingles?

Roof flashing can go over or under shingles depending on the flashing type and roofing material. Flashing should overlap asphalt shingles, but for aesthetic purposes, flashing that extends over shingles is typically covered with additional tabs. On metal roofs, flashing is installed on top of the panels.

How long does flashing last on a roof?

How long flashing lasts on a roof depends on the material used. Copper can last 100 years or longer while stainless steel can last over 50 years. Aluminum and galvanized steel have shorter life spans in the range of 20–40 years.

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