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What is Roof Flashing & How Does It Work?

Published on Tuesday November 22, 2016

What is roof flashing?

When it comes to your roof, shingles often get all the credit. Asphalt shingles look nice, they stand up to the weather, and they’re tough and durable. Shingles, though, are only part of the whole roof system.

The unsung hero of the operation is roof flashing.

Roof flashing is a thin layer of metal sheeting used to direct water away from seams and joints where moisture could penetrate the roof and cause water damage. 

What is the Purpose of Flashing on a Roof?

Underlayment and roof flashing are required to keep water from getting underneath your shingles.

Flashing is critical to certain areas of your roof —  namely, the places where the roof surface meets a wall (sidewalls and front walls), the low points where two roof slopes meet (called valleys), roof protrusions (bathroom/kitchen vents, skylights) and the roof’s edges (rakes and eaves).

Types of Roof Flashing

Most flashing roof materials are made from metals like aluminum, copper, or stainless or galvanized steel. Flashing can be used around windows, doors, chimneys, gutters — basically, any exterior joint where water needs to run off.

Common types of roof flashing include:

Base Flashing & Step Flashing 

Base flashing and step flashing are used where a vertical wall of the house intersects the surface of the roof deck. The two main types of walls are:

  1. Front walls (also called headwalls): Vertical walls behind a sloping roof deck
  2. Sidewalls: Vertical walls along the edge of a sloping roof deck

What is the purpose of flashing on a roof?

As wind blows and rain pours, imagine how water would be blown against the walls of a home. The water would run down the wall and right into the joint where it meets the roof deck below.

Thanks to base flashing at the joint, water is stopped before it gets between the wall and the shingles and is directed into the gutters. Roof flashing acts sort of like a gutter for your roof deck.

Some key points to remember:

  • Base flashing is the solution for front walls. Base flashing is one length of flashing that is bent (along the length) to match the pitch of the roof. Base flashing is installed over underlayment, but under siding and shingles, so you may not be able to see it from the ground.
  • Step flashing should always be used where the roof deck meets a sidewall. Pieces of step flashing are bent at an angle relative to the roof pitch and installed along the full length of the sidewall where it joins the roof deck. The flashing should be partially exposed along the wall. Since you can see step flashing, some homeowners choose a more expensive flashing material like copper to achieve a certain aesthetic.

Step flashing is considered a much better alternative than continuous flashing since it does a better job of protecting against leaks. If a single piece of the step flashing has loosened roofing cement or fails, then rainwater simply hits the next piece of step flashing and doesn’t turn into a massive leak.

The next piece of flashing directs any water onto the next shingle and away from your home’s roof.

Step flashing has been used on most one- and two-family homes with asphalt shingle roofs since the 1980s. L-shaped metal pieces that are a bit longer than the shingle overlap are installed over each shingle, adjacent to the sidewall.

Then the flashing is laced into each course to ensure that any water that makes its way under a shingle ends up on the flashing, then flows down on top of the next shingle. 

Step flashing helps ensure that water drains safely with the least amount of risk to your roof.

Valley Flashing

Roof flashing purpose.

Valleys are the places where two roof decks slope toward one another, creating a low line that looks like a valley between two mountains.

Water flows into the valleys on your roof the same way rivers wind through mountain valleys. Valleys, as you’d imagine, are a prime target for water penetration.

So what to do?

Some shingles can be woven to provide extra layers of protection to the roof valleys. However, many dimensional shingles are simply too thick and inflexible to be woven.

The solution: valley flashing.

Metal valley flashing is installed over the underlayment, but under the edges of the shingles. In this “open valley” style, the shingles over each section of the roof do not touch. They are installed to overlap and form a gap that exposes the valley flashing. This allows water to run off the edges of each roof, into the valley flashing, and down to the gutters.

Valley flashing is visible from the ground. Some homeowners choose a unique color of valley flashing to add a striking contrast to the color of their shingles.

Chimney Flashing 

Purpose of roof flashing.

Penetrations like chimneys, vents, and dormers are more complicated when it comes to flashing. Each feature requires a unique set of materials and installation techniques to provide a water-tight seal.

While the flashing needs at each penetration site may differ, waterproofing these locations is always critical.

“Flashing and penetrations in a roofing system are similar to keeping a boat afloat. In a marine vessel, you have penetrations in the hull that must keep the water out, or you will sink. In a home, you have extreme weather, winds, wind-driven rain, freeze-thaw cycles and summer rain cycles that constantly beat on the exterior of your home,” Jeff Frantz, Washington D.C. territory manager for CertainTeed Roofing Products explains.

“Using proper flashing components and maintaining penetrations like skylights are critical to the performance of the roof as a system…Newer technology in flashings and roofing materials, not to mention product performance levels, has improved greatly in the last 20 years, and flashing components need to be consistent with the new roof system.”

Drip Edge Flashing

Drip edge directs water at the edge of your roof so that it won’t get under your roofing materials or rot the wood at the edges. Drip edge is MOST important at the rake edges of your roof, but it’s still good to have at the eave edges, as well.

Did you know that the most common cause of all home insurance claims is exterior wind damage? According to The Travelers Companies Inc., 25% of all losses from 2009-2015 were due to wind damage.

During severe winds, shingles are more likely to lift along the rake edges where they don’t have the benefit of shingle overlap at the edge. Flashing wrapped at the rake edge gives an extra layer of defense.

Your shingles and gutters do most of the work at the eaves, so the drip edge is merely on-call support. Drip edge at the eaves protects the fascia board from water caused by clogged gutters or ice dams.

While some roofs may not have walls or valleys that need to be flashed (a hip style roof doesn’t have either), all roofs can benefit from drip edge at the eaves. Your roofing professional can explain the best options for your roof’s particular needs.

Roof flashing is different for every home because it is always tailored to your roof’s unique features. The type of shingles you’ve selected and the style of your home have a lot to do with the types of flashing that your roofing professional will recommend.

Skylight Flashing

Flashing is a very important part of any proper skylight installation. Skylight flashings are the first layer of protection against leaks and moisture. The entire purpose of skylight flashing is to prevent as much moisture from getting on the surface of your roofing material as possible. They accomplish this without relying on sealants that can break down, which can cause damage later on.

Without properly installed skylight flashing on all of your roof’s skylights, your skylight could end up causing a lot of problems for your roof. Skylight flashings are what help guarantee that your skylight won’t leak, so without them, you risk water seepage, roof rot, corrosion, mold, and even structural damage to your home.

Installing skylight flashing isn’t only recommended; it’s best practice and essential if you want to avoid roof leaks at a skylight.

Continuous Flashing

Continuous flashing is a type of roof flashing sometimes seen where the sidewall and roof meet. It is installed in a long run of flashing, often meant to prevent leaks in this corner space. It’s important to know, however, that continuous flashing isn’t a desirable type of flashing and doesn’t actually do a great job of preventing leaks.

The idea behind continuous flashing is that a single, continuous piece of flashing would offer more protection than several pieces. Unfortunately, the results are fairly poor. If a small section of the roofing cement fails, then the entire continuous flashing is compromised, and you have a leak that easily spreads.

If your roof has been installed using continuous flashings, you may want to get it replaced soon with something more reliable, like step flashing.

Kickout Flashing

Also called diverter flashing, kickout flashing works to divert rainwater and moisture away from the cladding of your home and into the gutter system. When kickout flashing has been installed correctly, it does a great job of preventing unwanted water penetration. 

If you’re dealing with rainwater intrusion near your gutter system, there are a lot of possible reasons for this, but a missing or faulty kickout flashing could be the cause. Missing or faulty kickout flashing is often responsible for large accumulations of water which can cause severe damage to your home’s exterior walls.

This damage may or may not be easy to catch since water penetration to your home’s cladding may not be recognizable upon a visible inspection. When your kickout flashing isn’t properly installed, you can end up with severe damage to your home without any visible evidence.

What Materials are Used for Roof Flashing?

You’re likely pretty familiar with the different material types for your roofing—asphalt shingles, cedar shakes, metal—but understanding flashing materials is a bit less common for homeowners to know about. Flashing is absolutely necessary to ensure your roof is installed well, and your home is protected, so here’s what you should know about the different materials used for roof flashings:


This is the most durable flashing material on the market today, and it’s also very eye-catching. Copper doesn’t tend to develop rust, even after years of exposure. It ages more gracefully by forming a green patina over time and still tends to look great with different roofing types. It doesn’t require painting or regular treatments to remain effective.


Galvanized steel is a popular flashing material type and works very well with metal roofs since it’s usually made of the same or similar materials. Steel flashings usually have a zinc coating to protect them against corrosion, allowing them to last much longer under exposure to moisture and other weather elements.


Lead is actually a very durable material for flashings, and many prefer it right behind copper. It naturally has long-lasting resistance to water damage and since it is a softer metal, it’s able to expand and contract with the outside temperatures easier than many other materials. Lead flashings are believed to last for more than 200 years, so they should be fine to last the entire lifespan of your roof.


This is a relatively inexpensive flashing material while also being malleable and durable. It’s a great material for trickier sections of a roof due to its malleability, such as chimney flashings, base flashings, and valley flashings. It does need a protective finish, however, since it corrodes easily when exposed to weather elements, wood, cement, or concrete.

Does Roof Flashing Ever Need to be Replaced? 

The only way to absolutely guarantee the quality of flashing is to replace it, so if you’re worried about the quality of your flashings when getting a new roof, feel free to have yours replaced. That being said, many flashings outlive the normal lifespan of a roof and can be reused.

Many roofing contractors will even recommend you reuse your roof flashings since they are designed to last and are often in fine condition. If your flashings have experienced weather damage, have holes, are corroded, or show any other signs of damage and wear, it’s best to replace them. If they’re in good shape, however, then they can be reused. 

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