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What is Roof Flashing, Why is it Important, and How Can I Install It?

IKO Roof Flashing
June 11, 2019 at 6:00 p.m.


Roof flashing is a thin material, usually galvanized steel, that professional roofers use to direct water away from critical areas of the roof, wherever the roof plane meets a vertical surface like a wall or a dormer.

Editor's note: Use this information to help educate your homeowners or share with a team member who is new to the roofing industry. 

Flashing is installed to surround roof features, such as vents, chimneys and skylights. Water should run down the side of the flashing and be directed to the shingles instead of finding its way into the roof deck.

If there were no flashing against these walls, water could slowly drip into the crevice between the wall and the roof, and potentially into the home.

Damaged or improperly installed roof flashing, especially around the chimney or dormer, is one of the most common causes of roof leaks. According to the Federal Emergency Management Agency, FEMA, improper roof flashing could lead to wood rot and even deck collapse. So, what is roof flashing? How do you install roof flashing? What about repairing roof flashing? You can find out here.

Roof Flashing

There are almost as many types of roof flashing as there are parts of the roof! After all, each roof feature needs protection. These are the key types of roof flashing you need to understand:

  • Continuous flashing: Continuous flashing is also called “apron flashing” because it acts a lot like an apron. It’s a long, single piece of metal that carries water down to the shingles below. Long pieces of continuous flashing will have trouble flexing as the home expands and contracts in the changing seasons. If left as is, it could break or warp and fail to keep water out. Therefore, long pieces have built-in expansion joints so they can move with the home.

  • Base flashing: Some roof features, such as chimneys, require two pieces of flashing. This ensures that rain always meets a flashing surface that directs it downwards. Plus, it is notoriously tough to install flashing around a chimney. There is another benefit to two-part flashing: When the roof materials naturally expand and contract with weather changes, the two pieces can move, so the whole system stays secure. The base flashing (or apron flashing) is the bottom piece.

  • Counter-flashing: Placed opposite to base flashing, or above base flashing, counter-flashing completes the two-part team.

  • Step flashing: Step flashing is a rectangular piece of flashing bent 90 degrees in the center. It is used for roof to wall flashing. Multiple pieces of the flashing will be installed in layers with shingles to ensure the water flows away from the wall. Learn how to install it below.

  • Skylight flashing: While some skylight manufacturers include flashing in their products, sometimes roofing professionals have to create it or purchase it separately.

  • Valley flashingOpen valleys have metal flashing to protect this critical area of the roof.

  • Drip edges: The edge of the roof has thin metal flashing that helps water drip off the roof without damaging the home or causing a leak.

  • Kickout flashing: Roofing professionals need something to bridge the gap between where step flashing ends and the gutter begins. Kickout flashing directs water away from the wall and into the gutter. Learn how to install kickout flashing below.

While there are premade flashing pieces you can purchase, many roofing professionals will cut their own roof flashing from sheet metal. They’ll use tinning snips to cut the sheets and bend them to the exact size they need while on the roof. However, if you walk into a store to buy your own flashing, be careful. There are other types of premade flashing that are not made to be used on roofs and may break if you try to install them on a roof. For example, head flashing or cap flashing protects doors and windows from water.

Roof Flashing Sizes

Pay attention to size. The flashing for a plumbing vent needs to be large enough to surround the vent, so it should be just wider than the vent’s diameter.

Further, the step flashing pieces need to be 10 inches (254 mm) long and at least 2 inches (50 mm) wider than the exposure of the shingle being used. For example, 3-tab shingles usually have a 5 5/8-inch (143 mm) exposure. Typically, 10- by 8-inch (253 mm x 203 mm) flashing is chosen. Those 8 inches are more than enough to cover the exposure of typical shingles. The same size can be used to cover the longer exposure on CambridgeTM shingles.

However, if you were using IKO’s Crowne SlateTM, with a 10-inch (254 mm) exposure, you’d need a 10- by 12-inch (254 mm x 304 mm) step flashing piece.

Roof Flashing Materials

In the past, roofing professionals would use lead, or lead-coated materials, as flashing. However, roofing professionals in North America now prefer other materials:

  • Aluminum: Aluminum flashing is easy for roofing professionals to form and is lightweight. However, aluminum must be coated if it is to be used with concrete and masonry, as plain aluminum reacts and degrades when it touches alkaline surfaces. In coastal areas, aluminum flashing should be coated even if it does not contact concrete or masonry, to prevent corrosion.

  • Copper: Copper roof flashing is also malleable and takes soldering well. It’s also highly durable and has a long life. On the other hand, it does discolor into a patina, which some homeowners dislike, but others favor. You will still routinely find copper flashing around chimneys.

  • Steel: Steel is usually the material of choice for flashing. It’s malleable, has aesthetic value and, when galvanized, is corrosion-resistant.

Building codes may demand the use of a specific material for flashing. They may also list a minimum thickness. Most building codes require 26-gauge galvanized steel as a minimum. Always check local codes to be sure you’re following them.

In fact, if you're in Florida, you may find that building codes call for a special flashing material, modified bitumen roofing tape:

  • Modified bitumen roofing tape: Modified bitumen is a watertight, tough material. When made into a tape, it can be used to aid roof flashing. The specific tape you use will have to be approved by your building code and of a certain width.

Roof Flashing Sealant Types

When you install roof flashing, you will need to use a sealant. While roofing professionals occasionally use nails when flashing, they must choose whether to nail to the roof plane or the vertical wall. If nailed to both, the flashing may deform under the pressure from shifting brick or wood. When nailed only to the roof plane or vertical wall, the flashing can stay in place while the other building materials expand and contract throughout the seasons.

Roofing cement: Roofing cement creates a waterproof seal. Roofing professionals usually apply it with a trowel.

Roof Flashing Techniques

Before you learn how to install roof flashing, you need to understand that there are three primary techniques, each suitable for different areas on the roof. You’ll find that some types of flashing correspond to a specific technique.

  • Step flashing: Step flashing is best for where the roof face meets a wall, for example, where a dormer projects out of the roof. In this spot, it’s possible that water could flow down the wall and slip past the shingles into the building below. Step flashing ensures that water is directed away from the wall and ends up in the gutter. It’s installed in steps, with layers of shingles between, so that the water pours down each step and down the roof.

  • Counter-flashing: Counter-flashing is often used to flash chimneys. It involves two pieces of flashing. The first, base flashing, sits around the bottom of the chimney. The second piece, the counter-flashing, is embedded into the masonry of the chimney. It sits over the base flashing to ensure water doesn’t slip behind the base flashing. Construction professionals use counter-flashing for many other purposes, but it always involves a second piece of flashing set off from the first.

  • Plumbing vent boot flashing: Vent flashing is a cylindrical piece of flashing that fits right around the vent. The shingles are installed over the base or boot. The height of the boot forces water to run around the vent.

How to Install Roof Flashing

Disclaimer: Roof flashing should always be installed by professional roofers, who understand best practices, safety requirements and the building codes and laws in their area. These instructions are only to help homeowners understand what to expect from their roofing professional.

Step flashing is the most time-consuming of all flashing jobs on the roof because you must complete it step-by-step as you shingle up the roof. There are a few general best practices you need to know. First, step flashing must be installed before the siding, so that the siding can cover the top of the flashing. If this is a repair job, the siding must also be removed and replaced with the flashing. Second, step flashing needs to extend 8 to 14 inches above the shingles, according to the National Roofing Contractors Association (NRCA).

Also, before you start installing your flashing, you need to look to see if the wall in question has a corner on the roof face, as in the image below.

If it does, follow our first installation procedure. If it doesn’t have a corner, and simply looks like the image below, follow the second installation procedure.

How to Install Step Flashing With a Wall Corner

If a wall corner is on the rooftop, you will need to create a corner flashing piece using a typical step flashing piece. These instructions will tell you how to create a corner piece and how to install step flashing generally:

  • Step One: Install your underlayment completely and your shingles up to where the wall begins, so that the first piece of flashing, or the corner flashing, will rest on a shingle.

  • Step Two: To make corner flashing, take your tin snips and cut a 45-degree line from an outside corner to the center fold. Then, cut along the center fold and remove the resulting triangle. This will allow you to bend the step flashing around the corner cleanly. If you do not wish to form your own corner flashing, you can buy pre-bent pieces and cut them to size, or use a piece of corrugated aluminum, which is easier to bend.

  • Step Three: Bend the corner flashing tightly around the corner. Ensure it sits flat and extends at least 8 inches above the shingles. Nail it in place with two nails, one on each side of the top edge.

  • Step Four: Take a second piece of flashing and set it down in place. Bend it back so that it overlaps the corner flashing.

  • Step Five: Remove this second piece and apply your sealant where the flashing will overlap. Then, set the piece down. Hammer one nail to the bottom of the flashing, high up so that the next row of shingles will cover it.

  • Step Six: Finish a whole shingle course above this piece of flashing.

  • Step Seven: Now you must apply a piece of step flashing where the next course of shingles will start. This piece must overlap the last piece of flashing by at least 3 inches. Apply sealant where the base of the flashing will sit, place the flashing.

  • Step Eight: Continue to alternate between shingles and flashing until you reach the top of the roof.

  • Step Nine: To flash the peak of the roof, you will need to create another custom piece of flashing. Take a usual piece of step flashing and cut into its fold, about halfway.

  • Step Ten: Leaving one side of the cut piece straight, bend the other side to match the peak of the roof, as shown below. Use roofing cement to secure this piece and a single nail in the base. Later, you will cover it with a ridge shingle.

Watch our Video on How to Prepare and Install Roof Flashings

How to Install Step Flashing Where There is No Wall Corner

If your roof face connects to the wall cleanly without creating a corner, then you don’t need to create a corner flashing piece. Instead, you will need to install kickout flashing to help guide the water into the gutter. While some professional roofers will make their kickout flashing by hand in copper, if you’re using galvanized steel, it will be too tough to bend properly. Instead, buy a premade kickout flashing piece.

Here’s how to install kickout and step flashing:

  • Step One: Place your kickout flashing on the base of the roof, snugly against the wall. Briefly remove the piece and apply roofing cement where it will sit.

  • Step Two: Place the first piece of step flashing over the end of the starter strip, ensuring it leads directly into the kickout flashing. Use roofing cement and two nails to secure it to the roof deck. Place the nails on the base of the step flashing piece, so you’re nailing into the deck. Also place the nails high, so the next course of shingles will cover them.

  • Step Three: Once the kick flashing and first piece of step flashing are secure, you have to apply a shingle. First, apply sealant to the base of the flashing. Lay a shingle on top of the flashing and nail it as usual. Notice that the bottom of the shingle covers the flashing base and nail.

  • Step Four: Finish the whole shingle course.

  • Step Five: To complete the step flashing, follow the same procedure as described above for installed step flashing with a corner piece, starting at step seven.

How to Install Counter-Flashing on a Chimney

Chimney flashing should be installed at the same time the mason is laying the brick and mortar, or the roofer will have to cut a ridge out for the counter-flashing. Then, the roofer will have to seal this indent up, after placing the flashing.

  • Step One: Ensure the chimney’s base flashing is in proper order, secured to the roof as the manufacturer or mason recommends, beneath the shingles. If you were not provided with base flashing, you can install step flashing up the side of the chimney.

  • Step Two: Cut an indent into the chimney with a diamond bit saw/diamond grinder disc. This is where you will hang the counter-flashing.

  • Step Three: Insert the counter-flashing to the indent. Be sure that it hangs so that it overlaps with the base flashing by at least 2 inches.

  • Step Four: Use roofing cement to secure the counter-flashing to the base flashing and the chimney.

  • Step Five: Seal the indent with roofing caulking, so the counter-flashing hangs securely.

How to Install a Roof Plumbing Vent Flashing Boot

Installing a roof plumbing vent is typically an easier task than flashing a chimney. Here is how you do it:

  • Step One: Install shingles as normal up to the base of the plumbing vent.

  • Step Two: Place the flashing or boot onto the plumbing vent, so the base is resting on shingles. Momentarily lift the boot and apply sealant to hold the flashing in place.

  • Step Three: Firmly push the flashing back down into place.

  • Step Four: Install the next course of shingles. When you reach the plumbing vent, allow the shingles to overlap the top of the flashing.

  • Step Five: To make room for the vent itself, cut out a circular piece of the shingles, as shown below.

  • Step Six: To secure the circular edge of the shingle, apply roofing cement beneath it. Ensure you also nail it where you normally would.

Do you need information about how to install the other kinds of roof flashing? IKO has already explained how to install drip edge flashing and how to install metal valley flashing.

When to Repair a Roof Flashing Leak

You might also wonder when you should replace roof flashing. Check over your flashing and look for:

  • Damage, holes or bending.
  • Corrosion or rusting.
  • Loose or missing nails.

If you find any of these problems, you may need to replace the flashing.

You may also wonder if you’ll need to replace your flashing when you’re having a new roof installed. That depends on a few factors. If the original flashing is in good condition and can be removed without damaging it, it is possible to reuse it. However, you’ll need to prioritize proper fit and seal over saving money if you want a watertight roof; so be prepared to purchase new flashing where needed.

  • Dried out or missing sealant.
  • Flashing that has come loose.

How to Repair Roof Flashing

Fixing roof flashing, just like installing flashing, is best left to roofing professionals. You should not take risks with your roof.

  • Step One: Gently pry up the shingles surrounding or covering the damaged flashing. When working with step flashing, this may involve removing undamaged pieces of flashing as well.

  • Step Two: Take your chisel and gently remove any asphalt cement you find.

  • Step Three: If you caught the flashing problem before a leak began, there should be no damage to the underlying roof parts and structure. However, it’s important to check for damage anyway. If you find damage, then you will have to get it repaired before reinstalling the flashing.

  • Step Four: Now, reinstall the new flashing and shingles. Be sure to follow the proper order, as described above, especially when installing step flashing, to ensure the flashing will stand up to water.

Though roof flashing isn’t the most dramatic element of a roof, it is one of the most important. If you find some flashing on your roof that you’re concerned about, we suggest you find a professional roofer through IKO’s Contractor Locator. A professional can help you solve all of your roof flashing problems or answer the questions you have about your specific roof.

Learn more about IKO Roofing Systems. 

Source: IKO

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