Roof Flashing: What It Is and How It Works – The Ultimate Guide

This guide answers some of the most frequently asked questions homeowners have about roof flashing. Whether you are tackling a complete re-roofing job, or simply need to repair the existing flashing on a leaky roof, this guide will help you find the right information in no time!

End-wall copper flashing on an asphalt shingle roof
via: Runyon and Sons Roofing

Table of contents:

Here is a quick table of contents to help you quickly find the right information.

  1. What Is a Roof flashing and Why Is It Important?
  2. Types of Flashing
  3. Roof Material Expenses (i.e., the cost of flashing and roof feature repair costs)
  4. Signs of Roof Flashing Failure
  5. How to Repair Roof Flashing
  6. Can Roof Flashing Be a DIY Project?

What Is a Roof Flashing and Why Is It Important?

While roofs are purposely constructed to allow rainwater run-off, they can often have several vertical features or protrusions, such as chimneys, parapet walls, half walls, dormers, skylights, vent pipes, and the like, where water can pool up and/or drip into the crevices between. The areas can eventually become damaged and allow water seepage and leaks into the house. To prevent this, professional roofers apply roof flashing to divert rainwater down the sides of the vertical surfaces, keeping the water from stagnating.

Roof flashing is typically a thin metal material fabricated from rust-resistant metal, such as G-90 galvanized steel, a frequently used flashing material, copper, or aluminum. Depending on the company, flashing may even be available in lead or zinc alloy.

Steel, copper, and aluminum are particularly used because of how malleable these metals are, rendering them easier to shape. However, the homeowner should note the different characteristics of each metal:

  • Galvanized steel is less costly than aluminum and copper, and is well resistant to corrosion. It is for these reasons that steel is most often chosen for flashing.
  • Copper is quite durable, but it discolors into a patina over time. Some homeowners appreciate the classic appearance of copper with patina.
  • Aluminum is easier for roofing professionals to shape and lighter in weight. Aluminum must be coated to be preserved. Bare aluminum breaks down when touching alkaline surfaces, such as mortar used to secure bricks. As long as the aluminum is coated, it can be used on concrete or stonework.

Flashing closes the joints between the roof and the roof features in order to protect the house from erosion and leaking. A lack of flashing or poor flashing installation can also lead to wood rot, potential shingle damage, or deck collapse, among other issues.

For installation, the roofer will have to choose between nailing the flashing or using a sealant. As professional roofers may explain, nailing can involve some risk. If flashing is nailed to both the roof plane and the vertical surface, it can cause the flashing to deform from the pressure of shifting brick or wood.

The flashing will stay in place when nailed properly to only one of the surfaces, but nailing can create unnecessary holes in the roof structure. An alternative that professionals use is roofing cement, which creates a waterproof seal between the roof and the flashing.

Flashing comes in various shapes and is used for different areas on the roof (see “Types of Flashing” below). For this reason, different flashing types will be installed in dissimilar ways, and the cost of flashing will vary.

If the homeowner is uncertain about having flashing installed, it should be noted that alternative materials, such as sealant or tar, are ineffective long-term. They break down, making the areas they are purposed to protect vulnerable to water. Flashing, on the other hand, is made for long-term use and, when installed properly, keeps the roof from blemishes and damage.

Types of Flashing

Flashing is vital for specific areas around the roof: where the roof abuts a wall, low points where two roof slopes converge (called valleys), roof protrusions (such as skylights), and roof edges (called rakes and eaves).

Flashing is shaped into various designs depending on where it is needed along the roof.

There can be a wide variety of names assigned to each flashing style, but the list boils down to the following:

Base flashing

is at the joint between the roof and a front wall, parapet, chimney, or other vertical surface.

Applying base flashing to a front wall – where asphalt shingles meet the front wall siding

For front end-walls where roofing shingles meet siding, the metal flashing strip should be bent, using a metal brake, to extend at least 2 inches up the vertical wall and at least 3 inches onto the last shingle course.

For chimneys, specifically, base flashing is the first of two types of flashing applied, and sits below the second type of flashing (counter flashing).

Base Flashing applied over shingles and set in roofing cement

Step flashing

is rectangular and bent at a ninety-degree angle, set between the roof and a wall or dormer.

Step flashing applied to the side-walls of a dormer

This flashing safeguards the house structure below the roof by directing water run-off away from the sidewall of a dormer or chimney into the gutters. It is installed in “steps” (ascending a slope like steps) with layers of shingles in between each piece of flashing.

The minimum dimensions for applying step flashing against vertical sidewalls like dormers, chimneys, or sidewalls in general (such as home additions) are listed below.

  1. The width of the step flashing over the roof deck must be at least inches wide.
  2. The height of the step flashing installed against the vertical surface must be at least 2 inches high.
  3. For a step flashing application, the pieces of flashing must overlap each other by at least 2 inches.
  4. The length of the step flashing pieces depends on the type of shingles being installed.

Counter flashing

is the second type of flashing used with base flashing. It is typically applied to chimneys during new construction projects or complete roof replacements. For all chimney re-flashing projects, counter flashing is applied after the base flashing and step flashing have already been installed.

Through-wall masonry chimney counter-flashing, applied over step flashing.

Continuous flashing

(also referred to as apron flashing) is a long, single piece of metal that protects the joint between a wall and a sloped roof, guiding the run-off down the shingles.

Valley flashing

is typically installed over the Ice-and-Water shield (waterproof winter membrane applied directly to the roof deck), running from the peak of the valley (the point in which two roof slopes converge) to the gutter. It is set below the shingles such that the rainwater from the shingles flows into the valley sheet metal flashing (with open valley applications).

Open valley application with sheet metal flashing

Note: There are also the “closed valley” and “woven valley” applications that don’t use metal flashing. However, all systems, do use an Ice-and-water style waterproofing membrane attached directly to the roof deck in the valley, regardless of whether there is a sheet metal flashing being used. That said, open valley applications with sheet metal flashing are far more reliable than woven valley and closed valley applications with no metal flashing.

Drip edge (flashing)

are installed at the edge of a roof to direct the water run-off from the shingles into the gutter, or away from the fascia (the wood below the shingles) if the gutter is not present. There are three types of drip edge flashing:

Type C

is also called “L style” since the flashing is shaped like an “L.” This style has a lower flange (a projecting flat rim that keeps the flashing in place) at the bottom.

Type D

is shaped like a “T” and is either called “D-metal” or “T style.” This style has a lower flange, as well, and directs run-off even further from the fascia than Type C.

Type F

is also known as “F style” or “gutter apron.” This flashing has an extended drip edge, making it easier for a roofer to install new drip edges over existing shingles or on rake edges.

Kickout flashing

is not as common to see, but it is located at the edge of a roof section that ends at a wall. The flashing can be soldered step flashing or a prefabricated piece of its own. Its purpose is to direct the water away from the edge of the roof, which will otherwise stream down and damage the side of the wall, and into the gutter.

Roof Materials Expenses Associated with Flashing Installations and Repairs

The national average cost to hire a roofer to replace the flashing, shingles, and to seal a leak in a small area of the roof is between $300 and $1,100. Larger roof fixes can cost between $1,100 and $1,500+.

New Shingle Roof

Average price
New Metal Roof

Average price
New Flat Roof

Average price

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Retail cost for flashing per linear foot is $0.25 to $1.00 depending on the brand and the type of flashing. For instance, valley flashing can cost as high as $0.75 per linear foot (lowest price at about $0.50), whereas step flashing can be priced as high as $1.00 (lowest $0.55) per linear foot. You can always find reasonable prices at the nearest Lowe’s or Home Depot.

The following chart lays out the prices of roof repairs — depending on the area of the roof and scope of the repair. Prices depend on the size of the area needing repair.

Most roofing companies and handyman services have minimum fees of $65 to $400 for repairs. The fee covers their time up to one hour.

Section of Roof/Roof Feature Approx. average cost of repair
Metal roof $550—$2,550
Composite/Asphalt shingle roof $500-$1,500
Gutters $5—$12 per linear foot
Asphalt Shingles (missing or damaged) $250—$750
Fascia $22 (per 12-ft boards)
Trusses $3.00—$7.50 (per sq. ft.)
Skylight $500-$2,500
Vents $500
Chimney $600-$1,500
Valley $250—$2,000

The homeowner should make sure to check his/her homeowner’s insurance when addressing a roof leak. It may be possible to file a claim and receive a reimbursement for the repair costs.

Signs of Roof Flashing Failure

The clearest indication that flashing is underperforming is by water leaking into the house. Of course, it is desirable for any homeowner to catch the issue before leaking occurs; in this case, here are some specific outside signs for the homeowner to look for that may imply flashing that is not protecting your home:

  • The shingles are curling, broken, damaged, have lost granulation, or are missing (which denotes that water is draining under the shingles).
  • The asphalt or wood shingles are moldy, decaying, or splitting.
  • Patches, cracks, or tears in the roof.
  • The flashing itself tears or buckles around the roof features. When this happens, chimneys, specifically, will have missing, damaged, or stained brick.
  • The sealant, if present, is crumbling or missing. Or nails, if used in place of sealant, are missing. Both circumstances will make the flashing loose.
  • The fascia is decaying or stained.
  • The gutters are rusting, sagging, bending, or have leaky seams. Gutters may even become clogged.

The homeowner can inspect the inside of the house as well.

  • In the case that the roof has patches, cracks, or tears, the homeowner may also find water spots, mold, damaged rafters, or leaks in the attic.
  • Moisture marks and/or brown, yellow, or gray stains are present along the walls or ceiling.
  • An increase in energy cost may denote the roof ventilation is compromised.

How to Repair Roof Flashing

Before getting into the specifics of various flashing repairs, it should be mentioned that it is wisest for the homeowner to have all previous flashing taken off before having new roof flashing installed. If only parts of the old or damaged flashing are replaced, it will likely fail before the new flashing lifespan is over.

However, it is possible for professional roofers to salvage flashing that is still intact when removing the old or damaged pieces. The roofer will have to first inspect the flashing to assure it will continue doing its job efficiently; then, if finding that some flashing pieces are still useful, the roofer will attempt to remove the damaged flashing without damaging the rest.

Not all types of flashing are easily removable without causing damage to each of the flashing pieces. In most cases, it makes more sense to remove the entire row or line of flashing and install fresh flashing.

The following is designed to help you understand what goes into flashing repair. For the homeowner considering installing or repairing the flashing him/herself, see the subsection below.

Flashing can be repaired in a few steps:

  1. The shingles surrounding the flashing should be gently lifted so as to not be damaged.
  1. The flashing needs to be removed, whether the homeowner decides on only pulling off damaged flashing or all flashing. This can be done by carefully chiseling away the sealant or taking out the nails.
  1. If the flashing is being repaired before leaking occurs, the roof substructure should not be damaged under the flashing or shingles. However, it would be a necessary precaution to inspect it to ensure that no damage was done. The roof damage would have to be repaired before the flashing itself.

The new flashing can be installed according to local/universal building code and with sealant or nails. Depending on the flashing being handled (namely, step flashing vs. base flashing), different installation processes will be in order.

For a visual understanding, watch the video below to see how a step flashing is installed:

Can Roof Flashing Be a DIY Project?

While the Do-It-Yourself (DIY) disposition can motivate homeowners to get the job done while saving money, it is in the homeowner’s best interest to hire professional roofers to install or repair flashing.

Improperly installed roof flashing leaves your home vulnerable to water damage that can range from moderate to devastating and costly. Besides, pro roofers understand the most efficient practices and the building codes involved with the process.

Without the proper precautions and expertise going into the work, the DIY process may not pass code inspection, and it is possible that the roof and the roof features will become damaged in the process, putting additional expenses on the homeowner. Working on a roof is also a dangerous task that may lead to injury or fatality.

This does not mean that the homeowner cannot find other ways to save money. Here is some helpful advice on how to avoid unwanted house expenses in the future:

  • In order to avoid roof flashing issues and greater house damage, it would be best to have a professional inspect the roof on a regular basis, especially after harsh and extreme weather events.
  • Keep attic ventilation flowing. If the ventilation is blocked off, it will cause moisture build-up in the attic. The moisture will cause mold and wood rot, and shingles will buckle.
  • Ensure that the gutters are always cleaned out. Gutters with debris will not allow run-off to drain, making water pool up under the shingles.
  • Trim and maintain overhanging trees above the roof. Moss and algae will grow on shaded, damp areas and will cause wood to deteriorate, shifting the shingles. These moving shingles may in turn damage roof flashing.
  • Immediately replace damaged or underperforming flashing. If it is not replaced in a reasonable time, more damage will come to the house.

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