Roof Flashing and Why it Matters

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 three 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).

Copper base flashing at base of chimney.

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 sidewalls 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.

   
   

   

   Step flashing on side of chimney

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 counterflashing applied overstep 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.

Copper valley flashing in conjoined valley.

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.

Copper drip edge flashing at eave of roof.

Kick-out flashing

Kick out 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.

Kick-out flashing at eave of shingle roof.

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.