What Is Step Flashing and Why Do Roofs Need It?

What is step flashing and why do roofs need it 1

Every time a roof meets a vertical wall, step flashing is required to properly complete the installation. While it may be tempting to save time by skipping this step because you don’t want to cut into the old siding, don’t make that mistake. This area is known as a transition point, and if not properly protected, the results can be disastrous.

In addition, attempting to install step flashing after the new shingles are on will be a complicated and expensive project.

Experienced pros can skip this next section, but it’s a good refresher for intermediate or those new to the industry.

What Is Step Flashing?

Step flashing is a piece of metal, bent at 90 degrees, that goes between the shingles and a sidewall, dormer or chimney. It prevents water from getting under the shingles and destroying the roof structure by directing it back to the shingles below and off the roof.


Step flashing can be made from aluminum, galvanized steel or copper. Aluminum is widely used because it won’t rust, hold paints well and can be easily bent for specific applications. While copper and galvanized steel are hardy materials that can withstand even the most extreme outdoor elements, aluminum still remains the most popular choice due to its longevity and ease of installation.

How Is It Sold?

Step flashing is typically sold in a pre-bent configuration. Most dealers and manufacturers will sell flashing in boxes of 100, or they can be purchased as individual pieces. The typical dimensions are 10” x 8”, though other sizes are available. Step flashing can also be purchased with a painted finish, which may be a selling point for your customers if they don’t like the idea of shiny metal showing at the edges. The painted product also has the extra layer of protection provided by the coating.

How To Properly Install Step Flashing

A zoomed in view of a home with off-white siding, soffit, and fascia and stone details.

Step flashing must be installed at every point where the roof meets a wall. This includes:

  • A first-story roof meeting a second-story wall

  • Dormers

  • Around the base of skylights

  • Where a chimney meets the roof

Step flashing is installed at the transition point between every shingle, making contact with the sidewall at the same time. For the best results, an ice and water shield should be installed on the side wall and run down on the underlayment before the shingles and step flashing are installed.

Installing the First Piece

After the first shingle is installed that contacts a side wall, a piece of step flashing is set on top of the shingle and against the side wall. Here are a few points to remember when installing the flashing:

  1. The step flashing should extend at least 2” up the side wall - longer is better.

  2. The step flashing should overlap the previous piece of flashing by at least 2 inches.

  3. The length of the step flashing should be at least 2 inches more than the shingle exposure (the typical size of a piece of step flashing is 10” x 8” but you should check with the shingle manufacturer’s recommendations as some shingles are larger than others.

  4. Once the first shingle is down, lay the piece of step flashing on it so that the top of the flashing is slightly above the top of the shingle and the bottom does not extend below the nail line on the shingle. Nail the step flashing as high up on the shingle as possible and into the roof deck.

  5. Do not nail the step flashing into the sidewall.

  6. As added extra protection, coat the bottom of the step flashing with roof cement before attaching to the shingle.

Step Flashing a Roof-Over Project

If you are installing a second layer of shingles over the first, avoid the temptation to rely on the step flashing installed with the original roof. Many contractors take this route to avoid cutting into the siding, but this can lead to leaks if the original step flashing fails.

If the home has vinyl siding, it is relatively easy to remove and reinstall. But if the siding is wood, it is almost impossible to remove it without causing damage. If this is explained to the homeowner when the initial estimate is given, the increased cost to replace the damaged wood will not surprise your customer. Rather, they will be grateful you took the extra steps to ensure that the roof remains leak-free for years to come.

What is step flashing and why do roofs need it 3

Damage That Can Result Without Step Flashing

As a contractor, you can’t make money fixing your own mistakes, so it is best to avoid them by taking the proper steps during the initial installation. A leaking roof caused by a lack of step flashing is not only a disaster for your customer but also a black mark on your reputation as a reputable roofing professional.

There are numerous locations, including dormers, skylights, chimneys and houses with multiple stories, that require step flashing. If the step flashing is not installed, it may be impossible to locate the leak afterward, and it can cause thousands of dollars worth of damage by the time it is discovered. Damage can include:

  • Rotted wood decking and substructure

  • Damaged insulation

  • Ruined drywall

All these problems can be avoided by the proper use of step flashing when the roof shingles are installed.

Alsco® Metals Is Your Source for Roofing and Siding Accessories

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Alsco has been providing quality roofing accessories for over 70 years. Contact our team of dedicated professionals today for expert advice and directions to a retailer near you.