How To Flash a Window

Flashing a window might sound complicated, but it’s an important job that helps keep water out of your walls. Even the most energy-efficient windows can let water in if they’re not flashed properly. In this guide, we’ll walk you through how to make and install custom aluminum window flashing just like This Old House general contractor Tom Silva does in the video above.

Types of Window Flashing

Window flashing is a thin, waterproof material installed around windows to prevent water infiltration and damage. It helps water flow away from the window and onto the outside of your house. Good flashing keeps your house dry and saves you money on repairs down the road. There are a few options to match your project’s needs:

• Aluminum flashing: This flashing is lightweight, corrosion-resistant, and relatively easy to work with. You can cut and bend it to fit any window size and shape, making it a favorite among contractors and DIY enthusiasts. We use aluminum flashing in this guide.
• Flexible flashing tape: This is a peel-and-stick product that you can mold to fit around the window frame. It’s good for tricky shapes and corners, making a continuous waterproof barrier with no need for multiple pieces.
• Flashing self-adhering membrane: This material combines the flexibility of tape with the strength of aluminum. It adheres to the window frame and surrounding area, providing a strong, waterproof seal. It’s often used in combination with other flashing materials for complete protection.
• Liquid-applied flashing: This is a brushable or sprayable substance that forms a seamless waterproof barrier around the window. It’s ideal for complicated or hard-to-reach areas where traditional materials may not give adequate coverage.

Tools and Materials Needed

Besides the flashing itself, you’ll need certain tools:

• Aviation snips: Aviation snips are great for cutting aluminum flashing to your desired length and width. They give clean, precise cuts without distorting the material.
• Clamps: Clamps hold the flashing in place while you make measurements and adjustments. They are especially useful when bending the flashing to create custom angles.
• Framing square: A framing square helps ensure precise measurements and straight cuts. You need it to create the correct angles and dimensions for your flashing pieces.
• Metal brake: You’ll use a metal brake to bend the aluminum flashing to the required angles. If you don’t have one, you can rent it from most hardware stores or use a homemade bending jig.

We also recommend having exterior-grade caulk, a hammer, a measuring tape, a utility knife, and some scrap wood on hand.

Steps To Flash a Window

Now, let’s go through the steps to flash your window.

1. Measure and Cut the Flashing

Begin by measuring the width of your window, including the trim. Cut a piece of 8-inch-wide aluminum flashing to this length using aviation snips or a sharp utility knife. Make sure you have enough material to extend slightly beyond the window on both sides.

2. Create the Leading Edge

The leading edge of the flashing hangs down in front of the window trim. This helps water flow away from the window and prevents pooling:

1. Measure how far the flashing should extend down the front of the trim (usually about 1/4 inch).
2. Mark this measurement on your flashing.
3. Use a metal brake to bend the leading edge just past 90 degrees. As Silva explains, “When I bend the metal, it’ll spring back, so I have to go past 90 to make a 90.”

If you don’t have a metal brake, you can make a homemade jig with two pieces of 2-by-8 lumber clamped together.

3. Bend the Wall Flange

The wall flange extends up the wall above the window:

1. Make a gauge block from scrap wood that’s as thick as your window casing and trim.
2. Use this block to mark where the wall flange should start on your flashing. Slide the flashing into the brake, using the gauge block to position it at the proper depth.
3. Bend the flashing at this mark to create the wall flange, again using either a metal brake or a homemade jig.

4. Test the Fit

Test the fit of your custom-made flashing before you install it:

1. Place the flashing over the window.
2. Check that the front edge covers the top of the trim and extends out a bit.
3. Make sure the wall flange sits flush against the wall above the window.
4. If it doesn’t fit quite right, adjust it until it does.

5. Install the Flashing

Once you’re satisfied with the fit, it’s time to install the flashing:

1. Apply a bead of exterior-grade caulk along the window trim’s top edge.
2. Place the flashing over the caulk. Make sure it’s level and centered.
3. Secure the flashing with roofing nails along the top edge of the wall flange.
4. Seal the top edge of the flashing with another bead of caulk or flashing tape.

You can spray your windows with water to check for water infiltration. Fix any leaks or gaps to prevent water damage.

Tips for Successful Window Flashing

Follow these tips to make sure your window flashing performs as intended:

• Always flash a window from the bottom up. Each higher piece should overlap the one below it.
• Consider using flashing tape at the corners for added protection.
• Make sure the flashing extends at least 3 inches on each side of the window.
• Follow manufacturer guidelines for your specific window and flashing products.
• Use compatible materials—some caulks and sealants can damage certain types of flashing.
• Regularly inspect the flashing for signs of wear, damage, or deterioration. Fix any issues quickly.
• Call a professional if you’re unsure about any step or if your window has unique features that may require specialized approaches.