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DIY Window Flashing

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DIY window flashing for a clean install

The last post closed with cleaning up the install from the interior; DIY window flashing is all about cleaning up the exterior. On the exterior of a brick house, there is brick moulding around the window. Think of this like the trim or exterior frame around the window.

This moulding is what your storm window frame screws into and there are window stops that keep your window from falling out, just like the interior stops keep your window from falling in.

Why you need to flash your windows

Flashing your windows is necessary to protect your new window, the frame, and the interior walls from moisture and the elements of precipitation. The good news about flashing a window is that it will last 20 plus years. There are 2 options aluminum or vinyl flashing. If you have vinyl siding, go with vinyl flashing. If you chose aluminum flashing it would still work and provide you with a clean install.

Flashing is molding to the specs of the frame of your window and the exterior molding. It is important to have the exact measurements to a T simply because this determines how clean your install will be. 

In the pictures below you will see the before and after and ultimately you will want to go from this:

DIY window flashing mulled window
How to Flash 2 windows or a mulled window

To that:

DIY how to Flash a mulled window or 2 windows
How to Flash 2 windows or a mulled window

Notice how the aluminum flashing covers the top and bottom of the window, the middle and the left and right side of the window. Also the top corners were cut at 45 degree angles to perfectly align.

Should you try Do It Yourself Window Flashing?

If you are looking to save big money on your window install, you want to clean up your install, and you can following directions, then yes! You should certainly try do it yourself window flashing. If you installed your own window, you should definitely flash it, because flashing is much easier. This is a fine detail project that involves measurements, cutting, and precision. The material is literally feather light, so it does not involve heavy lifting.

My experience with flashing

As I mention later, I could not find much reliable or helpful information on flashing windows. I pulled from everywhere including professional window installers, Home Depot & Lowes’ employees, YouTube, This Old House, and more. What I heard was, “you need some window casing,” “you need some window molding,” “you need to get someone to wrap your windows in aluminum,” or “you need to have your window capped.” These are all the things you may read or hear as you look for how to get your windows flashed or wrapped in vinyl or aluminum. Either way, when it comes to DIY window flashing these terms may be used interchangeably. (Please note I am not saying they should be, because the process was much more confusing because of that.)

Cutting the flashing

So truth be told, there is very limited DIY information available on flashing windows. For weeks, I’d go to work. Watch YouTube before driving home. Then stop at Home Depot and Lowes on my way home to look at aluminum, aluminum rolls, chimney flashing, etc. I had no clue what I was looking at and the YouTube videos were only How Tos not really “What to buy and from where”. And when I thought I found a good video, it was just a guy using a Brake to bend aluminum to make flashing for his own windows. More or less him bragging out how he was able to flash his windows. 

Where to buy Window Flashing

If you are DIYer on a budget like me, then you want to always keep quality and budget in mind. Moving forward, I finally found 1 video where the guy was talking about

He went on to show how simple it was to install flashing ordered from Trimbender if you can provide the exact measurements of the window. He even instructed on how to measure. The rest was history.

Trim bender refers to it as window trim (see image below). This is what you need to order.

DIY window flashing brick mold trim
DIY window flashing trim from

I measure every single window, I placed my order through their website and I paid their outrageous standard shipping fee of $69. Being on a budget and uncertain if this would even work I placed a small order for about 5 windows. Let’s just say Trimbender makes it very easy for you to get the flashing you need! They literally make it to the specifications and all you have to do is cut it because it comes in 6ft and 9ft lengths. You can even order the nails through them as well. Most important your order arrives in 7 days max.

Cutting your flashing

As mentioned earlier, if you buy an aluminum roll, you can cut it using a Brake. A Brake is a $1,500 piece of equipment that will cut your aluminum to size. It is about 6+ ft. long. Ever see those window installer guys with their trailers? They have a Brake in there! But if you are a do it yourself on a budget, don’t buy a brake. Just go to trim bender and save the hassle. Now that you have your order and it is time to unbox and install.

You are going to need a multi angle miter cuter tool. I bought mines on Amazon for $15. Also, you will need Tin Snips, which in my opinion worked better for me. You are going to measure the angle you need, 45 degrees in most cases and cut the corners. You are also going to cut the length and width of your windows to cut the flashing to those specifications.

Purchase Tins Snips for $23.99 at Amazon see below

DIY Tin Snip Aluminum Flashing Tool
DIY Tin Snip Aluminum Flashing Cutting Tool


One last tip about cutting. After I did the 45 degree cuts, I noticed driving around and visiting my parents, a lot of houses don’t have 45 degree cuts on their flashing. Their installers kept it square and it just overlaps and still maintains a clean look. Much simpler if you ask me because it is a pain standing outside in the winter cutting aluminum.

You can experiment, you may choose to keep it square. To do so, just account for the cut in your measurements. 

Either way, when flashing make sure you overlap the sides of the flashing on top of your bottom at the points where the sides and bottom meet, and overlap the top with the side at the points where the top meets the sides. This creates a protective barrier from moisture getting behind your flashing and becoming trapped. When I say the bottom, I am talking about the window sill flashing.

Congratulations you just finished learning how to prepare and complete DIY Window Flashing!

Other helpful links

DIY Window Replacement Installation

DIY Window Remodel & Removing an Old Window

This the Miter Tool I used from Amazon for $28.99

DIY Window Aluminum Wrap Trim cutting tool
DIY Installing Aluminum Window Wrap Trim Miter Cutter tool


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