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Installing Window and Door Cap Flashing :: Sealing Out Water on Wood Siding

Flashing for Wood Siding

One of the biggest challenges with saving the original Dutch Lap wood siding on an 1889 farmhouse (well, other than dealing with 13 decades of paint) is ensuring against water entry at the top of windows and doors.

Our Dutch Lap, where it was protected, has held up nicely to the elements through the years. I would suggest that in some places the old-growth, heartwood fir looks as good as the day it was installed.

When it was installed, however, I must assume easily workable sheet metal was not widely in use. Building today, we “flash” above and/or around all window and door openings, usually with aluminum sheet goods.

Aluminum Coil Stock is highly workable, completely resistant to water, and often comes from recycled material. You can feel comfortable that it is a great choice for flashing applications.

Editor’s Note:  We saved both our wood windows (and its window trim) with the plan of one day adding storm windows. If replacing these elements, a more effective means of flashing is removing a course to of siding and sliding, a z-type flashing up behind.

Flashing, Back in the Day

Back in the day, and on my house, the builder, a self-builder as they might call it in England, topped all window trim with a cap. This head cap, similar to what you would see in an Arts and Crafts-style interior trim build-up. It was smartly and purposefully sloped away from the house as to shed water. (Hopefully, the picture shows this.)

Today, windows installed with new construction (aptly called new construction windows) receive a z-flashing (named for its shape) prior to the application of the structure’s finished cladding (or siding). On my house, at some point in its life, coil stock had been bent (or broke) on-site and was applied to this window cap. (Again, hopefully, the pictures will show.)

While this type of application was certainly practical, aesthetically I had issues with it. Especially given my distaste for poorly painted metal, it had to go. As I have been working my way around the house, addressing paint issues, I have also been working to implement a more modern flashing solution.

Retro-fitting Window Cap Flashing

* Installing Window and Door Cap Flashing

While I have been around houses, and am sometimes known to do restoration work, I do not own a brake. This device, and you have likely seen one, is used to brake or bend sheet metal for flashing, and other exterior, installations. While I would love to one day own one, for this project – I wanted to avoid having to purchase, rent, or borrow one. So I looked elsewhere.

* First I Improvised

Most home centers and lumber yards stock a select set of aluminum edging profiles. I will admit again, I don’t always have time to do as much legwork as I would like. For example, when I did work around the windows in the back ell of the house last summer I had difficulty finding a profile that would work with my trim set-up.

In that case, and at the time I thought rather ingeniously, I grabbed a roofing eave edge (for the raking ends of a shingling installation), flipped it over, and made it work.

* Later I Found This Profile

I wish I would have spotted Amerimax‘s Window and Door Cap back then. (And this is about the closest thing I can find online right now Amerimax 3-1/2″L x 1″W Aluminum Cap Flashing. Check Amerimax on Amazon for more profiles.) As the pics will hopefully show, it seems that this stuff was designed with my house in mind, and it worked excellently for the windows on the front of house.

Again, I never claim to be a craftsman or a home improvement expert, but I would like to provide the procedure I used for installing.


One of my biggest priorities was trying to get lap (that is, coverage) at the ends of the window cap – and this proves to be the most difficult part of this installation. (I did take pics at each step of the work, but . . . truthfully they did not turn out great. I will not post them here, but I will email them to anyone who asks.)

Oh yeah — when working with sheet metal, watch out for sharp edges!

* The Procedure

1. Measure the Length of the First Window Cap

Warning: I learned long ago, and especially with old houses, not to gang cut any elements intended for the windows. Usually, there are a few factions of an inch of difference one to the next. I added one inch to the measurement here, to allow for a ½ inch fold at each end.

2. Cut the Cap to Length

I used my 10″ chop saw. I almost always do this. Loaded with a good carbide blade, cuts on sheet metal work out well (though a slight rough edge is sometimes left).

3. Find the Set Back

Working on the flashing itself, starts by marking for the set- back. To do this, I used the combination of a speed and a small carpenter’s square. (For marking the opposite end, I held it in place on the cap later — marking it there.)

4. Make Two Short Cuts Using Aviation Snips

First at the rear where the flashing begins its slope upwards, and at the front where the cascade begins its fold over. I then made another short cut at the lip in line with my marking. I finished this with a notch cut where the slope intersects the flashing’s deck.

5. Trim the “Ear”

I worked my flap upwards slightly with my fingers, I trimmed the ear, and then took the lip that was now free and folded it almost a full 180 degrees back on itself. This allowed me to then fit my hand brake in.

6. Use a Hand Seamer to Bend the Ear

Enter the hand seamer – a great tool made by Wiss (which I actually bought for HVAC applications)! I started shifted forward – and on the working line, I bent my flap upwards to 90 degrees. Then I broke the flap downwards. I had to reset the tool by shifting it to the back of the profile midway through the bend. (This was a simple matter of physics – the hand seamer I was working with was a little too large for the profile, but it worked nonetheless.)

 7. Finish with Caulking

Now with a fully shaped flashing in hand – I placed it into a bead of caulk at the top of the window cap. Using stainless steel trim nails from Maze Nails, I fastened the flashing at five points from above. (Obviously – I don’t love creating the additional penetrations on a horizontal surface, and it is debatable if I could have just glued the piece into place – for me, though, I later dabbed all nail heads with a glob of silicone caulk. And I was happy.)

*Selecting a Sealant

While I used an “elastamastic” on the rear, here, I chose instead a 100% clear window/door silicone to seal the flashing at the point where it meets the house. In these applications, I choose not to finish, or swipe, the bead as I feel it allows the bead to hold up longer under the stresses of expansion and contraction.

Anyway – this is how I did it. I would love to hear what you might have done differently.

* Lessons Learned – Rusty tools are the devil & never snap your pictures on the first item you do in a set. Work only gets better as you get into a rhythm.

For more projects and, well, hacks, see our category, What I Did. For more on this old house, the category, Old House. Thanks for reading. ~jb

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