Window Flashing How-To Tutorial

Window Flashing, Window Leaks and Window Leak Repair,How-to Pictures

Think of your home's windows as 'Vertical SkyLights'. Water gets at the sides and tops of them. The water must have a way to exit from the siding. Ever notice how SkyLights on the roof have the lower flanges/flashing exposed so water can escape? The same holds true with regular windows.

Letting water get to the house wrap is like letting water get to the roofing felt. It's already too late when that happens.

Here are pictures of window flashing. Metal used should be no less than the window length long x 14 wide. Notch out a 4 wide 'corner' so the side flashing reaches well around the window corner.

Though a poor mock-up, this shows how the metal is supposed to exit onto the siding. Not like this! I left the wood exposed because tyvek doesn't show the water that gets into your house. I only used a small piece of scrap flashing! For a window in a house, I ALWAYS run my metal to the top of the window, door, or other opening! White is the best and less likely to warp.

 For further info on how to flash a window, keep reading!>>>>

On the right side of the window, the window flashing forces the water to exit ONTO the siding and it NEVER reaches the Tyvek, felt or any other covering! This is how the siding becomes your FIRST line of defense against the weather not just a covering that is hiding the damage being done to your home.

Instructions for flashing a 'Pre-Flashed' window.

Tape and wrap the opening normally.  (For all sidings, use house wrap first, then 'wrap' the windows with plastic, or one of the bitumin products. Just let all the bottom wraps flap in the wind until some siding gets to them. Do the same with the metal flashing. Cut them all off so they lap the siding but are hidden by the next piece of siding.) Install a 14" piece of flashing across the bottom of the window opening UNDER the window wrap, and extending about 8" past each side of the window. Insert the window, level and check for square. Put a few nails across the top and upper corners to hold it in place. Insert the notched section of metal BEHIND the window flange on both corners, and over the piece of metal that runs across the bottom and nail upper edge of flange to hold it in place. Finish securing the window. ( If the windows get caulked, they can be caulked before securing the top and inserting the metal). DO NOT NAIL THE BOTTOM EDGE OF THE WINDOW FLASHING METAL AND DO NOT PUT ANY HOLES IN IT OTHER THAN THOSE REQUIRED TO SECURE THE WINDOW.

For the ultimate application, after wrapping the bottom of the window opening, install a sill pan and terminate the side wrap into the pan. Fold the outer pan edge down. How far out it sticks, if any, will depend on the siding being used. It can extend out over brick or stucco so water has good drainage OUT of the building envelope.

Now tape the window flange. I do believe I'm going to add to this step. Another piece of metal, with a 5/8" to11/16" lip turned up about 80 degrees (for a tight fit) and pushed into the channel, before any siding goes on, and running the length of the window and exiting below the lower flange. The only thing that could make that leak would be a bullet!

Another thing, I used black metal for contrast. I would recommend 'white on white' metal in real life use so it doesn't absorb heat. Aluminum nails would be the nail of choice too.

 Run the siding up the wall. The LAST PIECE OF UNCUT SIDING BELOW THE WINDOW GOES BEHIND THE Flashing METAL. Cut the metal off flush with the locking strip on the last uncut piece. LEAVE THE METAL UNNAILED. Continue to install siding normally, but for best results, cut a 3 strip off the bottom edge of next piece so water will flow out uninterrupted..

Pictures and Details:    Click on the thumbnails to enlarge them. ( For demonstration purposes only, I omitted the tyvek because the wood shows water better.)

In this picture, the left side exemplifies the way I find most windows. The right side is the way Albert's Roofing does it. For these pictures, I used a display window inserted into a faux wall and I omitted the insulation board, Tyvek and tape. I used duct-tape to prevent water from getting behind the flange, same way Tyvek tape would be used in a real installation. The left side of the window was installed the way some builders and siding installers do it. See how the water goes directly BEHIND the siding. You must depend on the Tyvek, felt or any other covering to never let water into the walls of the dwelling. Or rust nails. Or wick into the sill plate. These covering are supposed to be the SECOND line of defenses against the weather! DO NOT OMIT THOSE STEPS! Another piece crossing all the way under the lower flange, behind these corner pieces would be ideal. This type of detailing also works well with Hardi-Plank and other solid siding Just another view. The metal extends down about 10".  This view show how the last UNCUT piece of siding goes behind the metal. I'm putting water on top of window. Notice the wood getting wet on left side.

( If this bores you, just skip to the  Window Flashing Leaks | Siding leaks  and their after effects page.)

In this view, notice the wood is only wet below the window flashing  metal on R. side.   It's wet above, below, and behind the siding on the L. side.  A close-up of water following the metal. In real life, I wouldn't run bottom tape where the side tape crosses it. I would stop about even with the inner edge of the J-channel. The tape's only purpose would be more wind-break capability.  A close-up of water running onto the wood. If Tyvek was on the wall, water would be running down the Tyvek, behind the siding, looking for openings   I lifted the piece of siding to show that it's wet on one side, not the other. The display left about 4" of exposed wood below the siding. Notice it's the only wet spot on R. side of window. Now that I've shown how flashing works on a window, I've cut it off flush with the siding's locking strip. This is how to 'finish' the flashing and incorporate it into the siding as a whole system protecting your home.  A close up of the trimmed off metal with water running onto it.   Now, we bend another piece of aluminum 5/8" up on one edge, for the length of the window. Don't quite bend it a 90. Just under 90, it will press into place tightly. This piece is the counter-flashing. With it in place, not even normal power washing will cause water to get into the building envelope.   Here's a picture of the bottom. Notice it goes to the bottom of the outer trim.    I run the metal at least 4-5" above the highest point of the window. The vertical leg is cut off flush with the top of the outer trim. Another view of that last piece, from the top of the window.    To finish off the flashing, we add a drip-cap. It's corners are folded and tucked around the outer edge of the window trim to complete the job. The back of the drip-cap will extend 4-5" above the highest point of the window trim.  Not a great picture, but it's the back of the NEXT piece of siding. I've cut some locking lip off below the window corner so water can get out easily. If you have Vinyl or Aluminum siding on your house, go out and look at the lower corners of your windows. This gap is on all siding jobs. Metal flashing is not behind all siding jobs. Real window flashing is made of metal.  Like shingles, Albert's Roofing clips the tops of the siding to help guide water where I want it to go.  A picture of the other side done the 'normal' way. No evidence that clipping the tops actually helps. But I try to hedge my bets. Putting dry paper towels behind the siding for the 'acid' test. This one's going below the left corner. I put a sheet below each corner.  These pictures show how the base flashing will work if any water ever gets to it. I'm pouring water on the top J-channel. It seem to be everywhere down at bottom of window. Can you tell that more water is on front of siding at right corner than left corner?  Another view. See the water? A close-up of right corner. I pulled a sopping wet paper towel from below the left corner.

I pulled a dry paper towel from under the right corner. An edge of the paper towel was wet from water wicking onto the bottom edge of the lower panel of siding The proof is in the 'seeing'. The wood on left side is soaked. The only wet wood on the right side is the exposed section of the display BELOW the lowest panel. Nary a trace of moisture behind the siding.  Don't forget to install the drip-cap before crossing the window top with that last piece of siding for a completely water-proof system. And that's what it's all about. The walls, window, corners, and the roof must act as a system to be effective.

 Clarification after some e-mails;

"> Frank, I have watched many Youtube videos on installing windows.
> Your metal flashing method is unique. It makes sense that the water
> should drain to the outside of the wall instead of the inside.
> Installing a window like a vertical skylight is logical.
> Prediction: This will become SOP in 7 years.
> Does it matter whether the side galvanized flashing is placed in the
> J-channel of a vinyl window, or between the window and the OSB/house
> wrap?
> Thanks, Arnold


On 9/20/2013 4:38 AM, wrote:
> It should be under the flange and on top of the wrap. Then, I tape
> it. To protect the tape, I add another piece of metal with the J
> channel side turned uo about 9/16". This lip literally locks into the
> J channel so water should never get to the tape. It's a
> counter-flashing I feel comfortable with. I've seen too many windows
> have sills, jacks and studs all rotted out using tape and wrap only.
> No tape was ever invented to last the 100 years a house could last.
> Current philosophy is that people won't live in a home more than 7
> years before moving and voiding the warranty, or is the building
> expected to last more than 20-40 years either. As for SOP, many
> builders are adopting the flashing method, but the ICC is hesitant to
> adopt the policy because it adds $30.-$50. per window at the time of
> construction. Repairs can approach $2,000. per window in cases of
> severe rot if the leaks are allowed to continue too long. I get calls
> on homes less than 5 years old.


Older windows with Brick Molding

Use a 1/8" drift-punch to drive the finish nails through the brick molding, insert the flashing metal behind the molding, drive in new finish nails, J-channel, and continue on.

Though I usually charge about $280.00 per window to flash them after the siding is already on with a minimum charge of $300.00, I'm posting all the necessary details here for any D.I.Y.'er, or other person so they can fix it themselves.

If these repairs are made within 7 years of the home being built, there is a chance no major wood work will be necessary. The more time that passes, the more likely sheathing, studs, bands, joists and flooring/sub-floors will be seriously affected.

Remember! To work properly, the window has to be installed just like a flanged skylight. Roof or Side over the lower edge and it will bite you!

In the case of a NEW window with brick-mold and J-channel, the metal flashing is supposed to be installed BEHIND Brick-mold and before the J-channel. When siding an existing house and this can't be done, Install the flashing behind the J-channel. Assuming the J-channel is correctly installed, this should catch about 98% of the water.

Would you like to see what windows flashed according to code look like before the siding goes on your house? Click the links below.

Window Flashing Mistakes and No No's    Here are the results of doing it wrong:  Window Flashing Leaks | Siding leaks

A recent comment;  "Frank, you have one heck of a website---VERY-NICE...and informative, maybe you'll teach a few contractors the rights & wrongs---- Grin

Great pictures....!!!!


Who is Duff, you ask? None other than 'The Dale Duffy', NACHI Member #5101994

Phoenix Arizona Home Inspector, Scottsdale, Mesa, Chandler, Inspections


Some real life photos of a house I did. I think you'll agree, this is how it should be done:

The door needs to be shimmed and attached. I was closing it off for the night. Rough opening is already wrapped. Tyvek is in place, the opening is wrapped in plastic.

A doorway. There is a sill pan under the door too. The plastic wrap goes into the pan, not under it. The metal flashing goes in before and behind the door casing/brick mold.

Metal is covering the band. This pic was taken before wrapping the bottom and installing the pan.

I install flashing pretty high. 14" overhang on eave here.

You can now see a window in the wall near the door. Rough opening wrapped, metal installed. Still need to tape the flanges.

Opening was wrapped, Metal added, and then Butyl Tape added. There is danger of water getting behind the tape at the bottom corners. In hindsight, I believe it should be stopped flush with the inner edge of the J-channel. Even then, the metal would do it's job and direct the water out from behind the siding.

You can see how this is coming together now. Center metal was cut even with lower board exposure line. Side metal is getting cut same way on next level of siding. See the metal in the Hardie Plank joint?

With Hardie Plank, I use step flashing like I would on a roof. Eliminates 98% of potential water infiltration without even using caulk.

At the corners, I clip the tops to insure that any water getting to them will stay in the same area, not follow the top of the siding across the wall.

A finished window. The metal is barely noticeable. It has to be pointed out for people to see it. We hi-lighted it here.

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Need window repair in New York area? Call John Advanced Roofing and Contracting Corp.

Need window repair in Texas? Call Barry Adair. He is putting a crew together for the purpose.


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Comments from other contractors:

Thank you tinner. I have seen your info on flashing and it has been a help to me.

Wish you the best,



I've changed the way we did things, thanks to you.



Nice job, Frank. I do the exact same thing. That's what I was referring to in my post (3 months ago) when I said, "There are ways to direct this water out sooner, but I hesitate to go into that."

When I said "unpreventable" I was referring to the fact that water gets behind vinyl siding. That is not preventable. Additional measures can be taken to direct that water out as soon as possible- practices we both use.

At any rate, those are nice pictures and good documentation. Mind if we refer to your illustrations in the future?

If you read the entire post, I think you'll find that most people don't understand the need for techniques like that. They think that vinyl siding is "waterproof" or that more caulking will solve the problem.

When these extra flashing techniques are not used, it becomes imperative that building paper is used, because it is the only thing protecting the sheathing until water exits the wall at its lowest point.



That picture show's the answer to all your questions. See how the water sheds out over the vinyl?



And a great addition to the flashing. This product or a site-built pan should also be used.
Silltrays Drainage Tray   (Australia)

Rainscreen and Building Envelope Flashing Solutions including the Sillpan system and Weather stripping  (America)

Albert's Specialty Roofing and Window Flashing Co.


Q. I was reading your article on how to flash around windows. Thank you for the information. I had a question though as to why, in your expample, you only apply flashing about 2/3 up the side of the window instead of taking it above the top of the window. Why not extend the flashing all the way up the sides? Also, why not put flashing across the top of the window?

A. Good question! The windows in question were under a 14" overhang. Rain would have to be horizonal to cause a problem. Windows on the ends of the house would be fully wrapped and have a drip-cap. Experience has shown though that the water won't migrate more than an inch out within a foot of the top of the window. Each window must be address seperately in regards to prevailing storms, overhangs, etc.


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