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.

Here are pictures of window flashing. Metal used should be no less than 30” long x 14” wide. Notch out a 4” wide x 24” long piece.

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 as way Tyvek tape would be used in a real installation. 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.

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 them. Or rust nails. Or wick into the sill plate. These covering are supposed to be the SECOND line of defenses against the weather!

On the right side of the window, the water exits ONTO the siding and NEVER reaches the Tyvek, felt or any other covering! This is how the siding becomes your FIRST line of defense against the weather.

Instructions for flashing a 'Pre-Flashed window.

Tape and wrap the opening normally. 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 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 EGDE OF THE METAL AND DO NOT PUT ANY HOLES IN IT OTHER THAN THOSE REQUIRED TO SECURE THE WINDOW.

Now tape the window flange.

Run the siding up the wall. The LAST PIECE OF UNCUT SIDING BELOW THE WINDOW GOES BEHIND THE 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.

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.

Though I usually charge about $150.00 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 10 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.

 Pictures and Details:

In this picture, the left side exemplifies the way I find most windows. The right side is the way I do it.

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.

In this view, notice the wood is only wet below the 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.

A close-up of water running onto the wood. If Tyvek was on the wall, water would be running down the Tyvek 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.

Not a great picture, but it's the back pf 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.

Like shingles, I clip 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.

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.

Some real life photos:

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

A doorway. There is a sill pan under the door too.

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.

 

Here are some pictures from a thread I found on the internet that explain the consequences of not flashing the windows.

They were contributed to me by M.L. He took the pictures and had posted them on a forum looking for causes and answers to the problems.

Interior shots of what happens when the water gets into the walls. Sub-floors, carpets, and even furniture can be ruined.



Exterior shots. Look at the water trails. M. L. had said this was caused by leaks in the windows themselves, and he could be right in this case. I've found this same damage in both scenarios. Virtually zero difference in either case, including the amount of damage and cost to the homeowner.

Two lower vinyl panels removed and insulation barrier cut away.





 

A link to Four Seasons Industries with good pictures of several examples of siding leaks and the resultant damage.

When Siding Leaks - Ignorance is bliss if you earn your income installing vinyl siding

Some more pics;

 

 

 

 

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,

Dave

 

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

(anon)

 

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.

(anon)

 

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

(anon)

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

Any questions, just e-mail me.

 

Frank