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Old 07-17-2009, 01:00 PM   #1
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Where the siding meets the window


I just noticed how easy it is for rain water to get behind the vinyl siding where the vinyl siding meets the sideways j-channel on the sides of the windows on the my house. Just out of curiosity, what would be the correct way to build that junction in a way that isn't easy to leak?

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Old 07-17-2009, 01:38 PM   #2
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Vinyl siding is not meant to be a weatherproofing material. It hangs on the wall loosely and only diverts the water down. Wind will easily blow water up through the overlaps and into the vertical joints/channels. - It is expected and necessary.

That is why a primary moisture barrier is required under it and good flashing is required when a window is installed.

Vinyl is very weak and unstable and must be allowed to move with the changing conditions, so it cannot be sealed to anything more rigid.

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Old 07-17-2009, 03:56 PM   #3
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Ok, I understand what you say concretemasonry, but then that makes me think of another question. After the water gets behind the vinyl siding and runs down the primary moisture barrier behind the vinyl siding, where is it supposed to exit? Just curious; I'd like to understand.
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Old 07-17-2009, 11:17 PM   #4
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concrete is right. It's the same with stucco. The building paper is extremely important to shed the water. If you do want to go the extra mile, run some caulk between the j-channel and the window.
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Old 07-17-2009, 11:54 PM   #5
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J-channel can be sealed with silicone where it meets with a door, window And the special attention should be given to the corners around windows and doors as they are often cut in different ways that leave large gaps that can allow water to easily penetrate behind the siding. This is especially true when replacement windows have been installed as they are often installed without nailing flanges or flashing.
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Last edited by wrangler; 07-19-2009 at 06:38 AM.
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Old 07-18-2009, 09:04 AM   #6
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Just another proof that not everyone can install vinyl siding. To answer your question, a piece of L-shaped flashing needs to be installed at the bottom corner of the window. It should be overlapped onto the last full course of siding. When the water does get behind the j-channel and siding, the flashing will catch it and divert it to the outside.

Caulking and silicone only prevent the siding from contracting and expanding naturally, eventually fail, and don't look nice.
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Old 07-18-2009, 09:55 AM   #7
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Jaros, will you elaborate on "It should be overlapped onto the last full course of siding." ie: After I have attached the top part of the L-shaped flashing under the bottom corner of the window (which will be behind the siding), how do I bring the bottom part of the L-shaped flashing out from behind the siding?
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Old 07-18-2009, 01:27 PM   #8
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I usually leave the flashing long and cut it after the last full piece of siding is installed. You can then just cut it slightly above the fold where the next piece will interlock, which is slightly below the nailing fin. This way any water getting behind the siding will be redirected back on to the siding. The next piece will cover the flashing but the weep holes will allow it to escape. Those are all the tiny evenly spaced holes in the bottom of the siding.
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Old 07-18-2009, 02:57 PM   #9
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I see. That makes sense Jaros.
Here's the long version of the story, if you or anyone is interested:

I have recently had a roof added to my deck, turning it into an open-air porch. (The house is two stories with vinyl siding on the back where the deck/porch is). The porch roof was attached to the side of the house with a ledger board. To attach the ledger board, they removed a section of siding, and I think they cut away the foam board too. (There was no wood sheathing/underlayment, just foam board and I think felt paper behind the foam board. As far as I know they left the felt paper, but I'm not positive of that.) A few weeks after the project was done, I learned for the first time that vinyl siding relies on water barrier material behind the vinyl siding to protect the house from rain water. In my case I presume the water barrier was the foam board, and then behind that the felt paper.

They cut away a big rectangular area of the foam board to install the ledger board, so now the water barrier is "breached", and there are second story windows above the ledger board area that will let rain water behind the siding, due to the loose nature of siding. The thing that started me on the path of discovering all this was the fact that when they built the porch roof, they butted the porch roof flashing up against the exterior of the siding instead of putting it behind the siding, and they insisted that was the right way to do it even though I knew it was wrong. (By the way, I screened these guys really well before they did the job, and they seemed really good, so don't think that I just called someone out of the phone book or something.)

So basically there are two issues I have to try to fix. The first is that the flashing should have been put behind the siding. The second is that they should have done something to address the breach in the water barrier caused by the cutting of the foam board.

I'm thinking of trying to get someone to come remove the old piece of porch roof flashing and install one piece of new porch roof flashing that goes behind the siding and also behind the window flanges (the bottom of the windows are close enough to the porch roof to do this, only about 6 inches up from the porch roof), and tape the whole length of the top of the flashing to the foam board (because there is also a little bathroom window up higher that is too high for the flashing to reach to tuck under its flanges, but it will still leak water onto the foam board because it has siding next to it too.) I've seen a little water trickling down the siding under the porch roof during recent rain, so I know something's not right.

What do you think?
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Old 07-18-2009, 04:03 PM   #10
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Not too bad on the analysis. Yes, the piece of flashing should have gone behind the siding and even behind the foam board. Any liquid moisture traveling down the wall should have been picked up and brought back to the outside by the transition from the wall to the top of the roof. You should not see any liquid coming down the wall inside the porch if this was done right even if the windows above were flashed wrong. I like to run a piece of flashing tape over the top of the metal flashing so that anything traveling down the sheathing will have no choice but to run over the top of the metal and on to the roof instead of behind it. A piece of nice custom bent colored steel is the best choice or even a nice piece of copper if the budget allows. Aluminum is not a good choice.
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Old 07-18-2009, 04:32 PM   #11
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Jaros, a few follow up questions on your reply above.

At the point that the flashing exits out from under the siding, will the siding just stop and loosely lay on top of the flashing (this is my preference), or would the bottom of the siding have to have J-channel, or some other configuration. I think if J-channel were put on the bottom of the siding where the flashing exits, then that would mean that the J-channel would have to be nailed and that would mean nail holes in the flashing.

What kind of flashing tape do you use? I might want to buy some. There is a roofing supply store here in town I think I could go to.

Do you think I should ask whoever I find to come to put the new piece of porch roof flashing behind the foam board like you say above, or is in front of it ok.

Why is aluminum a bad choice for flashing?

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Old 07-18-2009, 04:34 PM   #12
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Quote:
Originally Posted by jaros bros. View Post
Not too bad on the analysis. Yes, the piece of flashing should have gone behind the siding and even behind the foam board. Any liquid moisture traveling down the wall should have been picked up and brought back to the outside by the transition from the wall to the top of the roof. You should not see any liquid coming down the wall inside the porch if this was done right even if the windows above were flashed wrong. I like to run a piece of flashing tape over the top of the metal flashing so that anything traveling down the sheathing will have no choice but to run over the top of the metal and on to the roof instead of behind it. A piece of nice custom bent colored steel is the best choice or even a nice piece of copper if the budget allows. Aluminum is not a good choice.
Aluminum is not a good choice???????????????

I think you could explain why not.
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Old 07-18-2009, 05:37 PM   #13
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Aluminum rips too easy, scratches, paint doesn't hold up like it does on steel, ice tends to push through it, it corrodes quickly and usually it doesn't come in the more sturdy guages that steel or copper come in. I also don't care for the expansion rate of aluminum, especially in the thin gauges. IMO, aluminum is used because it's cheap and convenient.

Aluminum flashing almost always needs replacing on reroofs and siding jobs whereas painted steel alloys and copper are generally still in fine shape to be reused.
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Old 07-18-2009, 07:30 PM   #14
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Jaros don't forget about me, you answered framer's question but not my questions.
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Old 07-18-2009, 08:37 PM   #15
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Quote:
Originally Posted by jaros bros. View Post
Aluminum rips too easy, scratches, paint doesn't hold up like it does on steel, ice tends to push through it, it corrodes quickly and usually it doesn't come in the more sturdy guages that steel or copper come in. I also don't care for the expansion rate of aluminum, especially in the thin gauges. IMO, aluminum is used because it's cheap and convenient.

Aluminum flashing almost always needs replacing on reroofs and siding jobs whereas painted steel alloys and copper are generally still in fine shape to be reused.
You must live in a different area than I. I can take you to roofs that I have done with aluminum flashing by me up to 30+ years that is still fine.
Aluminum comes in other thickness than the thin stuff at the big box stores.
I will agree that copper is probably the lifetime flashing, but not steel. By the way, steel is a lot less money than aluminun and as far as corroding, we are talking something that doesn't rust.
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