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Question DetailsAsked on 1/22/2018

should windows have J channel on outside? I had windows and siding done and it just doesn't look "finished"

When I asked my neighbors one of them said the J channel is missing around the window..... My question is: Is J channel mostly cosmetic issue ? Should I be concerned that contractor didn't install what he was supposed to (eg flashing or J channel or ????) I do have a picture but can't see how to upload the picture here.....

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3 Answers


If you go to Answer This Question (yellow button right below your question, just as if you were going to answer your own question) when you click it you have the Your Answer come up. At the far left end of the gray header bar above the text box in Your Answer, you will see a yellow icon like a mountain with a moon - click that and you can attach photos in GIF, JPG (including Mac JPEG), or PNG format.

Some sort of window termination is needed - sometimes a J channel, sometimes an integral waterstrip that lies over the ends of the siding or edge of the panel siding. Sometimes with "self-flashing" windows all the window flashing detail is under the end of the siding so all you see is the end/edge of the siding coming close to the window frame - in that case (which sounds like it may be your case) you need a window trim or brickmold around the window frame to stop water from getting in there and conceal the expansion gap on the siding - but NOT on the underside as that is where any water in the flashing will run out.

Check out the installation instructions for your window model on the manufacturer website - and also whether they did the rough opening water shield correctly. (Below are a few link to the general way it should be done - varies slightly with different window -

But the window detailing has to have a way to keep water from running down the siding and into the wall around the window (commonly two ways - one flashing or ice and water shield at the water barrier level, and one at the wall surface to keep impinging or surface runoff water from getting in under the siding), and has to have drainage at the bottom so any water trapped by the water barrier can exit at the bottom of the window and back onto the siding (this is commonly neglected ro done wrong, or caulked over by people who do not know that is supposed to stay open).

And no, J channel or equivalent is not just cosmetic, though it does conceal the expansion gap - it shouldkeep debris and water from getting into that gap. Unfortunately, most window manufacturers instructions do not "follow the water" and trace out where water can get in and out, so commonly there is a direct path off the siding (especially when the water is blowing or wicking sideways or flowing heavily on the siding) in under the surface trim or front flange of the J channel - letting water right in under the ends of the siding, leaving only the underlying water barrier to handle it under the siding.

Properly, in addition to flashing and C or J channel abross the top to handle runoff there, there should be a drainage channel alongside the window unit at the water barrier level (and blocked from lateral migration in under the siding, which is almost never done) to a drainage point at each lower corner of the window.

Then, there should also be (but almost never is) a water seal at the siding surface interface with the channel or built-in flange or brickmold - to keep water on the siding from migrating in sideways toward the window. This can be a compressible impermeable foam seal or putty or it can be caulk or better, both.

In the old days a putty was used for this - called "fenester (sp ?) putty" or "window frame putty" (different than window glazing putty though sometimes same was used for both) - today a compressible impermeable foam or caulk would be used. It is CRITICAL to be sure that any water seal like that has proper flashing or ice and water shield leading to an opening back to the surface at the bottom of the window, otherwise pollen and dirt and water will end up under the siding and cause rot and growth of moss/lichen/mold which can end up staining and sometimes even growing out from under the siding planks or panels - as well as into the wall.

Answered 2 years ago by LCD


Source: here is picture of one of the windows where it meets the siding.... is it possible to tell if it has necessary trim/flashing,etc?

Answered 2 years ago by nedunla


OK - nice picture. If you look just above the close rolled edge corner of the bottom trim, you can see a dark shape - that is apparently looking INTO the rough opening through the wall from the window. So it looks like they cut through the water barrier, if that is what the square-cut corner at the closest edge of the black shadowed shape is, put the windows in with zero watershield in the rough opening and zero flashing to keep water out of the wall, and just butted the siding up against the side of the window. I can see zero watershield around the windows, zero flashing, certainly no J channel unless it is an unusually deep integral channel, and zero integration of the window into the under-siding water barrier (if that is what the horizontal textured white stuff is - might be asbestos panel siding like Transite instead ? Looks thick for a water barrier unless one of those claiming to be insulation as well) - looks like they just cut a rectangular hole in that to put the window in through.

If that is what it is, then the question arises of whether there is any water barrier in the exterior wall at all - should show under the siding and be integrated with the window water shield and flashing to prevent water penetration.

You can see any water getting in to the window-end of the siding is just going to pour down along the side of the window frame (or J channel if that is an integral lhannel the siding is pushed into and you just can't see the back flange of it) and into the rough opening into the wall - because even if it is a J channel there is no bottom channel or sill piece.

So - my call - looks like this is the worst example I have seen in a long time of a window job - if they are all like this it calls for a total redo of the windows - which may require removing the adjacent siding first to do it right. This one also appears to be set too far to the of the outside, but would take a close look all around to confirm that.

This does not appear to be a type of window designed for installation with plank siding (unless the siding is shoved into a very deep integral J channel with the back flange of it concealed by the siding) - looks like a "flush mount" window to go over panel siding or brick (though would still need the water shield, flashing, etc in that case.). Though (not totally clear from that angle) might have been able to use J channel in combination with it to do the job roughly right.

One other thing - I don't see, unless this was done in very hot weather (so already expanded to full length) which seems mighty unlikely given the time of year, where is the expansion gap at the end of the siding ? Normally 1/8 - 1/2 inch depending on temperature when applied and type of siding material.

I certainly would not pay them anything more on this job, and would get an expert window contractor or licensed building inspector or Architect to act as your expert witness on dealing with this contractor on getting it done right.

In fact, I have zero confirdence that this contractor can ever do the job anything like right after that example, so I would probably call the city/county (as applicable) building inspector to have him inspect and fail the job, then with the inspector's report I would call the contractor's Bond to force them to pay another contractor to do the job right.

Whether you start with the contractor's owner first or not is your call - assuming he was not at the site during installation - but if his crew does work like this I would questions if the company is going to do any better job on the rework - which depending on how they fastened the windows in and whether they distorted them by not shimming correctly and such, might have to include new window units as well.

Sending this photo along with a few more of typical windows (perhaps a bit less of a closeup for those) to the manufacturer for their opinion might help too - they might well send a rep out to give their official opinion, which could only help your case here.

Answered 2 years ago by LCD

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