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New Const Window Integrated J-Channel

Earl57's picture

I'm looking at replacing my old alum windows and panel siding with new construction vinyl windows and fiber cement lap siding.   It seems like integrated brick mold/J-channel receptors are becoming very popular because of the time/cost savings compared to installation of traditional exterior trim with a drip cap.  They say once the header nail fin is sealed with flashing tape, you don't need metal flashing above the header brick mold because the nail fin has been incorporated into the building envelope. 

But to my way of thinking, it doesn't make any sense to channel siding run-off above the window into the header J-channel and down the jamb J-channel behind the terminated siding.  Sure the window perimeter is sealed with flashing tape, but the design virtually guarantees water behind the siding below the sill.  You could caulk the joint between the header brick mold and lap siding, but it will eventually fail, you could place a felt paper spline under the sill to redirect the water out to the siding exterior, but why would you want to allow water to be routed behind the siding in the first place? 

Am I missing something? 



(post #104891, reply #1 of 8)


This is a bump, partly because you got me curious. Your concerns seem well-founded, and the j-channel instead of drip cap at the head seems like laziness, not best practice.


(post #104891, reply #2 of 8)

I don't install windows like that, and I doubt I would if someone asked. It is very sound practice to use metal flashing over the head casing, and in fact it's required by the IRC. We do it on every window. I have Z metal flashings made by the various metal roofing manufacturers from painted galv steel, or sometimes from copper.

The building industry is full of "new and improved" products that claim to make things easier and let you skip a step here and there. Personally I haven't found that many places to skip a step.

Edited 10/5/2007 12:05 am by davidmeiland

(post #104891, reply #4 of 8)

Thanks for the feedback.  I suppose I could drip cap over the header brick mold with the first course of siding above the window flush with the top of the brick mold; the siding wouldn't be seated in the J-channel along the header. 

But it sounds like the best practice would be to purchase windows without the integrated J-channel/brick mold feature and do my own trim.



(post #104891, reply #6 of 8)

I buy them with nailing fins. Makes for a very easy and very good waterproofing job.

(post #104891, reply #3 of 8)

I have installed vinyl windows with the integral J channel on HfH homes.

The manufacturer's instructions for installation seemed shakey.

We did some extra felting / taping at the head jamb to direct water into the top jamb, j-channel.  Also extra at the sill jamb to direct water from the side jambs out away from the wall directly to the bottom, back of a siding lap.

This is tough to describe but I will try - can do it much faster than describe it!

1) Entire wall was covered with Tyvek and the RO wrapped and taped, including the head jamb.

2) Felt was cut into a sort of window pan at the sill. Strip of felt about 18" wide applied to sill, notched for side jambs and extends below the window RO by about 12". This felt flap is not stapled to the wall. The felt is held only by a few staples inside of the RO into the sill.

3) Window was set in the opening with caulk under the nailing flanges and nailed off. Felt is held now by window flange nails - do not add any more staples.

4) Tyvek slit horizonally above the  head jamb - about 6" up and the slit extends about 6" past the window on both sides.   

5) Two 6" wide strips of felt were applied to the sides of the window, side strips extend about 6" above the window and are tucked under the Tyvek at the top. Strips extend a foot or so below the window, lapping over the "window pan" flap.

6) Another piece of felt was cut about 12" wide and 14" wider than the window. This piece was applied at the head jamb, top edge of felt slides under the Tyvek, bottom edge rides in the integral j-channel.  This felt overlaps the side felt strips at the top.  We then taped (Tyvek tape) all of the joints and edges at the head and sides.  Bottom felt flap is still loose.

7) Later, when the vinyl siding was applied, we lapped the bottom felt flap over the siding nailing flange at the top of the last full width run of siding below the window. We then cut the felt flap off so it rode on the front of the siding latch.


The theory which we applied was that any water travelling down the face of the Tyvek above the window would be directed into the top integral j-channel of the window, then down the side j-channels, onto the felt flap at the bottom where it would be carried out away from the wall to the back of the vinyl siding and subsequently out of the siding drains.

I did this on 56 windows in 2 duplexes, 8 years ago.  There are no reported or observed problems at this time.  There may be a better way, but we did the best we could with what we had at the time.


Never underestimate the value of a sharp pencil or good light.

PS: You are kidding yourself if you believe water does not get behind vinyl siding whether there is a window present or not.  Providing there has been rain in the past week, upon removal I have always found water behind the siding usually along with a bunch of dirt / small leaves / etc. Can't pinpoint where it comes from - just that it is there.      

Edited 10/5/2007 10:02 am ET by JTC1

Never underestimate the value of a sharp pencil or good light.

(post #104891, reply #5 of 8)


Thanks for the details!  Today you might use self-adhered flashing tape instead of felt splines.  The technique you describe along the sill to move the water from the jamb J-channels over a pan skirt or spline to the exterior surface of lap siding is what I was calling felt splines.  To my way of thinking, that should be a secondary line of defense with a drip cap being the primary. 

I guess if the top of a window was fairly protected from wind-driven rain, say under an eave it wouldn't be a problem, but I'm not sure I would want to rely soley on felt splines on an exposed end-gable wall.

Thanks again!



(post #104891, reply #7 of 8)


The more I think about it, the system you detailed makes perfect sense when you are siding with vinyl.  I guess you pretty much have to use a J-channel around window and door frames where vinyl siding terminates.  With loose vinyl siding interlocks  the drainage plane you created under the window could probably handle a fairly large volume of water.  But with fiber cement lap siding the min 1-1/4" lap is pulled in fairly tight during blind nailing.  A large volume of water would probably run laterally along the lap joint until it was past the sill pan. 

I can certainly see why builders would use vinyl windows with integrated J-channel and brick mold with vinyl siding.  And your email was detailed enough to do the flashing without even looking at the window manufacturer's instructions.

I still might go with integrated J-channel/brick mold just to eliminate the trim work, but with fiber cement lap siding I'm thinking a bead of caulk between the brick mold and lap siding along the header and side jambs to minimize the amount of water that gets into the J-channel might be a good idea.





(post #104891, reply #8 of 8)

You are welcome for the details.

Sorry, as soon as you said windows with integral j-channels, I assumed you were planning to use vinyl siding.

You know what they say about ####-u-me. It's true!


Never underestimate the value of a sharp pencil or good light.

Never underestimate the value of a sharp pencil or good light.