Head Flashing (Drip Cap) - Horizontal Aluminum Siding

I have been unable to find anything detailing how water is kept from accumulating in J-Channel or snap-lok trim and running out to the sides of those gutters to get behind the siding at the side trim. Vinyl Siding Institute details this carefully for vinyl systems. Anyone out there have a detail for aluminum or does it just depend on underlayment and flashing (often missing on older buildings)?


See pic.

I found this manufacturer with this detail. Maybe it will be helpful.

Here is another that easier to read from the same mfg.

Thanks Brian.

I had already checked out Gentek’s manual and had not found anything about drip cap there. Just flashing details (attached). Where did you find the drip cap detail?

I don’t like Gentek’s detail for the wood fiber backed siding that i encounter. It does not seem to divert the water off the face of the window and anticipates it running down the side J channel where it could get at the edges of the moisture absorbent wood fiber. Their detail is pretty much the same as the Vinyl Siding Institute for Vinyl, except that the VSI calls for drip cap flashing with a gap between it and the J. Why would aluminum details differ?


I do not know.

There may be other things to consider such as; is it a finned window flashed with tape? Does the window profile protrude past the siding or is it recessed? Is the AL siding a retrofit over old siding? But generally speaking, I would have the same concern as you. I think J-channel is a poor substitute for drip cap.

Oddly, in one of the above illustrations drip cap is pictured but is not mentioned in the narrative.


all vinyl, aluminum and steel siding all reply on the power of housewrap, felt paper, etc to keep the walls dry. windows should be sealed behind the siding with tape and there should be felt paper or another type of membrane to keep the structure dry. the siding itself is not completely water proof. especially around trim. it is not meant to be.
but without tearing it off you can not verify this. But if the siding has been on for many years and the sill plate looks good from the crawl space, its most likely not leaking and getting into the structure.

Thanks Bill. Aware of those things. Vinyl Siding Institute recommends drip cap. Most houses here are not on crawlspaces and many quite old. Cannot depend on flashing and wrap.

Thanks Brian. Looks like we see this issue from the same perspective. I would be really temptedto put some good caulk on that top seam but mfgr instructions say no.

What you are looking at on the top of the window is finish trim covering the cut in the siding, no need to caulk.

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Thanks Scott. Yes, but it is a rain catching edge that leaves the cut end sitting in water that might cause corrosion. Generally with flashings and things similar no upward exposed edges are allowed. See SMACNA. why here? I know it is just the way things are done. But is it just a matter of expediency?

No caulk above horizontal flashing is typical. Cement fiber gives the same directive. The cladding needs a place to drain along horizontal flashing.

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