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Community and Q&A

How does one best trim a window when using metal siding?

Kim Goodwin | Posted in General Questions on

I’m planning to have a red metal building with metal roof and siding erected, part of which will be used as a small house.  My husband and I are hiring a contractor to erect the building, then planning to finish the interior ourselves.  We are using metal for some very specific reasons.  We are in SE Arizona, with about 12 inches of rain a year, monsoons in summer, and rarely a little snow in the winter.  Most of the rain is in the summer months, but can be driving rain.

The only contractor in our area licensed to erect metal buildings is only familiar with using J-trim all the way around the windows.  I’ve watched videos showing how people use J-trim, and it seems really inadequate for preventing water intrusion. 

In a previous post on GBA titled “Metal siding – window head flashing detail”,  J-trim on the top is said to be a bad idea.  In that post, it seems like the best option proposed is using a double angle trim or a drip cap trim at the window header.  Can anyone show me pictures of how that works and ties into the side trims?  Or just direct me to a site that shows this? 

I’ve been combing the internet for days, but almost everything I find is just J-trim all around.

Thanks so very much!

Here is a link to a document I’ve found with different trims diagrammed, which is where I’m getting the info on the names of different metal trim:

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  1. Zdesign | | #1

    Take a look at the ABC Metal Roofing website, they have a lot of details including the drip cap for windows and doors. Keep in mind though siding is just that, siding, your water barrier is at your sheathing level and if any water gets behind the metal its not the end of the world. I am installing ABC Metal Panels on my house as well and using just J channel around the windows/doors.

  2. Expert Member
    Malcolm Taylor | | #2


    Use this video for advice using J trim for the sill and jambs. The sequence of overlaps is very important.

    Substitute a drip flashing at the head as I did in the photo below. Note the siding is being installed onto furring to create a rain-screen gap.

    1. Kim Goodwin | | #3

      Thank you very, very much. The picture helps a lot. I'm familiar with the video, it was the best example I could find, as well. I still thought there was a lot of reliance on caulking, but it makes more sense with the drip cap now.

    2. Geir Gaseidnes | | #4

      Could one not consider the vertical channels in the metal siding enough of a gap to obviate the need for furring?

      1. Expert Member
        Malcolm Taylor | | #5


        Yes, and our code acknowledges corrugated siding installed vertically as meeting its rain screen requirements. I furred out the wall because I needed something to attach the siding to.

  3. GBA Editor
    Peter Yost | | #6

    Think of y0ur cladding and trim as protection against baseballs, hail, and ultraviolet light, and providing an aesthetic.

    Bulk water management is accomplished BEHIND the cladding and by connecting/weatherlapping flashings to your weather-resistive barrier (WRB) at all penetrations.


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