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Community and Q&A

Window head flashing

randygiles | Posted in General Questions on

I am preparing to install some windows. If I want to do best practices – I am assuming I should have end dams fabricated into the window head flashing. My questions are –

1) what is the typical height of the end dam – i.e. a 1/4 inch?

2) how far over the corner of the window should the flashing extend on each side of the window?

Thanks for your help.


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  1. Expert Member

    Randy, You don't need to get the end dams fabricated, they are easily made on site. There are two ways to do it.
    Either simply cut along the seam of the head flashing, bend up the ends approximately 1" and fold over to form a hem leaving you a 1/2" end dam which you then caulk the corner of.

    If you are more ambitious, you can try and fold the ends up without making the cut. This is the preferred way as you don't need to caulk, but I find it difficult to make the flashing exactly as long as I need it.
    Here is a link showing how to fold the flashing:
    The dams should be between 1/2" and 3/4". Cut slots in the head trim so it can be lowered over the dams and sits 1/8" above the flashing so any moisture can exit the cavity.

  2. GBA Editor
    Martin Holladay | | #2

    SureSill sells head flashing with integral end dams. The product is called HeadFlash. Here is a link to a web page with more information:

  3. GBA Editor
    Patrick Mccombe | | #3

    Forgive my ignorance, please. But what are the end dams on a head flashing supposed to do exactly?

  4. Expert Member

    End dams stop water, (especially wind driven) from making their way around the ends of window and door head-flashing. This is particularly a problem if you have a rain screen wall. Without dams the flashing can provide a direct route for moisture to enter the cavity

  5. GBA Editor
    Martin Holladay | | #5

    Here's what I wrote in an article on the topic in the June 2005 issue of Energy Design Update:

    "As many home inspectors have learned, the typical aluminum drip cap is often problematic. When siding is installed tight to a drip cap, as it often is, the pressure of the siding can pinch the flashing crease, creating a reverse slope in the flashing that conducts rain back toward the house (see Figure 3). The drip cap, now acting as a gutter, directs water to the window’s vulnerable upper corners. Drawn by gravity, the water dribbles down the sides of the window. On windy days, the water may be driven inward. If the sides of the window are inadequately flashed, water sometimes follows the trimmer studs down to the rough sill, which eventually begins to rot."

    [The photo below is the one that accompanied the article]


  6. GBA Editor
    Patrick Mccombe | | #6

    Thanks gentlemen. I'm sure I've never seen end dams on head flashing. I always make the head flashing a little wider than the casing and bend over the excess. I guess I should be doing more

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