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Community and Q&A

Head Flashing Design + Should I notch trim over end-dams?

jamesboris | Posted in Green Building Techniques on

Hello again! Zone 2A, new build. Nail-fin windows in plane with the sheathing/WRB. ½” rainscreen, with 5/4″ live edge siding and 3x trim (mounted to RS). Steel head flashings on *both* the windows *and* the trim. 3 questions:

1) With respect to the flashing on the head-trim: How far should it project? General rule for windows is 1/4″ I think? But I’ve heard that for trim, when the window below has a metal drip cap, zero projection is okay — because any water that runs down the face of the trim (and wicks underneath it) will just land on the *window’s* head flashing.

2) Does a 5/16″ L “drip leg” plus a 1/4″ L kickout sound okay? Making these elements as big as Hammer & Hand (and FHB!) recommends covers up a lot of the window frame’s head. Looks bad on the double hungs (I made a prototype).

3) My head flashings will have end dams, and I’ve seen builders like Malcolm Taylor here suggest that you should cut small notches in the head casing (trim) to allow it to be lowered over your end dams (to the normal ¼” gap b/w trim and window frame). What’s the point of having a greater-than-1/4” H end dam, if you’re blocking off the top ¼” of it? Why not just make it ¼” H?


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  1. Expert Member


    1. Yes, size the flashing for the window, not the trim.

    2. Yes that sounds fine to me.

    3. The height of functional end-dams is largely independent of the height of the gap you leave between the head flashing and trim above. Rather it varies according to the conditions they will see. So if there is a lot of wind-blown rain you need higher ones to stop water jumping the dams, and getting into the cavity on each side. Our code acknowledges this by making the required height of end-dams dependent on the local wind-driven rain pressure.

    I would reduce the 1/4" gap to 1/8". That will still allow any water to drain off the head flashing, but reduce the amount of wind driven moisture entering the cavity, and deter pests.

    1. jamesboris | | #2

      Thanks Malcolm... followups:
      RE: #1: I'll have flashing on both the window and the trim. I already plan to have the flashing *for the window* project 1/4" past the window... so here, I'm asking how far the flashing *for the trim* should project. I edited the question to make that clearer.

      RE: #3: I'm not quite following. Let's say Window #1's cap flashing has 1/2" H end dams. You cut little slots in its head-trim, then insall it so that it's 1/8" above the head flashing. Window #2's cap flashing has 1/8" H end dams. You install its trim right above, flush to the top edge of those end dams. How is #2 going to perform differently than #1? What is the point of #1's extra end dam height if it's just getting blocked by trim?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        Sorry, I misunderstood your first question. Why do you have flashing both above and below the trim at the head? It's usually an either/or situation. If the trim sits on top of the siding, the head flashing goes over the window. if the trim sits directly on the rain-screen battens, the head flashing goes over the trim, and no additional flashing is necessary over the window. The trim can be caulked to the window frame on all four sides.

        The primary purpose of end-dams on head flashing is to stop the water that accumulates on the head flashing from being driven sideways by wind, and entering the rain-screen cavity. That water sits at the back of the flashing at the junction between the sloped horizontal leg and the vertical one of the flashing. That's also the back of the rain-screen gap, well behind the trim.

        So in your examples: Yes at the trim layer the depth of the end-dams doesn't matter much, but it's behind the trim and cladding, at the unprotected cavity where the end-dam is the only thing providing protection, where the height is important.

  2. jamesboris | | #4

    "Why do you have flashing both above and below the trim at the head? It's usually an either/or situation."

    I've always seen it as an either/or situation too -- until this building, my first since I started studying GBA, etc. I would *love* to skip the cap flashings on the windows, but I'm getting mixed messages. Hammer & Hand agrees with you. Matt Risinger recommends cap-flashing above the window *and* the trim. So does my window manufacturer, Marvin -- even though their windows include an integral vinyl drip cap!

    Check this out -- here's Marvin instructing you to *cut off the integral vinyl drip cap* and replace it with a metal one: And here's Marvin saying to put cap flashing above both window and trim: even though the caption above it says "Don't." Finally, here's Marvin telling you to glue the vinyl drip cap to the top of the window (p. 6, Step 3)... and also showing it cut-away and replaced with a metal one (Step 4):

    Maybe I should start a thread just about that Marvin foolishness!

    A final thought: The top edge of the window head is flat (even with the integral vinyl cap). So if water blows in, it will perch there until... So, the metal head flashing gives you a way to add a drainage slope.

    Again, I would love to save $1000 and a bunch of hassle and omit the drip caps above the windows (and just use one above the head-casings)... I'm just not sure what to do.

    1. Expert Member
      MALCOLM TAYLOR | | #5


      That is a dilemma! A whole other topic would be where all the how-to videos manufacturers supply fall in terms of warranty. They often contradict the written installation instructions, and sometimes our code. But I guess it is what it is. So going back to your original question #1, I don't see why a zero overhang on the trim flashing would be a problem if you decide to go that route.

      As an aside: with all due respect to Matt Risinger, who has introduced me to a lot of materials I'd never heard of, he does tend to over-complicate things. If I had to choose between his advice and Hammer and Hand, I'd opt for the latter.

      1. jamesboris | | #6

        It's a tough call for me (though Marvin's advice means very little to me!). I'm leaning more toward omitting the cap flashing over the window (and keeping the one over the trim of course). But the plot thickens... one of the only places I see a detail drawn showing 2 head flashings is from the BC coastal region! Ever built there? ;) And here's one from Michael Maines:

        As for Matt Risinger, I completely agree -- he'd be a better builder if he'd spent a couple years with a nail gun in his hand (like he pantomimes in that intro!)... but I've learned a lot from him too.

        1. Expert Member
          MALCOLM TAYLOR | | #7


          Both those details do need two sets of flashing. Because they are mounting the upper one on the rain-screen battens (it's really only there to stop water getting into the joint between the trim and cladding above), that leaves the cavity behind open, meaning water can still get down to the window head. The question is if the flashing above the trim goes right back to the sheathing, what would be the function of the lower one?

          To further muddy the waters: If the trim is not mounted on the cladding, is there any p0int in furring out behind it, or just mounting the window trim directly on the sheathing? I've done it it both ways, and don't see much difference.

          The Warranty Approved Details were one of the first to come out almost two decades ago. They were very useful at the time, but still included some advice we now see was incorrect - like drilling holes in your sheathing to allow drying - and need to be used selectively. If you want a good laugh, search GBA for the discussion I had with Martin where I defended the idea of drying holes, until John Straub finally popped up and called it nuts.

          1. jamesboris | | #8

            Good eye Malcolm, can't believe I overlooked that (that the upper flashing is mounted on the rainscreen). I'm gonna omit the metal cap flashing over the window and just use one on the head-trim. The flashing will mount to the WRB, behind the rainscreen. Thanks for helping me think through this. Saved me a lot of time and money. Although now I need to think of something else to do with this spare sheet of paint-grip 24ga steel!

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