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Window cap/head flashing: 0, 1, or 2?

tannerc | Posted in General Questions on

Hi all, 

New construction project here and I’m getting ready to install window trim and then siding. I have ply gem vinyl windows installed which have an integrated nailing flange. Windows protrude 1 3/8″ and are fully flashed with peel and stick. Siding and trim will be installed over 3/8 in Rainvent battens. 

I’ve seen many different variations now and I’m wondering how many aluminum flashings should be installed above the window. One over the window head and another over the wooden head trim or just one of these or none? Water does currently pool on the flat window head when it rains which suggests to me that a cap flashing there would be a good idea.


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  1. tannerc | | #1

    Hmm tried to upload a photo to no avail...

    1. Expert Member
      MALCOLM TAYLOR | | #2


      It depends on whether the head-trim is installed directly on the sheathing, or on top of the siding.

      If it is the former you should stop the battens above the trim, provide a head-flashing with end-dams on top of the trim and caulk all four sides of the trim to the window frame.

      If it is the latter, install the head-flashing onto a bed of caulking on the window, run the battens right down to the flange, then install the siding and trim on top - but don't caulk between the flashing and the siding at the head.

      In both cases make sure the flashing is installed on the sheathing and lapped by your WRB.

  2. rockies63 | | #3

    I have seen this window head flashing detail before that has a little tab at the end of the flashing that is bent down to help protect the sides of the window, but I would go further and weld the front edge seams together to prevent leaks. I would even weld a small flat piece of metal (behind the corner where it touches the wall) to seal that seam too. You could get these made at a metal shop.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      That was the detail suggested some years ago. The problem with it is it dumps the water behind the siding at the jambs. Current best practice is to bend the ends up not down, to make end-dams.

  3. Peter Yost | | #5

    HI Tanner -

    Just about every window cap installation I see fails because the projecting plane of the cap is either level or even tipped back to COLLECT water rather than sloped down to shed. Note that both references above suggest a 10 degree or even 15 degree slope down.

    I have never tried the end dam fold as shown in the HammerandHand detail but I do note that as you scroll down the second detail shows the fold down and around the corners.

    We end dam rough opening flashings; if it makes sense there, sure seems to make sense with window cap.


    1. Expert Member
      MALCOLM TAYLOR | | #6


      End-dams are mandated by our building code here in BC. There are even requirements for different heights depending on the climate zone.

  4. tannerc | | #7

    Thanks guys! I was planning on installing the trim first and butting the siding up to it unless anyone has reasons I shouldn't. I've attached 2 schematics, one from a BC govt rainscreen detail reference guide and another from one of Martin's articles on flashing windows in a rainscreen wall.

    Malcolm I like your suggestion but wonder if the trim would benefit from being furred out as shown in these drawings.

    I think I'm coming to the understanding that I can't go wrong with metal flashing above both the window and the head trim but could substitute a caulk joint if I'm willing to keep an eye on it and maintain it over the years. Does that seem about right?

    Finally does anyone have any strong opinions on flashing below the window? I have attached the BC reference guide version which includes yet more aluminum!

    1. Expert Member
      MALCOLM TAYLOR | | #8


      You can mount the trim on the rain-screen battens, but one of the flashings has to go back to the sheathing to divert any water that gets into the cavity.

      If you look at how the two details dealt with that, the one uses two layers of flashing, the other (from FHB) pretends it isn't a problem and butts the siding to the trim. One way or another, if the trim isn't on top of the siding, you need some way of dealing with that joint above the head-trim.

      If you are providing flashing above the trim, I don't see any advantage to having the second piece below the trim, or mounting the trim on the battens. The function of the flashing is to move water to the outside, but at the same time it is more vulnerable to allowing water into the wall than a caulked joint between the trim and window frame.

      Flashing below windows is one of those apocryphal details you see drawn, but never actually see in real life. It achieves nothing that sloping the top of the trim below the sill already does.

      Edit: This may be a useful guide:

  5. rockies63 | | #9

    Malcolm, the link you provide in comment #4 shows both an upturned end and a downturned end to the window head flashing. The link in comment #8 doesn't show either method (neither does the further link on that page under step#14 "Note: See alternate rain screen head flashing detail in section 4.2 Top of Window" .

    So in BC I'm guessing that the mandated end dams have to point up, but what about the gap between the back of the turned up end tab and the part of the flashing running up the wall behind the siding? Would water still leak through that point and dump water behind the siding?

    1. Expert Member
      MALCOLM TAYLOR | | #10


      I haven't looked into whether the requirement covers all of BC, or just the costal region where I build - as is the case with most details associated with rain-screens, which are in force only for a specific marine climate zone. Either way , end-dams are good practice everywhere.

      There is no gap between the turned-up ends and the vertical leg of the flashing. It is folded, not cut. You an see that in the link in my post #4.

  6. tannerc | | #11

    Hey folks I've some follow-up questions:

    A friend pointed out that because my siding is tongue-and-groove and relatively flat it might be easier to put the window trim on top of the siding so that imperfect siding cuts around windows could be covered. If I went this route I would follow Malcolm's advice from reply #2 and install a head flashing directly over the window before siding and trim.
    1. Should the trim be caulked to the siding as it is installed?
    2. Should the trim be caulked to the window frame at the sides and bottom(if no rainscreen vent?)
    3. In the case of the door where the trim will be directly on top of the housewrap/flashing tapes (this would also be the case if I put window trim down first) should the trim be caulked to the house wrap?
    Thanks again!

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