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Community and Q&A

Window flashing before or after housewrap

quantumgirl | Posted in General Questions on

DuPont has two different manuels for flashing windows. I think the more common way is to install and flash the windows after the housewrap. Is that the best way though? What happens if water gets behind the housewrap? Would it not make more sense to install and flash the windows to the sheathing, then install the housewrap over that?
my sheathing is plywood. I plan on taping the joints. Windows have a nailing fin. Sheathing will be covered with Tyvek, rain screen and hardie siding.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Quantum Girl,

    There are many ways to install and flash windows. It sounds like you will be installing flanged windows in a wall with housewrap and using flashing tape. You are correct that the housewrap should be installed before the windows. Then, the housewrap, window, and tape are integrated and lapped in a number of ways. Here is an article that does a better job of explaining how to execute these details than I can here:

    1. quantumgirl | | #3

      That's the way I have done it in the past. I just thought taping the nailing fin to the sheathing would be a better idea than taping it to some flimsy housewrap.

      1. GBA Editor
        Brian Pontolilo | | #6

        Quantum Girl,

        One way to think of it is that the housewrap is the drainage plane. (That's why a drainage gap behind siding is so helpful. Without it, water can get trapped against the WRB, which is one of the causes of WRB failure) Anyway, all the other flashing details are incorporated to get water out to this layer. Here's a drawing showing how this works with the head flashings, that you are asking about below.

  2. Expert Member


    Just to be clear - we could be talking about two different things. Flashing the applied to the window, or head-flashing above - so I'll address both.

    There are dozens of ways to install windows. The one thing that everyone agrees on is there needs to be a sill-pan in place before the windows are installed. I don't think whether the rough opening is wrapped before installation, or is taped after, is as important as that the work is done correctly. We should assume that water will not get past your WRB. If we can't assume that, what's the point of the WRB at all? You can see examples of both approaches in these videos by Dupont, which are probably the ones you watched:

    Somewhat surprisingly to me, neither approach shows the installation of a metal head-flashing. This should be installed in a bed of caulking on the window frame before the WRB, taped to the sheathing, and then lapped by the WRB. It should include end-dams, and if the wall includes a rain-screen cavity, a small gap (1/8" or so) should be left between the flashing and head-trim to allow moisture to escape.

    1. quantumgirl | | #4

      Any chance u have a picture of where exactly the metal flashing is supposed to go?
      I've done a few windows in the past for a playhouse I'm working on. I'll attach a picture. I put metal flashing over the window trim, not over the window itself. Housewrap goes over the metal. So that's wrong? I should have put the metal right on top of the window, underneath the window trim instead? I only caulked the gap between window and trim.

      1. Expert Member
        MALCOLM TAYLOR | | #5

        No - you got it right. If the trim is applied directly to the sheathing, then the flashing goes above. If the trim is installed on top of the siding, it goes over the window.

    2. JC72 | | #7

      I've never seen, in the wild, windows with metal head flashing. I suspect it's because we do not receive constant rain.

      1. Expert Member
        MALCOLM TAYLOR | | #8


        I have friends who build on the Canadian prairies who say the same. Out here on the Wet Coast we have no choice as they are mandated by our code, however I think you may see more head flashing as rain-screen gaps become more common in other regions.

        1. JC72 | | #13

          Ya. it's odd because per Building Science Corp based upon the amount of annual precipitation my climate zone has sufficient rainfall to warrant a rain screen. I guess they just don't build them because most of the heavy rain is intermittent (ie. Hurricane/Tropical Storm).

          Residential builders would flip out if they had to add metal head flashing to windows.

          1. Expert Member
            MALCOLM TAYLOR | | #14


            Done improperly it can cause more problems that it solves. Sloped back it can dump water at each end onto the window jambs, or if not bedded in caulking water can get blown up under the flashing.

            With most windows now being either vinyl or fiberglass, I wonder whether it wouldn't make more sense for manufacturers to build in a head flashing, complete with drip edge and end-dams, into the top extrusion?

      2. GBA Editor
        Brian Pontolilo | | #9

        I've traveled all around to look at houses, and I can't say that I have noticed any consistency with cap flashing. Some architects seem to spec it and some builders included it in their window flashing details, other don't.

        When I have used it, I have not integrated it with the WRB. I've run the WRB down below the trim, and installed the metal flashing simple as protection from water getting in between the top of the trim and the bottom of the siding. But I can see how it is better to lap the WRB over it. That way, it directs water that is already behind the siding out before it reaches the window.

        1. Expert Member
          MALCOLM TAYLOR | | #11

          Your comments and those of my builder friend bring up an interesting p0int. In the absence of a rain-screen gap where the siding or trim is attached directly to the WRB on the sheathing, in most climates, could enough water be reasonably expected to make it's way down behind the cladding that it needs re-directing to the outside? If not then the flashing can be used as a transition between materials, or safely omitted altogether.

          1. GBA Editor
            Brian Pontolilo | | #12

            I like the metal flashing in this spot because I think there should be something to keep water out, but don't like the idea of caulking when a mechanical flashing can do the job. Also, the metal flashing needed for this detail is readily available, at least in the NE US. If there is a rain screen siding detail with furring strips however, I think you have to bend your own flashing if you want to take it back to the WRB.

      3. user-6809423 | | #10

        Andersen includes metal head flashing in the box with their windows. I've seen plenty of windows without head flashing, but never installed one without it.

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