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Community and Q&A

Flashing Around Existing Window Openings

fall50 | Posted in General Questions on

I have existing window openings with orginal sash windows.  Attached to exterior/outside stop is older storm windows that came with the house.  While there is plenty of literature on  flashing new construction openings.   I came across the following on FHB for flashing openings when dealing with a situation similar to my setup.

Is this diagram considered best pratctie

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  1. Expert Member


    It's one of several methods that work. What it neglects is the most important part, which is including a sloped sill-pan and integrating that with the flashing and WRB.

  2. fall50 | | #2

    Thanks Malcolm. Do you have a picture of a detail that includes sloped sill pan. All of the window sills are currently sloped and wrapped on the house. But I'd like to see the specifics visually related to your suggestion.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      The first 33 seconds of this video shows how to install a membrane sill-pan. The most important part is to make sure it laps over the WRB below.

  3. woobagoobaa | | #4

    Question MS. Are you asking how to best flash with the old window and storm in place, or are you removing and re-installing the window?

  4. fall50 | | #5

    I have spent the better part of the weekend reviewing different flashing techniques. Pretending the brickmold didn't exist in this photo, is in essence what I am dealing with as I am keeping the original double hung sash windows with angled wood sills. The plan is to use flashing tape underneath the sill, next tape the vertical sides of the window and then use a final piece across the top which would extend 1" beyond the vertical flashing.

    At this point, trimming out the windows would ensue. In the attached picture the WRB butts up to the outside edge of the window casing (does not extend underneath the trim). Would best practice have been for the WRB to have extended underneath the window trim? Or does this not matter? Bewteen the 10:40 - 10:50 of the linked video, shows both options (left side WRB extends all the way undeneath the trim whereas right side is WRB stops roughly at the outside edge.)

    Now that the window is trimed out. I would take a 2nd piece of tape across the top and and lap it over the edge of the head trim ( see 11:37 of the video This is also evident in the attached picture.

    Finally as evident in the youtube video at the 12:35 mark, it shows that after the z flashing is installed, a 3rd piece of tape is then used to lap the edge of the z flashing to the existing piece of header tape. This step is NOT evident in the picture, however I drew it in. Taping this seam makes a ton of sense to me as you would want that z flashing to have no chance of water get behind the top edge. The question is, if you tape the Z flashing edge, do you really have to use a piece of tape over the head flashing of the trim as , like he did at the 11:37 mark and in the attached pic? If so, why?

    Details matter and I want to make sure I have this correct.

    1. GBA Editor
      Brian Pontolilo | | #6

      Hi MS.

      If the window has integral trim, flash it as shown in the video. If you will have the trim removed, the tape is installed before the window trim, sealing the window flange to the sheathing. The metal head flashing is over on top of the trim, to keep water from getting behind the trim. Redundancy and shingle-style lapping for drainage are the goals when flashing a window; not always easy when remodeling around existing windows, but still worth shooting for.

      I hope that makes sense.

  5. fall50 | | #7


    For clarity, no fins. Orginal sash window. I am clear on the metal head flashing and its purpose. I was more looking for a conversation on whats the point of taking a piece of flashing tape across the top and and lapping it over the edge of the head of the window trim as they do in the picture, considering the metal z flashing protects this. Is this done to account for any wind driven rain that could find its way underneat the head flashing?

    1. Expert Member
      MALCOLM TAYLOR | | #8


      Yes it's for wind-driven rain, although It's not commonly done. What I would do is bed the head-flashing in caulking, as that is a much more common route for water to get into that area. i would also omit the sealant they show on the WRB above the window to allow any water that gets behind to drain

  6. fall50 | | #9

    Thanks Malcolm. I was bothered by the caulking of the WRB detail to the Z flashing. If water gets behind the WRB, you want to allow it to get out. Skip taping or Skip caulking seems to be the way to solve for that when attaching the WRB to the flashing

  7. fall50 | | #10

    Following up on this thread and looking for one more item I need feedback on. As indicated earlier in this thread I have original sash windows with beveled wood sills original to the house 1940's.

    1) As I run my vertical flashing tape down to the sill, am I lapping over at the 90 degree point (sheathing meets the sill) ? See Pic 1

    2) The sills will be wrapped in aluminum (pic 2). As you see can they have 90 a flange where sill meets the sheathing. Would flashing tape simply go over the flange? Window casing then sits on top.

    Feedback would be appreciated. Details matter!

  8. fall50 | | #11

    As I think about this further. It seemingly would make sense to use flashing tape on the side of the sill and also use it at the intersection of where the sheathingmeets the sill like picture #1 in my previous post.

    I also cant see any harm in use a piece of tape on the alunium flashing that legs up.

    Anyone have a different approach?

  9. GBA Editor
    Kiley Jacques | | #12

    There are some related comments and resources in this Q&A thread that might prove helpful in your situation: Flashing Around Existing Windows.

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