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Community and Q&A

Installing and sealing windows without a flange

nvman | Posted in Green Building Techniques on

Does anyone have any suggestions on how to re-install and seal windows without a flange?

These were replacement windows that were installed 10 years ago and the flange had been removed to facilitate installation into the existing opening.

We are redoing the siding and would like to re flash the windows.


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  1. GBA Editor
    Martin Holladay | | #1

    It's difficult to provide recommendations without knowing more about the existing installation.

    Ideally, the rough openings were flashed in such a way than any water that gets into the rough opening is directed to the exterior. This requires a sill pan (either sloped or equipped with a back dam), as well as jamb flashing to protect the rough opening jambs.

    If that work was done, you are 90% of the way to your goal. If the window installers forgot to do that work, it's best to remove the windows and flash the rough openings. Once the rough openings have been properly flashed, you can reinstall the windows. How to flash the gap between the window frame and the siding (or exterior trim) depends on your siding choice and trim details.

  2. nvman | | #2

    Martin, thank you for your answer. Sorry for the tardy reply as have have been busy with the demo work.

    The windows are wood but with no flange. The opening has not been flashed properly so the windows will be removed for that to happen. They were installed flushed with the sheathing. There will be a rain-screen and fibre cement siding.

    So back to my question. Once the opening has been properly flashed and a back dam added, how do you seal a flangeless window to create the drainage plane? I see all the examples of adding a bead of sealant or tape but there is no flange on these windows.
    Hope that is enough info. Maybe I am using the wrong terminology.


  3. GBA Editor
    Martin Holladay | | #3

    Q. "How do you seal a flangeless window to create the drainage plane?"

    A. With flashing.

    The drainage plane is usually your WRB; for this WRB to allow drainage, you need to include a rainscreen gap between the WRB and your siding.

    The window will probably be installed so that the plane of the window frame is aligned with the plane of the outer edge of your furring strips. In most cases, you will be installing exterior casing at the jambs and the window head; this casing bridges the gap between the window frame and the edge of the rough opening.

    Traditionally, asphalt felt splines were used as flashing behind exterior jamb casing. These days, most builders use a peel-and-stick product.

    The head casing is flashed with Z-flashing (aluminum or copper). The top vertical leg of the Z-flashing is lapped by the WRB. the bottom vertical leg of the Z-flashing overhangs the head casing.

  4. davisk6615 | | #4

    Hi Martin, are their any section details of what your are talking about, I have the same issue as Aaron and am residing this fall.

    1. patrick1 | | #5

      FHB recently did an article on flangeless windows that includes some details and suggested products to use. You can find it here:

      The manufacturer of the recommended air sealing tape, Siga, also has a lot of detail drawings for flangeless windows here but they are specific to Siga products of course:

  5. davisk6615 | | #6

    Thank you Patrick, the siga instructions are exactly what I was looking for my inset window install.

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