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Community and Q&A

Detailing Flanged Windows With Zip Sheathing

mpd_architect | Posted in Green Building Techniques on

I am building a new home with Zip-R Sheathing. The windows are flanged Andersen 400 series casments and awnings.  My question has to do with the bottom flange. I plan to shim the btm flange out with 1/16 shims to allow for water to weep out. The windows will all be trimmed out on the exterior with 1 x 4s all sides and bottom.  I plan to also place three small grooves on the back side of the bottom trim again to allow water to weep.  Just curious as to how others have detailed this? Also are 1/16 shims deemed adequate to weep.  Other opinions are much appreciated.
Mike Dolce, Architect

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  1. Expert Member


    Here's my thinking on this:

    When our codes first mandated rain-screen cavities many builders mounted their windows on the furring strips, which provided a natural drainage gap below the window, or drilled several holes in the bottom flange as an exit for water that got to the sloped sill.

    These approaches presupposed that en0ugh water could accumulate in the sill-pan that it needed a dedicated drainage path, and in the case of the drainage holes, that the water would take that path.

    Experience over the next decade seems to show that the amount of water that makes its way to the sill was almost incidental, and too small to actually "drain". The rot at the sills was from the framing being constantly damp, not seeing bulk water intrusions sufficient that it would pool. The important features which seem to make a difference are: sloping the sill, and providing a sill-pan. The small amount of water that may make its way there either dissipates, or seeps between the sill-pan and bottom flange. I don't think any additional effort to provide a dedicated drainage path is necessary.

    If you do decide to shim the window, I'd suggest shimming the whole thing. Otherwise your interior casings and exterior trim will be a lot harder to install.

  2. andy_ | | #2

    Doing a good job taping the entire rough opening and then taping the top and side flanges would be a better use of your time.
    I get that you want to make sure there is an exit path between the flange and the sheathing, but by shimming it I'd think you'd risk creating other issues with the connection of the flange; potential for warping, air leaks, etc. I know that 1/16th isn't much, but leaving that gap is the starting point. With any flex that 1/16th could become 1/8th or more.

  3. mpd_architect | | #3

    Thank you for the comments. This is helpful. Probably will omit the shims and just have a pitched sub-sill and not tape bottom flange.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      The other problem with creating a drainage path behind the trim, is that without a rain-screen cavity, you are just moving the problem from the sill to the siding and sheathing below.

  4. mpd_architect | | #5

    I agree Malcom. And this house will be vinyl sided which I know is not a true rain screen but should offer some draining and drying should limited moisture get behind it. Thanks again for the thoughtful comments.

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