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Community and Q&A

Shims in Flanged Windows

BrunoF | Posted in General Questions on

My simonton casements and fixed windows are going to be installed Friday and I was curious about the need for shims in flanged windows…especially the picture / fixed sash windows.

it seems like putting shims on the sill plate would make sense as it would center the window vertically within the rough opening but beyond that, with a nailing fin fully secured, will shims be needed on the sides?


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  1. nynick | | #1

    You have to center it and level it before nailing. Yes.

  2. Expert Member


    No. If the sill is level, or had shims added to level it, you don't need any shims on the jambs. The only exception to that I can think of is if the window itself is not square, you may need some horizontal pressure on it prior to nailing off the jambs and head.

  3. Expert Member
    DCcontrarian | | #3

    When the window is perfectly level, flush and centered, moving a sixteenth of an inch will make it imperfect. It's very easy to bump it while nailing. While the shims aren't absolutely necessary it's easier to have the window firmly held while nailing.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      I think we are talking about different things. The OP is asking about whether you need shims on the jambs to support the window once it is nailed off. You may well need some temporary supports to keep the window in place before that.

      I often work by myself so my process is:
      - I level the sill with shims before installing the window.
      - The window gets set in the RO, and a short piece of 2"x4" with a kerf in it gets screwed above the top flange to stop the window from falling out.
      - I go inside and center the window in the opening.
      - Outside I nail off the bottom flange, and one nail through a top corner.
      - Back inside for a final check, then nail off the other flanges.

    2. nynick | | #5


  4. Uiloco | | #6

    Just make sure it's even and nailing it is an important aspect as well!

  5. BrunoF | | #7

    Thx folks!

  6. monkeyman9 | | #8

    Just went through this, installing myself. Even with sill level, you'll find the window frames are not perfect. I had to level them a little...but im being picky with 1/16....3/32 probably too. I shim the sides. You want something to hold them when you cut the strap and nail them in. I dont shim all over the place....enough to align. Your gonna spray foam them and that provides support too.

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