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Green Basics

Flanged Window Replacement in a House with Wood Siding (5/5)

Step 5: Flash, Trim, and Seal the WindowWatch remodeler Bill Robinson as he shows how to use a combination of materials to flash over the flanges, and install a pre-built casing over the new window.

Bill Robinson: Now that the window is installed, I’m ready to flash it. If you remember, I cut back the siding a little over three inches. I’m using a 4-in. self-adhesive butyl flashing tape. I’m using my cutting table to cut my tape to 48 inches. I always start by pulling back the edge of the release paper enough so I can work with it when I get it into position. Because this window has an applied flange, I need be sure to roll the flashing tape about 1/2-in. up onto the window frame itself to make sure we have a water-tight seal here. I’m going to use my speed square to to push the tape up against the frame and get the overlap tight into the corner. Then I’m going to cut where the tape runs past the corners of the window frame so the tape can lay flat. And I’ll just go to the other side of the window and repeat these steps. I also use my J-roller to make sure all tape is tight into the corners. Next, Bill cuts small pieces of flexible corner-flashing tape to seal off the corners at the top of the window. He also cuts a piece for the joint where the two windows are mulled together. Bill: Now the rigid metal flashing will go over the top of the window. I’m going to apply a bead of sealant to the back of that, then center it over the top of the window. You can see how the flashing will take any rain that gets under the eaves and deflect it out away from the window. I’ll put a couple of fasteners through the flashing to hold it in place. The final step for flashing is to put a strip of flashing…


  1. Kevin McGuire | | #1

    Good detailed article.
    I know that he didn’t use metal flashing over his header trim, but is that only because of the overhang? Is it a good idea to z flash on top of trim and top of window? I’ve seen a construction instruction video do this...

    Is good practice to pre-fab the window trim when you’re able?

  2. Expert Member
    Malcolm Taylor | | #2


    Yes, it's because the trim is right up against the soffit.

    In general, if the head-trim sits on top of the siding, the flashing is installed where it is shown in the video. If the trim is applied directly to the sheathing, and the siding butts up to it, the flashing goes above the head-trim. The only situation where I can imagine using flashing in both locations would be if the dimension from the flange to the outside of the window frame was more than the depth of the trim being used.

    It is easier to prefab the trim. It keeps the pieces square, otherwise they tend to tilt as the window flange creates a bump. Another alternative is to shim the trim.

    1. Kevin McGuire | | #3

      Malcolm, thank you for the response. Good point on the trim. Now I plan to pocket hole and glue my trim beforehand

      You got me thinking when you said:
      “The only situation where I can imagine using flashing in both locations would be if the dimension from the flange to the outside of the window frame was more than the depth of the trim being used.”

      I think what you are saying is basically if the window sticks out further from the sheathing than the trim does you might consider flashing. Correct?

      Where I’m at almost all window frames protrude beyond trim - or they’re at least flush. I was even a little concerned because with my rainscreen and cladding and trim all put together they’d be depthier than the window frame by about 3/4”. (And I don’t really have the option to just shim the trim to wherever I want to because I need the trim to be flush with the battens) I was concerned this might “look funny” but perhaps to some it looks good.


      1. Expert Member
        Malcolm Taylor | | #4


        Pocket screws work great. Depending on how wide the trim is you might want to skip the glue to avoid cracking across the grain.

        The flashing does two things. It protects the top of the window frame (that protrudes beyond the flange) and channels water to the outside. As you can see in the video, the 5/4 trim they are using is slightly proud of the window frame, so if it was on a wall with no overhang above, it would be fine to put the flashing above the trim. If that was 3/4" trim you would need flashing to protect the window head too.

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