New double casement windows came from manufacturer bowed inwards at the center mullion so that window has an a very, very slight "V" shape when viewed from above.

The windows need jam extensions made so that they are flush with the adjoining drywall surface and must fit flush to existing jam, but the inward bow of the window near the center prevents the jam extension from making contact all the way across for the top and bottom extension pieces.

This means I can't make a jam extension with a flat surface fit the jam without a gap at one end (it rocks near the center). I've tried sanding down this area (see pic) but it is a tedious and I am not getting anywhere, and I am not sure this is the best way to do this. Feels like it would be better and easier to make the jam extension fit the jam but have no idea how to sand/plane/cut the extension surface to fit the jam. How would I do this? Is there a way to trace the jam surface onto a new piece so that I can cut it similar to scribing a bookcase to a wall?..I don't know how to do this if so!

top of window where you can see where I have tried to sand down raised area!

gap between jam extension and jam caused by inward bow of 2 windows!

1 Answer 1


The right way to fix this is to cut out the spray foam insulation and move the window into proper position so that the two units align on a single plane. This isn't as difficult as it sounds. You can pilot and countersink some construction screws through the window frame at an upward angle into the framing to pull it outward if necessary. Work slowly and use caution and the window will move easily due to flex in the mounting flange. Be sure to keep shims in place to prevent the frame from bowing upward.

It looks like you have screws running through the window frame into the header. Many window manufacturers specifically prohibit this, as it increases the chance that settling will bow or jamb the window. I suggest removing them and reviewing the installation instructions. Andersen windows, for example, call for nailing through the outer flanges only (except where special clips are provided). (Correction: frame fastening is allowed, but is only done through the sides of the window.)

Otherwise, you'll have to use a jointer, powered hand planer, or sander to shape your extension jamb to the window. I wouldn't try to remove material from the window itself. You'd do this by holding the extension in place with the same gap to the window frame at each end. Measure that gap, and there you have the amount of wood that you need to remove at the center of the extension jamb, tapered evenly to the ends. How you remove the material depends on the tools you have on hand and your skill set.

  • Thanks. I tried this flexing the window when I installed it and it did flex some but not very much. There are steel plates that join the 2 windows that prevent the window from flexing any further. To remove those would be more than I would want to get into. The installation instructions do not mention not fastening through the top but it does say to fasten through pre-drilled holes of which there are 2 on each window at the top. Nevertheless I will remove these screws.
    – Roberto
    Nov 22, 2016 at 21:38
  • Ok. I've been wrong before. It could be that the units were joined poorly at the factory, and you'll need to deal with the outcome by creating a custom extension jamb.
    – isherwood
    Nov 22, 2016 at 21:40

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