How do I Make Extension Jambs for Anderson Casement Windows?

Written by david brown
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How do I Make Extension Jambs for Anderson Casement Windows?
A casement window. (Plastic Window image by Russian Reviews from

Extension jambs are used to make a window or door fit in a thicker wall than it was originally designed for. For instance, adding extension jambs to a window designed for a two by four wall will allow the window to fit in a two by six wall. Extension jambs are usually made using screen stock, which is eleven-sixteenths inch thick, surfaced on four sides, and available in widths from 1 ½ inches to 5 ½ inches.

Skill level:

Things you need

  • Table saw
  • Carpenter's glue
  • Tape measure
  • Hammer
  • 8d finish nails
  • Screen stock

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  1. 1

    Rip (cut lengthwise) the screen stock to the appropriate width. For instance, to make a window with a 4 9/16 jamb fit a 2 x 6 framed wall, you'll need stock 2 inches wide. The rip fence is the guide parallel to the saw blade. Set the table saw's rip fence to two inches, and rip enough wood to go around the perimeter of the window.

  2. 2

    Cut the top and bottom pieces to length. Measure the width of the window, from outside of jamb to outside of jamb; cut the top and bottom extension jambs to this length using the mitre gauge on the table saw, or using a mitre saw.

  3. 3

    Attach the top and bottom extension jambs to the window. Place a small amount glue on the back of the extension jamb, and position the extension jamb on the window jamb, leaving a 3/16 reveal or setback to the inside of the window. Nail in place using #8 finish nails.

  4. 4

    Measure from the inside of the top extension jamb to the inside of the bottom extension jamb. Cut the side extension jambs to this length.

  5. 5

    Attach the side extension jambs as you did the top and bottom extension jambs.

  6. 6

    Nail through the top and bottom extension jambs into the ends of the side extension jambs. This will keep the corner joints tight.

Tips and warnings

  • Pre-drill holes for the nails to avoid splitting the extension jambs. Use a drill bit slightly smaller in diameter than the nails you are using.
  • Always use eye and ear protection when using power tools.

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