How to Build Window Jamb Extensions

Determine the width of the extension piece by measuring between the installed jamb to either the wall face or to the back face of the window casing or trim. Transfer this measurement to each end of the lumber you'll use to create the jamb extension. Stretch a chalk snap line from one mark to the other and snap a straight line across the length of the lumber.

Insert a fine-tooth cutting blade into a circular saw. Adjust the saw's cutting table so the blade will cut only slightly deeper than the extension material. Put on a pair of safety glasses.

Set the cutting table's front edge at the end of the lumber. Line up the blade and start the saw by squeezing its power trigger. Cut 12 inches deep into the line and stop the saw. Attach the rip guide to the slots on the upper front end of the circular saw and press the foot flush against the outside edge of the lumber. Lock the rip guide into place.

Pull the saw back slightly to free the blade. Squeeze the saw's trigger and wait until the blade rotates at the full rpm. Push the saw forward, keeping the rip guide tight against the outside edge, to cut the entire length of lumber.

Measure the length of lumber you'll need for each extension jamb section and cut it out of the ripped lumber.

Apply a generous amount of wood glue to the edge of the extension material that you'll press against the installed jamb. Press the extension material to the jamb, making sure the face's edges are lined up.

Hammer brad nails through the extension material and directly into the window jamb every 12 to 16 inches. Sand the face jamb and extension face with 120-grit sandpaper to blend in the butted edges when the glue is dry.