Measure the length of each side of the window. Extend the measuring tape from the upper edge of the jamb down to the window sill for either side of the window, and record the measurements.
Installed along the top and sides of windows, jambs serve as window frames. However, jamb extensions, or lengths of wood attached to the window jamb, are necessary in wider window openings, such as those found in older homes, to fill the space. These extensions increase the depth of the jambs to match the dimensions of the window opening. Before installation, accurately measure the opening and cut it carefully so the jambs fit snugly inside the available space.
Extend the measuring tape across the top of the window, from side to side. Butt the measuring tape flush against the side jamb where it meets the top window jamb and record the measurement.
Extend the measuring tape from the end of a side jamb to the end of the wall to determine the jamb depth, and record the measurement. Repeat the process at the other side.
Extend the measuring tape from the top jamb to the edge of the window and record the measurement. Take the measurement at three separate locations to ensure the window is plumb.
Lay pressure-treated, 3/4-inch-thick wood board on a table. Transfer the measurements for each extension on the board and cut along the lines carefully with a saw.
Lay the cut wood pieces on the table. Select the side on each board that will be visible after installation and mark it lightly with pencil. The pencil line helps identify which side of each jamb extension faces the window.
Butt the corresponding ends of the jamb extensions together to dry-fit the assembly. If required, trim excess length around the edges so the boards butt against each other, forming straight corners. Join the board using wood glue.
Set the jamb extensions in the window opening, flush against the jambs. Hammer evenly spaced galvanized wood nails to secure the extensions to the jambs.
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