How to Measure for Jamb Extensions

Jamb extensions are wood extensions attached to the window jamb. Window jambs form the frame around the window. There is a top jamb and two side jambs. The jambs come in standard 4 9/16-inch widths. This generally matches the thickness of the house. The studs, sheathing and drywall combine for the 4 9/16-inch measurement. For older houses, wider studs are sometimes used and jamb extensions need to be created to fit the opening. Measuring for a jamb extension is the first step of the installation process.

Things You'll Need

• Tape measure
• Pencil
• Paper

Instructions

1. Measure the the length of each widow side. Run a tape measure from the top of the window jamb straight down to the window sill for each side of the window. Record both measurements.

2. Measure across the window from side jamb to side jamb at the top of the window. butt the tape measure up against the side jamb where it meets the top jamb of the window. Record the measurement.

3. Measure the depth of the jamb. Push the edge of the tape along either side jamb. Measure the distance from the end of the jamb inward to the end of the wall. Record the measurement in inches. Repeat the process for the other side jamb.

4. Butt a tape measure up to the top jamb and measure the distance from the top jamb to the edge of the window. Measure three separate locations along the top jamb and verify that the window is plumb in the opening. If the measurements vary, the window is not level in the opening.

Tips & Warnings

• The same measurement techniques apply to measuring for a door. The main difference is the door measurements for the length will generally be much larger than the window measurements.

References

• Creative Homeowner: Making and Installing Jamb Extensions on Windows
• Home Addition Plus: Installing Interior Door Trim