MEASURING FOR YOUR DOOR NOTE: On single wide doors, a reinforcement strut must be installed if an automatic garage door opener is used. MEASURE WIDTH & HEIGHT Measure existing door width and height in feet and inches. This determines the size of door needed. The rough opening should be the same size as the door. MEASURE FOR SIDEROOM Measure for sideroom. MEASURE HEADROOM Measure area labeled “headroom” – distance between the top of the door opening (“header”) and the ceiling (or first obstruction, e.g., light fixture). MEASURE BACKROOM Measure area labeled “backroom”– distance is measured from the garage door opening toward the back wall of the garage. Backroom required = door height plus 18″ (except for High Lift applications). Common garage door widths include 8', 9', 16' and 18'. Common garage door heights include 7' and 8'.