An expandable flashing device comprises first and second pieces of flashing material that overlap one another with hems or folded edges located on opposite lateral sides of the pieces that hold the pieces together, yet allow the pieces to slide or shift with respect to one another to selectively change the length of the device. In a first stowed position, the pieces overlap one another and in an expanded second position, the pieces may be pulled apart to increase an overall length of the pieces. The maximum length of material that overlaps between the pieces is defined as the overlap range. The overlap range defines the range to which the device can be shortened or lengthened to fit a particular installation. A flashing system and a method of installing flashing are also disclosed.