The entire area within a single continuous perimeter enclosing the extreme limits of lettering, representations, emblems or other figures, together with any material or color forming an integral part of the display or used to differentiate the sign from the background against which it is placed, shall constitute the area of the sign for computation of size. Structural members bearing no sign copy shall not be included. Only one side of a freestanding or projecting double-faced sign shall be included in calculating sign size, provided that the two display surface areas are joined at an angle no greater than 45°.