In administering this section, the area of any sign shall be computed as the total square foot content of the background upon which the lettering, illustration or display is presented, including in such background any decorative motif or moldings. If the lettering, illustration or display is not affixed to a background but is affixed directly to the wall of a building or other structure, the area of a sign shall be computed as the product of the largest horizontal distance and the largest vertical distance of the lettering, illustration or display. The supporting members of any sign, which are used exclusively for such purpose, shall not be included in the computation of the area of the sign. Signs having more than one display face may have the maximum area permitted on each face.