Sign Height Measurement. The height of a sign shall be computed as the largest vertical height of the background upon which the lettering, illustration or display is presented. If there is no background, the height shall be the largest vertical height of the lettering, illustration or total display. This shall not be construed to include the supporting members of any sign which are used solely for such purpose. If the letters, illustrations or displays are attached directly to the face of a building, the height of the sign shall be the height of the largest letter, illustration or total display, whichever is greater.