Roof signs shall not exceed 15% of the front wall area of the building or structure on which the sign is to be mounted. Roof signs may extend above the roof, or top of the wall, a distance equal to 1/2 the height of the roof or wall, or 10 feet, whichever is the smaller height. No roof sign shall be placed upon the roof of any building so as to prevent free passage from one part of the roof to the other thereof, or interfere with any openings in such roof. All roof signs must comply with the structural height limitations required in each zone. No roof sign erected upon the roof of any building shall project beyond the edges of said roof in any horizontal direction.