“Floor area” means the sum of the gross horizontal area of the floor or floors measured from the exterior faces of the exterior walls, including elevator shafts and stairwells on each floor and all horizontal areas having a ceiling height of seven feet or more, but excluding all parking and loading spaces, cellars, unroofed areas, roofed areas open on two sides, areas having ceiling height of less than seven feet and basements used exclusively for storage or housing of mechanical or central heating equipment. (Ord. 583 §2.09(C) (part), 1980).