At least 30 percent of the gross roof area of a building containing 15 dwelling units shall be provided for recreational use. For each additional dwelling unit, 100 square feet of additional roof area shall be provided for recreational use, up to a maximum of 50 percent of the gross roof area. This recreational area shall be accessible to all the occupants of said building and their guests.  No fees shall be charged to the occupants or their guests.  The provisions of this Section may be modified pursuant to Section 15-30 (MINOR MODIFICATIONS).