Any roof area of a parking garage not otherwise covered by a building and larger than 400 square feet shall be landscaped. Up to five percent of such roof area may be used for mechanical equipment, provided that such mechanical equipment is screened from view by a fence which is at least 75 percent opaque or by at least three feet of dense planting. Up to 25 percent of such roof area may be accessible solely from an adjacent dwelling unit and the remaining roof area shall be accessible for the recreational use of the occupants of the building in which it is located. Hard surfaced areas shall not cover more than 50 percent of such roof area. Schools shall be exempt from the provisions of this paragraph (b).