At the time of construction, alteration, or enlargement of a structure or building, off-street loading areas shall be provided and maintained for all uses as follows:

   1.   One 1,000-square-foot, off-street loading area shall be provided for each use, building, or structure of a size between 5,000 and 20,000 square feet.

   2.   For larger uses, buildings, and structures one 1,000-square-foot, off-street loading area shall be provided for each additional 20,000 square feet or fraction thereof.

   3.   Where the off-street loading space borders a residential district, it shall be set back five feet and shall be effectively screen planted.