Laboratory, warehouse and industrial and wholesale establishment: one space for every one employee; or, if work shifts are used, one space for every one employee in the two largest work shifts combined; provided that, and in either event to allow for expansion, there shall be sufficient unpaved ground space, if necessary, so located, graded and maintained in grass which, when combined with the paved area, will provide for total parking of one space for every 100 square feet of usable building floor space, excluding lavatories and utility rooms.