The roof deck shall have only one lobby, whether for an elevator, a staircase, or both, which shall not exceed an area of 100 square feet. The area of the elevator and/or staircase shall not be considered as part of the lobby for the purpose of calculating the 100-square-foot limitation. If not for communal use, access shall be by a bulkhead for each unit immediately below, whether for an elevator, a staircase, or both, that leads directly, without lobby, onto the roof surface.