It is unlawful for any person to own or use any packing house or cold storage plant where articles are kept which rats might eat unless such house or plant shall be so protected by impervious material as to prevent rats from gaining access thereto.  All vents, windows, doors, holes, or openings thereto shall be so covered and protected by impervious material that rodents and especially rats may not gain access thereto.  The doors shall be equipped with self‑closing devices, which shall at all times be maintained in good operating condition.