All signs, other than permitted temporary signs, shall be constructed of durable materials and shall be adequately maintained. All cracked, warped or broken members of a sign shall be replaced. Deteriorated surfaces which evidence rusting, flaking or cracking shall be replaced or repaired. All broken or cracked glass shall be replaced. Any sign which fails to meet the maintenance provisions of this section shall be repaired or removed within 30 days of receipt of written notification from the zoning officer.