All storage of building, contracting, or plumbing materials, sand, gravel, stone, lumber, equipment, and other supplies in the I-2 District shall be located within an area not closer than 150 feet from any street right-of-way line. The storage of lumber, coal, or other combustible material in the I-2 District shall not be less than 20 feet from any interior lot line, and a roadway shall be provided, graded, surfaced, and maintained from the street to the rear of the property to permit free access of fire trucks at any time. All such open storage shall be screened from all streets, and on all sides which abut any residential or business district, by a solid six-foot wall or fence sufficient to serve as a permanent retaining wall or fence.