The building sewer shall be constructed of cast iron pipe, ASTM specifications A74-42, or approved equal; vitrified clay sewer pipe, ASTM specifications C13-44T, or approved equal; or an approved plastic pipe. Any plastic pipe to be installed on any building sewer shall not be approved by the city until the owner has furnished descriptive literature and typical sample section of the plastic pipe proposed for installation, to the city for inspection and review. All joints on all pipe installed shall be tight and water- proof. Any part of the building sewer that is located within ten feet of a water service pipe or city water main shall be constructed of approved cast iron soil pipe with approved joints. No building sewer shall be installed within three feet of existing gas lines. If installed in filled or unstable ground, the building sewer shall be constructed of cast iron soil pipe, except that non-metallic material may be accepted if laid on a suitable concrete bed or cradle as approved by the city.