Old or existing building sewers may be used in connection with the main sewer only when they are found on examination and test by the Safety-Service Director to meet all requirements of this chapter including minimum size of four inches and joints passing the standard water or air test for compression type joints. If an existing building sewer is not or cannot be tested, it shall be deemed to be unusable and a new six inch sewer with compression type joints shall be installed as a building sewer.