The size and slope of the building sewer to be installed shall be subject to the approval of the City Inspector, but in no event shall the diameter of the pipe be less than four inches. The slope at which a six inch pipe is to be laid shall be not less than one-eighth inch per foot and for four inch pipe, not less than one-fourth inch per foot. Any grades for the pipe, which are proposed for installation at grades less than these specified, shall be approved by the City Inspector prior to placement.