Whenever possible, the building sewer shall be brought to the building at an elevation below the basement floor. No building sewer shall be laid parallel to or within three feet of any bearing wall, which might thereby be weakened. The depth shall be sufficient to afford protection from frost. The building sewer shall be laid at a uniform grade and in straight alignment insofar as possible. Changes in direction shall be made only with approved curved pipe and fittings, including cleanout fittings.