All work in this Article provided for must be done under the supervision and to the satisfaction of said Director; and said Director shall require all materials used in such work to be in accordance with the specifications therefor; and all contracts provided for in this Article must contain a provision to the effect hereinbefore in this Section set forth, and also a provision to the effect that in no case, except where it is otherwise provided in this Article, or the Charter of said City and County, will said City and County or any department or official thereof, be liable for any portion of the expense of said work, or for any damages resulting in the course of the performance thereof, or for any delinquency of persons or of property assessed.
When any such work shall have been completed to the satisfaction of said Director, he shall so declare by certificate, which shall be made a permanent part of the records of his office, and thereupon he shall cause to be delivered to the contractor a duplicate of such certificate.