(1) It shall be the duty of the board of commissioners of a suburban improvement district to have the amount of work done by any contractor estimated, from time to time, as may be desirable, by the engineer selected by the board.
(2) The board shall draw its warrants in favor of the contractor for not more than ninety percent (90%) of the amount of work so reported, reserving the remainder until it has been ascertained that the work is completed according to contract and is free from liens.