The county highway engineer shall have direct supervision over all public roads of the county, and over the road overseers and of the expenditure of all county and district funds made by the road overseers of the county. He shall also supervise the construction and maintenance of all roads, culverts and bridges. No county commission shall issue warrants in payment for road work or for other expenditure by road overseers, or in payment for work done under contract, until the claim therefor has been examined and approved by the county highway engineer.