Most contracts permit the engineer to certify payment on account of materials delivered to site but not yet incorporated in the works. Section 16.5 sets out the matters to be taken into consideration when assessing what payment can be allowed. Usually a contractor will only ask for payment on account of relatively expensive items, such as steelwork, reinforcement, pipes and valves, etc. Before agreeing to payment on account the RE will need to inspect the materials to ensure they conform to specification, and may need to get confirmation the contractor has paid for them or otherwise has ownership of them.