(a) Contract containing provision for retainage.--A contract containing a provision for retainage as provided in section 3921 (relating to retainage) shall contain a provision requiring the architect or engineer to make final inspection within 30 days of receipt of the request of the contractor for final inspection and application for final payment. If the work is substantially completed, the architect or engineer shall issue a certificate of completion and a final certificate for payment, and the government agency shall make payment in full within 45 days except as provided in section 3921, less only one and one-half times the amount required to complete any then-remaining uncompleted minor items, which amount shall be certified by the architect or engineer and, upon receipt by the government agency of any guarantee bonds which may be required, in accordance with the contract, to insure proper workmanship for a designated period of time. The certificate given by the architect or engineer shall list in detail each uncompleted item and a reasonable cost of completion. Final payment of any amount withheld for the completion of the minor items shall be paid upon completion of the items in the certificate of the engineer or architect.

(b) Interest.--The final payment due the contractor from the government agency after substantial completion of the contract shall bear interest at a rate of 6% per annum for all contracts without provisions for retainage and at a rate of 10% per annum for all contracts with provisions for retainage, the interest to begin after the date that such payment becomes due and payable to the contractor. However, where the government agency has issued bonds to finance the project, interest shall be payable to the contractor at the rate of interest of the bond issue or at the rate of 10% per annum, whichever is less, but in no event shall the interest payable to the contractor be at a rate of interest less than the legal rate of interest.