24 CFR § 241.580 - Application of payments.
(a) The security instrument shall provide that all monthly payments to be made by the borrower shall be added together and this aggregate amount shall be paid by the borrower upon each monthly payment date in a single payment. The lender shall apply the payment to the following items in the order set forth:
(1) Premium charges under the contract of insurance;
(2) Interest on the loan;
(3) Amortization of the principal of the loan.
(b) Any deficiency in the amount of any monthly payments required under paragraph (a) of this section shall constitute an event of default and the loan shall further provide for a grace period of 30 days within which time the default must be cured.