An unintentional roof fall was found on 3rd shift while examiner was making his returns. Fall measured 60'x19'x8' high. Fall affected ventilation. It knocked out part of a return air stopping. Fall did not affect passage of men or cause injury. Fall won't be cleaned up. The stopping was rerouted & ventilation reestablished. Location of fall was 31+50 entry #4 in the 4C return.