Unplanned fall discovered reportable at 11:50 am on 8/22/17 located in the first intersection outby the back up ventilation control for the #4 seal of the 3 North Bleeders Seal Set. The fall was approximately 7' high, 8' wide and 15' long. Area developed in 1965. Fall occurred due to gradual roof deterioration to the point of failure just above the 5' anchorage zone.