ROOF FALL WAS DISCOVERED WHILE EXAMINING AIR COURSES. FELL AT A 3 WAY INTERSECTION. SOME SLIPS PRESENT IN AREA, ROOF WET, SANDSTONE CLOSE TO SHALE. FELL 30' LONG, 18' WIDE, 6-8' THICK. THIS AR EA HAD BEEN BOLTED WITH 6' ROOF BOLTS & STRAPS. NO INJURIES. NO DAMAGE TO EQUIPMENT, DID NOT BLOCK AIR.