Miner oper. had taken a cut of coal out of face of No.3 hdg. that was approx 15' in depth. After the coal had been removed, a section of mine roof in unsupported area of newly mined cut fell. Size of section of roof that fell was approx 14' wide, 12' long, 0-2.5' thick. The material was sandstone. The ignition occurred when miner oper. started to cut the material. Put out 3-5 mins.