AN UNINTENTIONAL ROOF FALL OCCURRED DURING RETREAT MINING. AS CONT. MINER WAS BACKING FROM CUT THE ROOF MEASURING APPROX. 15' W X 20' L X 3 1/2'H, FELL ONTO MINER. A RETRIEVER WAS USED TO REMO VE MACHINE FROM FALL AREA, NO INJURIES OR PROPERTY DAMAGE OCCURRED. AREA WAS TIMBERED OFF AND MINING RESUMED ON NEXT PILLAR.