INJURED WAS PINNING HANGING IN 3100 MPIC HEADING USING THE MECHANICAL PINNING JUMBO. HE GOT OFF THE JUMBO TO LOAD PINS IN THE CAROUSEL, WHEN A PIECE OF LOOSE APPROXIMATELY 18"" X 12"" X 1"" THIC K ROCK FELL FROM THE ROOF FROM IN BETWEEN PINS AND STRUCK HIM IN BACK AND RIB AREA.