INJURED AND ANOTHER EMPLOYEE WERE INSTALLING MILL BALLS IN RAW #1. THIS AREA (HOOKING UP BARRELS) IS WITHIN 20 FT OF THE HIGH BAY HOIST BARRICADED AREA. CONTRACTOR WAS HOISTING 5 SHEETS OF PLY WOOD OF HIGH BAY HOIST TO THE 4TH FLOOR WHEN THELOAD SLIPPED AND BROKE LOOSE FROM RIGGING. PLYWOOD SHEETS FELL ALL THE WAY TO GROUND FLOOR AND SAILED OUT STRIKING BOTH EMPLOYEES.