EMPLOYEE WAS POSITIONING HIS ROOF BOLTER TO INSTALL BOLTS IN THE INTERSECTION OF THE #2 ENTRY. A SECTION OF RIB MATERIAL MEASURING APPROX 4' X 3' FELL OUT OF THE CORNER OF THE INTERSECTION AN D STRUCK HIM FROM BEHIND, CAUSING HIM TO STRIKE HIS FACE AND HEAD ON THE BOLTER AS HE FELL TO THE GROUND.