EE WAS MOPPING WET FLOOR IN CONTROL ROOM. FLOOR WAS WET BECAUSE ROOF WAS LEAKING. WHILE MOPPING FLOOR A 2' BY 4' CEILING TILE SOAKED WITH WATER FELL OUT OF THE CEILING STRIKING THE EE IN THE H EAD AND KNOCKED HIM TO THE FLOOR. THE INCIDENT RESULTED IN A BACK INJURY TO THE EE. HE WAS UNABLE TO RETURN TO WORK HIS NEXT SCHEDULED SHIFT.