EE WAS PERFORMING HOUSEKEEPING ACTIVITIES ON THE ROOF ABOE #4 SCRUBBER ON A SCHEDULED DOWN DAY. EE LIFTED A LARGE METAL PLATE (APPROX 135 LBS.) EXPOSING A 4' DIA. OPENEING TO THE PUMP ROOM BEL OW. AS HE LIFTED THE PLATE AND MOVE FORWARD, HE STEPPED INTO THE OPENING. EE FELL TO THE FLOOR BELOW.