A CONTRACTOR EE WAS PULLING A PLASTIC SHEET WHILE WALKING BACKWARDS ON A CATWALK. THE EE FELL THROUGH AN OPENING(2' X 3') IN THE FLOOR GRATING AND FELL APPROX SEVEN FEET TO GROUND LEVEL. AS TH E EE FELL HE STRUCK THE BACK OF HIS HEAD ON THE FLOOR GRATING CAUSING A LACERATION WHICH REQUIRED STITCHES.