EE WAS WORKING 4'-5' OFF THE BOTTOM OF THE TRUCKBIN WELDING AR PLATE ON THE SIDE WHILE STANDING ON A SCAFFOLD BOARD. THE BOTTOM OF THE BIN WAS CLOSED. THE SCAFFOLD BOARD SLIPPED BACKWARDS WHIL E HE WAS SHIFTING HIS WEIGHT TO WELD A PLATE IN THE CORNER. HE FELL THE 4' CATCHING HIS SHOULDERON THE ANGLE HOLDING THE BOARD AFTER LOSING HIS BALANCE.