EMPLOYEE WAS SHOTCRETING OFF BACK SCISSOR TRUCK. HE HAD JUST HANDED HOSE TO ANOTHER PERSON. WHILE HIS ARM WAS EXTENDED, A ROCK 6 1/2"" IN DIAMETER WEIGHING 2.5 LBS FELL FROM THE ROOF STRIKING THE BACKSIDE OF HIS RIGHT HAND CUTTING IT. EMPLOYEE WAS TAKEN TO THE EMERGENCY ROOM, THE ROCK CUT SEVERAL TENDONS AND BROKE 2 SMALL BONES IN HIS WRIST.