BUILDING A TIN STRUCTURE, THE TOP ""Z"" CHANNEL SLIPPED OUT OF POSITION WHEN THE MIDDLE CHANNEL WAS CUT LOOSE. SO TO PUT TOP CHANNEL IN POSITION, WHEN TOP CHANNEL FELL AND HIT EMPLOYEE ON THE HEAD. CHANNEL FELL 12 INCHES. HE DIDN'T HAVE HIS HARD HAT ON. ANOTHER MAN WAS HOLDING THE OTHER END OF THE ""Z"" CHANNEL UP. CHANNEL WAS 18' LONG AND WEIGHED ABOUT 80 POUNDS.