An employee was installing steel decking to the roof a warehouse while standing on a roof truss. The employee lost his footing and fell. He was wearing personal fall protection which included a full body harness, a D-ring extension, a retractable lanyard, and a beam anchor clamp. After falling approximately 21 feet, the retractable device caught and stopped his fall but then the D-ring extension s threading failed and it broke. The employee then free-fell approximately 27 feet to the concrete floor. The employee was hospitalized with injuries including: the right knee shattered into twenty pieces, the right leg fractured in three places, fractures to the front of the skull/forehead, a broken nose, five teeth knocked out, and numerous facial lacerations.