Employees were performing steel erection activities and placing clips, insulation and roof sheeting onto a pre-engineered metal building. The employees were walking and working from a partial bundle of roof sheeting (approximately 3 sheets). During the operation, an employee stepped on an inadequately supported area of the roof sheeting partial bundle; the sheets bent down and the employee fell 21 feet to the ground below. The employee was hospitalized with a broken arm and tailbone.