Abstract: On November 6, 1985, Employee #1 was cleaning up a partially decked out roof area. The jobsite was new construction for a 160,000 square-foot office warehouse building. The construction consisted of a concrete slab floor, concrete tilt-panel exterior walls, steel columns to support glue lambeams, and a flat "berkley" roof system, 26 feet above the slab floor. Various roof openings were covered temporarily with plywood panels and marked by a "t-p" system of 32-ft by 4-in boards erected over the cover to identify the opening. Employee #1 was assigned to clean up the partially decked out roof area. Upon arriving at one covered opening that was covered but not marked by the "t-p" system, the employee attempted to move the plywood panel for clean up. As Employee #1 lifted the plywood, he fell through the opening to the floor below and received a concussion. Although other employees were working nearby, the employee was working alone and no employees witnessed the accident. Another employee found the employee directly below the opening in an unconscious condition. Employee #1 was transported to a hospital.