Abstract: Two two-man crews were installing wallboard to enclose vertical ducts in a new multi-story retail building. The opening in the floor that the ducts went through was approximately 20 ft by 10 ft. The distance between the floor and vertical ducts was approximately 40 in. The crews took down the perimeter cable protection that was up and took no precautions to protect themselves from falling through the space between the floor and vertical ducts. Lower levels were approximately 50 ft below. Each crew had one employee installing the wallboard and one employee cutting and supplying the wallboard to the installer. There were three planks set on angle irons and the floor at one corner of the opening. The planks were not secured in place. An employee of one the crews slid the planks from the floor to facilitate installation of wallboard. The other crew was not notified. Employee #1, the employee cutting and supplying material from that crew stepped onto the planks in his travels and he and the planks fell. Employee #1 fell approximately 50 ft and crashed through a ceiling before coming to rest on a concrete floor. Employee #1 sustained serious fracture injuries from the fall and was hospitalized.