Abstract: At 2:31 p.m. on December 10, 2017, Employee #1 was assigned to clean the HVAC duct work by using a temporary work platform in the interstitial space with a permanent cat walk with railings. Not all of the ducts could be reached from the cat walk, so the employee was instructed to build temporary platforms in the interstitial space, as a method to safely access the duct systems from the cat walk. The temporary platforms were constructed by laying two 3.5 inch by 3.5 inch timbers across the steel structural members of the building, spaced approximately 24 inches apart, and then placing a 32 inch by 48 inch section of half inch plywood over the timbers. There was an insufficient quantity of timbers available to build a temporary platform that spanned the entire distance from the catwalk to a section of duct that Employee #1 was directed to clean. While the employee clambered across the incomplete platform he was trying to access, he lost his footing and fell through the clean room ceiling, 12 feet onto the first floor. During the fall, he struck a Metro metal shelf bolt, rising up approximately 5 feet, 7 inches from the working surface of the clean room, before coming to rest on the working surface. Employee #1 suffered a deep laceration to his back and was admitted and treated for three days at a nearby hospital. The employer did not provide fall protection, nor require its use.