While descending on a fixed ladder from conveyor 54 down to the rooftop of Air Separator 1 (AS1), EE was unaware of the last 2 rungs that were missing (26"" gap), as he continued down, he had to catch himself from falling onto the rooftop of AS1 and injured his right shoulder. Employee continued to work and thought the pain would go away. On 4/3/13 EE sought medical treatment.