Nigel Chester, aged 46, from Nottingham, suffered head injuries when he fell 13ft to the floor at Premier Foods, Claylands Avenue on 1st May, last year.
The inquest in Nottingham heard Mr Chester, who worked for ADT Fire and Security, had a master key and walked through a door which had a sign warning of a four metre drop the other side.
He hit his head on a metal machine used to make gravy granules and OXO cubes.
The inquest heard Premier Foods has made the door safer.
A jury returned an accidental death verdict.