He was on the ninth floor. If he were on the 10th, 11th, or 12th floor, the sentence would be: "He was in a building above the ninth floor."

Also note that in the United States floors are counted from the ground. Typically a person enters on the ground floor and only climbs the stars or takes an elevator to the second floor or higher. Many buildings do not have a 13th floor but number 9, 10, 11, 12, 14, 15. In other parts of the world floor numbers start with something that means "Ground Floor" and the first floor is not on the ground but above by at least one flight of stairs.