During the occupation of Norway by Nazi Germany, Røtjer participated in the resistance as a member of Milorg's Group 131134.
After the war he studied at the Norwegian Institute of Technology, graduating in machine engineering in 1949.
[1] In 1963 he became technical director in the company acquired by Kværner, Moss Værft & Dokk, being promoted to chief executive in 1968.
[1][2] Røtjer was a fellow of the Norwegian Academy of Technological Sciences,[1] and was a member of the gentleman's club Det Norske Selskab.
[1] His son Tom Røtjer became a part of the corporate management of Norsk Hydro.