Terahertz (THz) vacuum travelling wave tubes require slow-wave structures (SWSs) realisable by microfabrication processes, which assure an effective interaction with the electron beam. A corrugated rectangular waveguide with the corrugation narrower than the width of the waveguide is proposed as a slow-wave structure for THz travelling wave tubes (TWTs). A complete cold parameter analysis by a 3D electromagnetic simulator is presented, and the validity of the structure is demonstrated.