Abstract The resistance of a slip resistant steel joint depends essentially on the preload applied in the bolts and the slip factor developed between the joined plates. The slip factor depends on the surface roughness, a parameter associated to the superficial treatment. Although some indications are already provided in the codes for the design of joints, such as the Eurocode 3, some doubts remain among researchers, designers and steelwork companies. Some surface treatments are not yet well characterized and in general the values for slip factor provided in the codes were calibrated for current mild steel only. The evaluation of the influence of the surface treatment on current mild steel, on weather resistant steel and in particular on S 690 high strength steel is presented in this paper. The slip factor was obtained through experimental tests according to EN 1090 — Part 2, carried out in the University of Coimbra — Portugal. The results obtained and discussed in the scope of this paper were also compared with values reported in the literature.