The effect of surface roughness on the transition from regular (RR) to Mach reflection (MR) over straight wedges in pseudo steady flows was investigated both experimentally and analytically. A model for predicting the RR ⇄ MR transition in the (Miθw)-plane was developed (Miis the incident shock wave Mach number and θwis the reflecting wedge angle). Its validity was checked against experimental results. Since the experimental results are limited to the ranges 1 < Mi < 2 and surface roughness heights of 0 < e ≤ 0.2 cm, the proposed model is applicable to these ranges only. In the proposed model (the boundary layer displacement thickness model), the RR ⇄ MR transition is related to the boundary layer thickness which in turn depends on the surface roughness.