The individual and coupled effects of incoming free-stream turbulence (FST) and surface roughness on the separation of a boundary layer over a flat plate, with an imposed pressure distribution representative of low pressure turbine (LPT) loading, is numerically investigated using Large eddy simulation (LES) approach. The formation of streamwise streaks in the presence of FST and roughness and their role in promoting earlier transition are captured. The streamwise evolution of the boundary layer parameters has been compared for different cases and the synergistic effect of FST and roughness in suppressing the separation bubble has been reported. The potential „roughness benefit? obtained in the case of highly loaded turbine blades in terms of its significant reduction of profile loss is also shown.