In recent years, the research on roughness has focused on various roughness features, rather than the roughness height only, in order to improve the understanding of roughness effects on wall bounded flows. A special focus is placed on the skewness of the roughness height profile. The skew-ness measures whether the height profile is dominated by negative or positive roughness elements. Surfaces with both features can be found on worn blades: On the leading edge, roughness is caused by the impact of particles resulting in a negative skewness. Rough surfaces around the trailing edge, however, develop due to depositions leading to a positive skewness. In this paper, rough surfaces taken from a compressor blade of an aero engine are systematically varied to investigate the isolated effect of skew-ness on aerodynamic losses. By direct numerical simulations of a periodic flow channel. The results show that the skewness has a major influence on loss generation. Based on these results, an existing model which essentially uses the shape-and-density parameter, is extended by a skewness factor. The modified correlation predicts the influence of the rough surfaces investigated well.