The shear strength of rocks along discontinuities is one of the key parameters for the determination of rock slope stability, the stability of rocks during underground space development and tunneling. Its value is influenced by numerous factors including surface roughness, which is one of the most widely investigated discontinuity property. The present paper introduces a simple graphical methodology for the classification of the surface roughness of rocks, based on the example of two different rock types, Bátaapáti Granites (Hungary), and Mont Terri Opalinus Claystones (Switzerland). The 3D surface of 24 rock samples was digitized using a photogrammetric surface detection method with the help of the ShapeMetrix3D software. The plane of each rock surface was defined by fitting a linear regression plane to the surface data. The distance between the data points of the surface roughness model and the regression plane was measured, and cumulative frequency diagrams of the measured distance values were constructed. This procedure allowed to define three surface roughness categories. The methodology proposed represents a promising new approach to surface roughness quantification, which could improve shear strength estimation.