Aims. Cometary surfaces can change significantly and rapidly as a result of the sublimation of their volatile material. Many authors have investigated this evolution; topographic data from all comets visited by spacecrafts have been used previously to derive a quantitative model that relates large-scale roughness (i.e. topography) with the evolution state of the nucleus for Jupiter-family comets (JFCs). Ground-based observers have published measurements of the phase functions of many JFCs and reported a trend in the phase darkening, with primitive objects showing a stronger darkening than evolved objects. Methods. We used a numerical implementation of this previous topographic description to build virtual comets and measure the phase darkening induced by the different levels of macro-roughness. We then compared our model with other published values. Results. We find that pure geometric effects such as self-shadowing can represent up to 22% of the darkening that is observed for more primitive objects, and 15% for evolved surfaces. This shows that although physical and chemical properties remain the main contributor to the phase darkening, the additional effect of the topography cannot be neglected.