A proper evaluation of shading is of great importance for predicting solar gains in buildings. A correct design of shading systems can lead to sensitive energy savings especially during the summer months. In this paper, in order to investigate how shading influences the building energy need, a sensitivity analysis of the monthly shading factor was carried out. The calculation of the shading factor was performed with a model capable of accounting for complex boundary conditions, such as a horizon profile, generic-shaped obstructions and vegetation. Through the ANOVA-FAST technique, the building design variables which have the highest impact on the variance of the shading factor were evaluated for a set of urban external contexts and geographic locations. The design variables that were considered for the sensitivity analysis are related to the geometrical features of shadow-casting objects (overhangs, fins, parts of the building itself, etc.). The geometry of surrounding buildings was taken into account for the characterisation of the urban context. Results for all the orientations showed that the design variables which should be carefully considered for the control of solar gains through shading in buildings are mainly the distance and depth of an overhang. The reveal depth plays an important role too, especially in a dense urban context