PV installations on flat roofs offer a wide range of design options, which are usually neglected in urban-scale assessments as these typically assume horizontal or other fixed arrangements. In this study, we analyse the influence of common design parameters (tilt and inter-row distance) in evaluating the potential of PV arrays installed on flat roofs, using three different performance indicators. By comparing optimised arrangements to horizontal ones, we show that the latter could be misleading, unless building- and indicator-specific correction coefficients are applied.