To understand the favourable locations for maximum yield of urban wind energy, understanding the flow pattern on the roof region of high-rise buildings is crucial. However, very few studies in the literature report on the wind energy resource using high-fidelity methods, which combined with the lack of wind tunnel investigations, explains the failure of urban wind energy as a viable contributor to the renewable energy mix. This work aims at proposing a strategy for the reliable evaluation of the wind energy resource by building on validated Large Eddy Simulation and performance maps based on the wind power density and the turbulence intensity above the roof of a prismatic 1:3 square building. Analyses include the effect of flat and decked roof shapes in two wind directions. Results around the facade reveal zones of accelerated flow close to separation edges, while above the roof, a higher potential is noticeable at 45°, than at 0°. Decking of the roof shows insensitivity to the change in wind direction. The tested methodology can be extended to other configurations to predict the wind energy potential as well as used to indicate the positioning of wind turbines in the built environment.