We investigate the effect of roof boundary fence shape on the flow over a cubic building and on the power and thrust performance of a roof-mounted wind turbine via wind-tunnel experiments. The effect of fence height, porosity, angle, and curvature is investigated, with the turbine placed at several streamwise positions along the mid-span of the building. For the flow without the turbine, the flow separation and shear layer growth from the roof leading edge are significantly affected by the fence shape. Vertical fences resulted in a decrease in the mean power and thrust with the increase in the fence height and streamwise distance from the roof leading edge. Increasing the fence porosity improved the turbine performance for streamwise distances greater than one rotor diameter. Inward-angled fences improved the turbine performance, with the 60∘ inward angle leading to a power performance similar to a case with no fence. Rounding the fence outward also led to a smoother flow, which resulted in higher mean power and thrust compared to inward-curved fences. The power and thrust coefficients were computed using rotor equivalent and hub height velocity magnitude as reference velocity, where the former one yielded physically feasible results for all the cases.