Shadow area distribution of vertical elements mounted on flat roofs was studied regarding its impact on opportunities for harnessing solar energy and its positive effect on obstructing undesirable heat transfer. Various cases were assumed for the position of a penthouse on flat roofs of dwelling units. In each case the spatial distribution of the shadow was studied on the host roof and its neighbors. The general constant ratio between the average annual shadow and lateral area of a rectangular shape was found to be 0.88. The north direction and the back penthouse position receive the highest quantity of annual shadow on the host's roof (2.85 times its roof area). The left corner roofs offer better opportunities compared to the middle and right corner roofs for collecting solar energy, on average 1.5 and 1.3 times, respectively. Left corner roofs with front penthouse position receive the highest amount of insolation energy per unit area, which makes it the favorable position for harnessing solar energy. The highest positive obstructed insolation is in 0 degrees orientations and 180 degrees penthouse position by 413 kWh/m(2)/year. Generally, the back penthouse position performs better than the center and front positions in terms of obstructing negative heat transfer. (C) 2019 Elsevier Ltd. All rights reserved.