Temporal Changes of Green Roofs Retention Capacity.

Green roofs experience an evolution over the years of physical and chemical properties of the substrate layer that may lead to substantial changes in their hydrological behavior. This study, benefiting from a 5-years monitoring period, aims at assessing changes in the retention capacity of two experimental green roofs (GR1 and GR2), located in Southern Italy and different in drainage layer, by comparing pairs of similar rainfall-runoff events which occurred respectively in 2018 and early 2019, one year after installation, and in 2022. To this end, once identified the retention capacity of each, differences among similar events occurred four years apart were assessed and compared for the two green roof configurations to detect: possible changes in their hydrological performance, the configuration most affected by aging and the evolution of their differences over time. The results obtained so far suggest a general decay of the green roofs retention capacity with some differences according to the drainage characteristics. GR1, with a drainage layer made by 5-cm depth expanded clay, reported a 79% reduction of retention capacity while GR2, with a drainage layer made by MODI' plastic panel filled with expanded clay, experienced a reduction of about 53%. Differences between the two green roof configurations also increase due to aging effects because the substrate, initially crucial in the retention dynamics, seems affected by a progressive performance loss. Conversely, the drainage layer, that appears to play a secondary role in the early operational period, becomes determinant in the medium observation period.