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Abstract

Several authors agree that the physical and chemical properties of the green roof substrate layer are subject to an evolution over time. It inevitably causes changes in substrate soil hydraulic properties which affects green roofs hydrological behaviour and in turn the retention capacity. The specific literature review reports on contrasting results, that is some case studies point on an increase in the retention whether some others present a reduction. This study aims at an assessment of the variations in the retention capacity of two experimental green roofs (GR1 and GR2), located in Southern Italy, University of Salerno campus, by comparing the average retention capacities estimated in two target years, 2018 and 2022. The 2018 measurements set up an early-stage condition as the green roofs were installed in 2017. The results obtained so far suggest a general decay of the retention capacity with no significant differences according to the drainage characteristics. GR1, with a drainage layer made by expanded clay, reported on average a 12% reduction of retention capacity while GR2, with a drainage layer made by MODI’ plastic panel filled with expanded clay, experienced a reduction of about 13%. Understanding possible changes over time of the retention capabilities is challenging, as it is also difficult to distinguish substrate changes effect from those of climatic variability. Indeed, an increase in rainfall intensity and cumulate rainfall was also detected during the mentioned period. A comparison at the event scale furthermore highlighted potential influence of soil moisture conditions prior to the rainfall events.