Several authors agree that the physical and chemical properties of the substrate layer on green roofs undergo changes over time. These changes affect the hydraulic properties of the substrate soil, which in turn affect the hydrological behavior and retention capacity of green roofs. The existing literature provides conflicting results, with some case studies showing an increase in retention capacity and others showing a reduction. The goal of this study is to assess the variations in retention capacity for two experimental green roofs (GR1 and GR2) located on the University of Salerno campus in Southern Italy. The study compares the average retention capacities estimated during two different periods: 2017–2019 and 2022–2023. The measurements taken from 2017–2019 reflect the early stage condition of the green roofs after installation in 2017. The preliminary results suggest a general decay in the retention capacity of both the GRs, approximately 12%. However, identifying the factors influencing the retention capabilities change over time is challenging, as it is often difficult to distinguish the effects of substrate changes from those caused by climatic variability.