With the growth of cities and the consequent reduction in infiltration area, fighting against the intensification of rainfall has become increasingly challenging. Knowing this, the water retention capacity of green roofs and the possibility to install them in urban areas make them excellent instruments for both managing rainwater and avoiding urban shortcomings such as flooding. To understand how this characteristic is affected by the composition of the green roof represents a relevant topic, which this study seeks to assess through the investigation of the relationship between the composition of the growth layer and the water retention capacity of green roofs. The present paper is a preliminary approach framed in a PhD interdisciplinary project focused on that objective. This will be done by experimentally testing 4 soil compositions for extensive, semi-intensive and intensive tests in full-scale models under a Mediterranean climate and numerically. The tests will allow to classify the influence of soil granulometry on the effectiveness of green roofs in stormwater management. The classification will be made through variables such as the reduction in peak flow and delay in surface runoff, thus measuring the consequent reduction in absorption required by the urban drainage system. This way, the study aims to investigate the relationship between soil composition and the effectiveness of green roofs in managing rainwater, resulting in a relevant task for their effective design.