## 1 Introduction  
In recent years, there has been a remarkable increase in green roof use, and knowledge of the benefits that they can provide is now far more widespread. As known, green roofs can play a central role in ecological integration projects and in the spread of urban biodiversity [1; 2; 3; 4; 5; 6]. Moreover, they help to regulate building temperatures, and their insulating properties help to save on energy costs. Energy savings and the positive effect on urban climate are not the only benefits provided, they have also absorbed rainwater, regulating the drainage process and its natural retention/recharge of water [7; 8; 9]. The amount of water that a green roof can absorb and recycle depends on a number of factors including climate, type of installation, and the composition of species present [9; 10; 11]. The plants, therefore, can protect the surface of buildings by absorbing a significant amount of solar radiation through photosynthesis, respiration and transpiration [12; 13; 14; 15; 16; 17; 18]. However, the plant structure and composition influence the performance of the green roof as heat insulation, protection by ultraviolet (UV) component, reduction of the heat island effect, and water retention [12; 18; 19; 20]. It is important consider that a combination of different species is more effective than monocultures in favouring the thermal and hydrological properties of a green roof [20; 21]. For this reason, many studies have attempted to understand how certain species influence the specific aims of a green roof, selecting and testing several species, different respect to _Sedum_ genus, and in certain environments [4; 22; 23; 24; 25; 26]. Several studies, instead, have focused the attention on the evaporation and photosynthesis activity of the species in order to define the thermal properties and energy performance of the green roof [27; 28; 29]. Moreover, a number of studies examined the relationship between different types of vegetation and rainwater retention in green roofs suggesting that the plants' structural characteristics are influential in the interception and storage of rainwater [21; 22]. These studies involved artificial rainfall and different types of vegetation, observing significant differences between amounts of rainwater runoff according to the type of vegetation. Species native to meadow habitats showed greater efficiency in reducing the flow of rainwater: _Silene uniflora_ [PERSON] and _Anthoxanthum odoratum_ [PERSON] displayed the highest levels of rainwater runoff reduction with respect to the _Sedum_ genus. Other studies investigated the relationship between roots and the water quantity in substrate [21; 30; 31; 32; 33; 34; 35] and not all of them agree. [PERSON] and [PERSON] [33] observed that some species can hold large quantities of water in relation to the density of their fibrous roots, reducing the soil's average porosity and its water retention capacity. [PERSON] et al. [21], on the other hand, in their study on the retention of water after rainfall, observed that plant height and root biomass are important factors in enabling a higher degree of rainwater retention. On the other hand, [PERSON] et al. [32] concluded that rainwater retention in winter was not affected by vegetation, and recommended planting a higher percentage of mosses and plants from humid climates, which remain active for most of the autumn, winter, and spring, even if it is very difficult to establish mosses that generally come naturally with time. Recently, some studies carried out in the Mediterranean area--characterized by sharp seasonal and daily temperature variations--have evaluated the benefits of green roofs [4; 19; 30; 35; 36; 37; 38; 39]. These studies considered the thermal and hydrological effects related to prolonged drought and irregular rainfall as well as looking at the capacity for survival of different species. Indeed, the selection of wild species typical of the Mediterranean area is a focal point of these studies.