Usefulness and Effectiveness of Retention Layers in Green Roofs

Green roofs are an emerging technology for urban climate mitigation, rain-water management and energy saving. Even though they are long time used, there are few data on the relation between its construction and performance for lightweight extensive systems in Mediterranean climate. This is particularly relevant for existing buildings as the opportunity of converting conventional roofs in green ones is limited by the extent of the new applied loads. The most part of the mass of these systems is due to cultivation substrate and retention layer, thus proper design would let a wider adoption of this technology. Moreover, data available in the open literature are mainly referred to northern temperate zone where drought periods are of little concern. In this framework, experimental light load (80 mm substrate) green roofs have been built up to check their performances in Mediterranean climate (Rieti -inner central Italy) with different retention layers. Drainage performance have been measured in natural weather as well as qualitative analysis of vegetation endurance to draught periods with a specific focus on the relevance of retention in drainage layer. Results show that, contrary to state-of-the-art statements, water retention actually occurs more in the substrate rather than in the retention layer that thus appears to be far less critical in green roof systems set up.