Component design optimization of green roof substrate layer based on the assessment of multifunctional performance

A high-quality substrate layer is the cornerstone of supporting that green roofs (GRs) can become an efficient and sustainable nature-based solution to urban environmental problems. In the present study, three lightweight substrate materials commonly used in GRs of peat soil, vermiculite and pumice with four appropriate proportions of the nutrient substrate and the mineral substrates were selected to install twelve substrate modules. The lightweight property, water-holding, nutrient retention and rainwater reduction performance of the substrate modules were investigated by the laboratory determination methods and the simulated rainfall experiment. An assessment model based on the multifunctional performance established by analytic hierarchy process (AHP) was used for the component design optimization of GR substrate layer. The results showed that the substrate modules based on peat soil and vermiculite (PV) as the mineral substrate, which the dry volumetric weights and the average water content were 1.40-1.70 kN m^-3 and 47.80%-49.06%, always exhibited better lightweight properties and water-holding performance compared to those composed of pumice. PV-40 had the highest value of the multifunction index even while none of its functional performance was optimal among all the substrate modules. The present study emphasizes the necessity of optimizing the GR substrate layer component based on the assessment of multifunctional performance to better promote the sustainable development of GRs in urban areas.