Effect of substrate depth on plant and thermal performances of a light-weight green roof for rural buildings

Green roofs offer energetic and environmental benefits and their implementation in rural and urban buildings has been widely studied. However, solutions for rural buildings with roofs of limited bearing capacity and slope up to 30 % have not been fully investigated. A low-weight vegetated roofing covering for agricultural buildings has been studied to assess the effect of different substrate depth (3, 5, and 7 cm) on vegetation and thermal performances. The pilot structure consisted of a frame structure fixed on concrete blocks, bearing 20 modules in two rows facing north and south with a slope of 25 %. A mixture of white clover (Trifolium repens L.) and bermudagrass (Cynodon dactylon L.) has been used as green species. The vegetation has been monitored during the season and the thermal performances were determined by measuring the temperatures on the top and bottom sides of the modules. The thicker substrate significantly affected both fresh and dry biomass. Similarly, plants grown with a thicker substrate showed a high number of flowers and southern exposure appeared to promote flowering. The temperature differences, compared to a standard insulated sandwich panel, were up to 10 °C, making them effective in mitigating heat stress during high-temperature periods. The best results for the vegetation growth have been obtained with a substrate depth of 7 cm that entails a load of around 70 daN m−2, but, from the thermal point of view, the performances were similar also with 3 cm substrate depth. In this case, the load of the cover (around 30 daN m−2) was only slightly higher than the insulated sandwich panels generally used (around 12 daN m−2) and seems still compatible with the structure used for rural buildings. The results on the vegetation growth and thermal performances of the simplified green roof confirmed its suitability for application on rural buildings, as cattle barns.