Effect of substrate depth and roof layers on green roof temperature and water requirements in a semi-arid climate

Green roofs have been built and studied mainly in humid climates and provide many benefits, from thermal insulation to biodiversity. However, little is known about their performance in arid and semiarid regions where irrigation requirements can affect their sustainability. In order to examine and evaluate the performance of green roofs in a semiarid climate, 114-m2 green roof modules were built and monitored for a year. The experimental setup consisted on nine different roof designs or modules, one of them with two additional replicates (11 modules in total). The study was performed in Santiago, Chile (33°26′S, 70°39′W, 570 MASL), a region with a typical semiarid climate. Three substrate depths (5-cm, 10-cm and 20-cm) and four commercial drainage systems were evaluated. The modules were implemented with a sprinkler irrigation system and instrumented to record air temperature, precipitation, and substrate water content and temperature at 5-min intervals. The results showed that substrate depth controls the amplitude in substrate temperature. The 10-cm and 20-cm depth modules showed a significant damping effect on substrate temperature. In addition, the 10-cm and 20-cm depth modules showed maximum daily temperatures up to 13 °C below the air temperature. In contrast, the 5-cm depth modules increased the amplitude in substrate temperature, reaching in average 13.8 °C more than the daily maximum air temperature in spring and 2.6 °C less than the daily minimum air temperature in summer. The 10-cm and 20-cm depth modules provided a stable and suitable substrate water content during the entire study period, while the 5-cm depth green roofs were more affected by the atmospheric demand. Although their limitations were partially overcome by increasing irrigation rates or adding a retention fabric to improve the substrate water holding capacity, the 5-cm depth green roofs experienced daily thermal amplitudes beyond the recommended value for plant development. Consequently, extensive (or thin) green roofs (less than 10 cm of growing medium) are unlikely to be recommended in arid or semiarid climates.