Green Roof Aging Effect on Physical Properties and Hydrologic Performance

Many studies focus on the hydrologic performance of green roofs, but the aging effect remains largely unknown due to the scarcity of long-term monitored data. This study characterized virgin and 7-year-old growth media and assessed the impact of observed changes on hydrologic performance. Comparison of new growth medium in 2011 with identical testing in 2018 showed that the organic content had shifted from 2.7% to 4.3%, while the maximum water holding capacity had increased from 39% to 46%. The aged sample showed a shift to smaller particle diameters: the fraction of particles with diameter smaller than 0.05 mm increased from 5.9% to 8.4%. The performance monitoring period included days of rain with no snow on the roof from April 2015 through June in 2018. Meteorological conditions were similar in 2015 and 2017, yet slightly better retention performance was detected in 2017. Furthermore, the HYDRUS-1D model was applied to simulate rainfall-runoff for four 24-h design storms. The monitored data and simulated results both showed improvements in retention and detention performance in the aged green roof. Because the aging effect of green roofs is relatively poorly understood, these findings can inform building managers and engineers about possible changes in green roof performance over time so they can plan accordingly.