Assessing Urban Green Roofs for Microclimate and Runoff Reduction in Cities

Thesis Abstract

Urban heat islands and stormwater challenges intensify in dense cities, stressing energy systems, air quality, and flood risk; green roofs offer a nature-based solution that can simultaneously modulate microclimate and reduce runoff, yet empirical evidence across different urban contexts remains fragmented. This study aims to quantify the microclimate effects and runoff reduction potential of green roofs in urban environments and to identify the drivers of variability across building typologies, substrate depths, and climatic zones. The specific objectives are (1) to quantify surface temperature, air temperature, and relative humidity differences between green roofs and adjacent conventional roofs across five city neighborhoods; (2) to estimate stormwater retention and peak discharge reductions attributable to green roofs under varying rainfall intensities; (3) to model the influence of substrate depth, vegetation type, and roof slope on thermal performance and hydrological response; (4) to assess residents’ perceived comfort and thermal satisfaction linked to green roofs through a mixed-methods approach; and (5) to develop a transferable framework linking green roof characteristics to microclimate regulation and drainage outcomes for urban planning. The research adopts a mixed-methods design combining empirical field measurements with simulative modeling. The population comprises commercial and residential buildings equipped with extensive and intensive green roofs in the metropolitan area of City X. A stratified random sample of 40 buildings (20 with extensive green roofs and 20 with intensive green roofs) was selected, with 8 sites per district, ensuring representation of different roof configurations, substrate depths (80–250 mm), and vegetation compositions. Data collection combines (i) in-situ meteorological sensors deployed for 12 months, capturing surface and ambient air temperatures, relative humidity, solar radiation, and wind speed; (ii) rainfall simulators and rooftop runoff collectors to measure runoff volume, peak discharge, and retention capacity during simulated and natural storms; (iii) substrate and vegetation surveys to document depth, soil moisture, plant species richness, and evapotranspiration indicators; and (iv) a resident survey (n=400) assessing thermal comfort, perceived changes in microclimate, and willingness to adopt green roofs. Instrument validity and reliability are ensured through calibration against city meteorological station data, pilot testing, and Cronbach’s alpha assessment for survey scales. Data analysis integrates quantitative and qualitative approaches. Descriptive statistics summarize microclimate and hydrological metrics; inferential analyses use multilevel mixed-effects regression to evaluate the effects of roof type, substrate depth, and vegetation on microclimate indicators and runoff performance, controlling for district climate and roof orientation. Time-series analyses and ANOVA test differences across roof configurations and rainfall events. Structural equation modeling examines causal pathways linking green roof characteristics to measured microclimate benefits and drainage outcomes, while scenario analysis simulates storm intensities up to 100-year return periods. The qualitative component employs thematic analysis of resident interviews (n=40) and survey open-ends to contextualize quantitative findings within human comfort and perceived sustainability benefits. The study also applies sustainable urban drainage theory and the urban ecology framework, drawing on the concepts of thermal mass, evapotranspiration, and hydrological retention to interpret results. Expected findings include statistically significant reductions in roof albedo-related heat gain and near-surface air temperatures for green roofs, with larger effects on extensive roofs with deeper substrates; substantial stormwater retention and delayed runoff peaks, particularly under moderate rainfall and in roofs with substrate depths exceeding 150 mm; positive correlations between plant diversity and evapotranspiration contributing to microclimate cooling; and varied resident perceptions aligning with measured cooling and moisture benefits. The research anticipates identifying thresholds for substrate depth and vegetation complexity beyond which marginal gains decline, informing cost-benefit analyses. The study contributes to knowledge by providing robust, field-based estimates of microclimate and hydrological benefits of green roofs across diverse urban contexts and by offering a transferable framework and decision-support tool for urban planners and policymakers. It advances empirical evidence on how roof engineering (substrate depth, vegetation) interacts with city climate to influence thermal comfort and stormwater management, bridging gaps between theory and practice. Recommendations include guidelines for optimal green roof design tailored to climate, building type, and urban drainage needs, integration of green roofs into municipal adaptation plans, and considerations for maintenance and biodiversity enhancement to sustain performance over time. The conclusion highlights the strategic role of green roofs in urban resilience and outlines avenues for future research, including long-term performance monitoring and the examination of economic incentives to accelerate adoption.