Assessing the impact of urban green roof systems on urban heat island mitigation and stormwater management in a subtropical city.

Project Abstract

Urban green roof systems have emerged as a multifaceted strategy to combat urban environmental challenges by moderating building temperatures, reducing heat flux to adjacent spaces, and enhancing stormwater capture in dense subtropical urban environments. This study assesses the performance of extensive and semi-intensive green roof installations across five representative buildings in a subtropical city with high rainfall variability and hot-humid summers. Using a mixed-methods approach, we quantify changes in roof surface temperatures, albedo, and radiant heat flux through in situ infrared thermography and surface sensors over a 24-month monitoring period, complemented by computational simulations to upscale findings under varying solar radiation and meteorological conditions. A paired, controlled design compares green roofs with conventional impervious roofs to isolate cooling effects attributable to vegetation, substrate depth, and green roof engineering components such as drainage layers and substrate chemistry. Simultaneously, stormwater management performance is evaluated by measuring runoff volumes, peak discharge, soil moisture dynamics, and evapotranspiration rates during both dry and wet seasons, enabling calculation of reduction potentials for combined sewer overflows and local flooding risk. The study further investigates microclimate interactions by analyzing changes in adjacent pedestrian-scale thermal comfort indices, wind profiles, and humidity gradients downwind of green roof sites. Life cycle assessment and cost-benefit analysis are incorporated to evaluate long-term sustainability, maintenance demands, and payback periods relative to conventional roofs, considering energy savings from reduced cooling loads, degraded heat island intensity, and ancillary benefits such as urban biodiversity support and improved air quality through particulate matter deposition and biogenic volatile organic compound interactions. Data integration employs a hierarchical Bayesian framework to address spatial heterogeneity across buildings and to assess uncertainty in parameter estimates, with scenario analyses that project performance under future climate projections and policy-driven retrofit incentives. Results anticipate meaningful reductions in roof surface temperatures during peak heat periods, corresponding declines in ambient air temperatures within the building envelope, and measurable reductions in runoff volume and peak discharge due to increased infiltration and detention provided by the substrate and drainage configurations. The research aims to identify optimal green roof configurations—balcony-accessible extensive systems versus deeper, semi-intensive setups—for maximizing thermal benefits while sustaining stormwater management efficiency in subtropical climates characterized by heavy rainfall events and synergistic urban heat island effects. Findings are expected to inform urban planning guidelines, retrofit standards for commercial and residential buildings, and climate adaptation strategies that integrate green infrastructure with water-sensitive design practices. The study also highlights potential trade-offs, such as maintenance complexity, nutrient leaching concerns, and the need for ongoing monitoring to preserve performance in the face of episodic extreme weather. Overall, the work contributes to a nuanced understanding of how urban green roofs can simultaneously alleviate heat stress and manage stormwater, advancing evidence-based decisions for resilient, livable subtropical cities.

Project Overview

What This Project Is About

---

A plain-language overview of how green roofs in cities can affect two big environmental issues: the heat that makes cities feel hot (urban heat island) and how rainwater is handled on rooftops. The project looks at a subtropical city and asks whether adding plants and soil on roofs can cool buildings and reduce stormwater runoff, which can cause flooding and pollution.

---

---

The Problem It Addresses

---

Cities very often get hotter than surrounding countryside because buildings, pavement, and lack of vegetation trap heat. This increases energy use, air pollution, and health risks. Rainwater can overwhelm drainage systems, causing floods and water contamination. The project fills a knowledge gap on how well green roofs work in subtropical climates to combat both problems and guides city planners and property owners on practical benefits.

---

---

Objectives of the Project

---

1. Assess how green roofs change rooftop and surrounding air temperatures in a subtropical city.
2. Evaluate how green roofs affect stormwater volume and quality during typical rainfall events.
3. Compare performance of different plantings and substrate depths on cooling and drainage.
4. Provide practical guidelines for implementing cost-effective green roofs in similar cities.

---

---

What You Will Do Step by Step

---

1. Review existing research on urban heat islands, green roofs, and stormwater management.
2. Select a sample of buildings with green roofs and comparable buildings without roofs for comparison.
3. Install or use sensors to monitor temperature, humidity, and rainfall impact on roofs and surroundings.
4. Collect data during dry and rainy periods, then analyze trends in temperature and runoff.
5. Analyze plant and soil setups to see which configurations work best.
6. Discuss findings in the context of local climate and city needs.

---

---

Expected Outcome

The project should show whether green roofs significantly reduce heat and manage rainwater in a subtropical city, identify the best plant and substrate combinations, and offer practical recommendations for policymakers and builders to apply this approach effectively.