Rooftop Greening Units Framework and Design Roadshow with Load-Bearing Capacity for Low-Carbon Urban Renewal

In the context of urban renewal, rooftop greening in old buildings plays a vital role in reducing carbon emissions and supporting carbon neutrality goals. This study evaluates the carbon capture capacity (TCO2) and efficiency (ηCO2) of rooftop greening under load constraints by developing standardised greening units and mathematical models to quantify CO2 absorption and self-weight load. Rooftop carbon capture performance is assessed by comparing the quantified TCO2 of each unit with the theoretical maximum and calculating overall ηCO2. Three representative species—Hibiscus mutabilis, Prunus cerasifera ‘Atropurpurea’, and Gardenia jasminoides var. radicans—were examined. In single-planting mode, ηCO2 was approximately 14% with no significant differences among groups, though TCO2 varied considerably, ranging from 6061.32 g·d−1 for Hibiscus mutabilis to 1515.52 g·d−1 for Gardenia jasminoides var. radicans. In mixed planting, ηCO2 showed greater fluctuation, with species combinations of similar TCO2 enhancing efficiency. Findings demonstrate that rooftop greening can effectively expand urban green spaces, improve species diversity, and promote ecological balance, while providing a systematic evaluation framework for low-carbon renovation strategies in urban renewal.