Biochar boosts green roof performance by cutting runoff and improving plant growth, real-world study finds

Cities around the world are searching for practical ways to adapt to climate change, manage stormwater, and reduce urban heat. A new field study offers compelling evidence that a simple material could significantly improve one of the most promising urban solutions: green roofs.

Researchers have demonstrated that adding a small amount of biochar, a carbon-rich material made from biomass, to green roof substrates can dramatically enhance water retention, reduce runoff, and support healthier vegetation. The findings come from a year-long experiment conducted on a high-rise rooftop under real environmental conditions.

Green roofs are widely used to capture rainfall, cool buildings, and create urban green spaces. However, their effectiveness is often limited by the properties of the substrate, which must balance water retention with structural weight. The new study shows that biochar can help resolve this trade-off.

By incorporating just 5 percent biochar by volume into a standard green roof substrate, the researchers observed a 7.7 percent increase in maximum water holding capacity and a 14.2 percent increase in plant-available water. These improvements translated into tangible environmental benefits at the system level.

Using both field monitoring and hydrological modeling, the team found that green roofs amended with biochar increased evapotranspiration by 23.5 percent while reducing stormwater runoff by 54.7 percent. This means more water is retained and returned to the atmosphere, and significantly less flows into urban drainage systems.

“Biochar acts like a sponge within the substrate, holding water that plants can access over time,” said one of the study’s authors. “This not only supports vegetation growth but also improves the overall hydrological performance of green roofs.”

Importantly, these gains did not come at the cost of increased structural load. In fact, the biochar-amended substrate was lighter than the conventional one, even when fully saturated with water. This finding addresses one of the key barriers to wider adoption of green roofs, especially on existing buildings with weight limitations.

The study also found that vegetation cover increased more rapidly in plots containing biochar, indicating improved growing conditions. Over the monitoring period, plant abundance rose by 10 percent in biochar-amended areas compared to 4 percent in standard substrates.

Unlike many previous studies conducted in laboratory settings, this research was carried out on an operational rooftop exposed to real weather conditions, including seasonal changes. The results therefore provide strong evidence that the benefits of biochar can be realized in practical applications.

As cities continue to expand and climate pressures intensify, solutions that combine water management, carbon storage, and urban greening are becoming increasingly valuable. Biochar offers a unique combination of these functions. In addition to improving soil properties, it also stores carbon in a stable form, contributing to climate mitigation.

The researchers conclude that integrating biochar into green roof design could play a key role in building more resilient and sustainable urban environments. With relatively low application rates delivering significant benefits, this approach may offer a scalable pathway for enhancing green infrastructure worldwide.

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Journal Reference: Petreje, M., Sněhota, M., Šípek, V. et al. Quantifying the benefits of incorporating biochar in green roof substrates: field study on the highrise rooftop in temperate climate setting. Biochar 7, 7 (2025).   

https://doi.org/10.1007/s42773-024-00409-z  

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About Biochar

Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field. 

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