Green roofs in Beijing were studied for their ability to manage stormwater. The researchers looked at how different types of soil on green roofs affect water retention during storms. They found that roofs with at least 10 cm of soil can hold back most of the rainwater, delaying peak runoff by 1.5 to 3.5 hours. Green roofs with 15 cm of soil work even better at keeping water and reducing peak runoff. However, the performance doesn't change much between these two soil depths because the city mostly gets light rain and doesn't have long dry spells. The study also predicted that adding more than 15 cm of soil won't make a big difference in preventing floods during heavy storms. So, using green roofs with 10-15 cm of soil in Beijing could be a good way to manage stormwater in the city.