Green roofs can help reduce stormwater in cities. A study looked at how well green roofs retain water in different Canadian cities. They found that the amount of water a green roof can hold varies depending on the location. Green roofs in Regina and Calgary work best for reducing stormwater, while those in Halifax and Vancouver perform less well. Toronto and London have the highest water retention, while Vancouver has the lowest. The type of plants on the green roof affects how much water it can hold more than the depth or porosity of the soil.