Hydrological performance

Green roofs are effective at reducing peak stormwater runoff. Green roofs can 1) delay peak runoff times by absorbing rainfall that directly lands on them, and b) reduce the total runoff volume by retaining a portion of the rainfall in the vegetation and substrate layer.

Various studies has found the water retention in green roofs ranged from 55% to 88%.

The infographic below shows what factors affecting the hydrological performance of the green roofs.

Water quality treatment performance

Green roofs can effectively reduce the pollutant loadings in runoff by absorbing and filtering the pollutants from wet and dry atmospheric deposition. Various studies found a positive effect of green roofs on runoff quality in terms of nutrient removal and in the retention of heavy metals. Nevertheless, there are instances where the green roof might contribute to pollutant release in runoff. For example, the application of commercial fertilizer onto a green roof may result in mineralized nutrients being leached from the substrate, thus contributing to runoff pollution.