Green Roof Retention

Green roofs provide a vegetated layer that can capture and temporarily store storm‑water, reducing peak runoff rates and mitigating downstream flooding. The amount of water retained depends primarily on the roof’s surface area, the depth of the growing substrate, and the substrate’s porosity, which determines how much void space is available for water storage.

When a rain event occurs, the volume of water that reaches the roof is the product of the roof area and the rainfall depth. The substrate can absorb water up to its storage capacity, which is calculated from its physical depth and the fraction of that depth that is void space (porosity). Any excess water beyond this capacity will flow off the roof as runoff.

Designers use these relationships to size green‑roof systems for a target storm event, ensuring that the retained volume meets regulatory or site‑specific stormwater management goals while also providing ancillary benefits such as insulation and habitat.