Green Roofs – What are they?

Green roofs are a type of SuDS feature used in many developments for both pollution control and attenuation purposes.

They are typically comprised of a vegetative layer underlain by a substrate growing medium with permeable filter fabric, a reservoir layer with a controlled outfall, and a waterproof membrane.

The vegetative layer works to intercept rainfall, facilitating water storage within the roof’s reservoir layer. During warm and dry conditions, high evapotranspiration rates from the vegetative layer work to utilise this stored water, preventing the roof from becoming saturated and therefore reducing the volume of surface water runoff discharging into the subsequent drainage system.

During cooler and wetter conditions however, green roofs can remain saturated for long periods. Instead of reducing runoff volume during these times, the green roof works to slow down the rate at which runoff is discharged into the drainage system by offering a medium for temporary retention.

 

The overall annual performance of a green roof, taking into account seasonal performance variation.

This can therefore aide small reductions in attenuation requirements within a development. Alongside runoff control, green roofs offer water quality improvement opportunities through silt separation and pollutant trapping within their vegetative layer. This is particularly beneficial for sites discharging to ground.

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