Green Rooftops

Green roofs tackle stormwater – Part II

Rain in urban area

Green roofs have many advantages: as eco habitats, insulation, or even producers of food. However, as was mentioned in the previous blog post, another often overlooked benefit of green roofs is their reduction in rainwater runoff. With high rain fall and increased flooding, in certain areas green roofs make a real difference.

The Sheffield Green Roof Centre website has some statistics which bear this out:

Up to 95% of the ground surface area in cities is now sealed off due to urban development – this is ground space through which rainwater cannot be lost by permeation. This leads up to 75% of rainwater becoming run-off in urban areas.

rainwater data 1

rainwater data 2

In summer, green roofs can retain 70-80% of rainfall and in winter they retain 10-35% depending on their build up. The difference is due to a combination of more winter rainfall and less evapotranspiration by the plants because growth is not as vigorous during the winter months.

rainwater data 3

rainwater data 4

By incorporating a green roof into a building, old or new, you will see a fall in the amount and cost of drainage needed for the development. Wondering if a green roof is right for you? Speak to a professional that can advise you on installation and costs. You will need to be sure your home can support the extra weight of the soil after a rainstorm—about 20 pounds per square foot. Take a look here for some basic guidance on green roof systems.

By Olivia

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