Green Rooftops

Green roofs tackle stormwater – Part I

Green roofs are an attractive building feature for a number of reasons, both ecological and economical. They help conserve energy, combat the urban heat island effect, and help stormwater management. Let’s be honest, they’re also pretty to look at.

Green roof drainage

Storm water retention

Urban areas are covered with impervious surfaces, such as roads, sidewalks and parking lots. These materials seal off soil and block water absorption. When it rains, rapid runoff from roofs can cause flooding, increase erosion, and can cause sewage to spew into our rivers. The major benefit of green roofs is their ability to absorb stormwater and release it slowly over a period of several hours – they can retain up to 60-100% of the water that falls on them.

Types of green  roofs

Green roofs come in three different types, listed below. They are categorised depending on their use and how they are constructed. Their distinction largely determines the types of plants chosen for the green roof and how the vegetation will look.

  • intensive
  • semi- intensive
  • extensive

The advantages and disadvantages of the different types of green roofs vary. While some need less maintenance, others have a greater energy efficiency. Irrigation and drainage systems may or may not be required, which partly determines the cost of the roof. You can find a useful chart detailing the pros and cons of the different types of green roof systems here, on page six.


The type of roof will also dictate the choice of vegetation for planting. Extensive roofs, for example, are low maintenance and are generally made up of a very thin layer of soil. As a result, the plants will need to be adaptable, self-sustaining and resistant. To select the right plants, a range of factors are taken into account.

Plant groups used on green roofs include mosses, grasses, herbs, herbaceous and woody plants. The selection of vegetation on a green roof will determine its water retention. Below you have a couple of charts showing how much water extensive and intensive roofs can retain, which depends on the vegetation planted. The data used for these charts can be found in a manual detailing best green roof design practices.



Intensive roofs average a better water retention, ranging from 60-90%, than extensive roofs, which retain only 40-60%. This is mainly due to the depth of the soil, or medium, that the vegetation sits in – intensive greenery sits in a deeper medium as shown in the graphs above. In reality, these differences will vary depending on the season. It’s more distinct in summer, when it rains the most and most water retention occurs. When it cools down, it rains less and there is less water absorption by plants. As a result, water run-off is at a maximum in the winter. Our next blog post, will explore this season variation further – stay tuned.

By Katherine


One thought on “Green roofs tackle stormwater – Part I

  1. Pingback: Green roofs tackle stormwater – Part II | The Green Rooftop

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