The properties of a green roof
A green roof is a roof covered with vegetation. This could be practically anything. Sedum (small succulents, ‘extensive green roof’) is commonly used but today roofs are also built to support trees and complete (vegetable) gardens. These are called ‘intensive green roofs’. The characteristics and advantages are as follows:
- Plants purify the air and convert CO2 to oxygen
- A green roof absorbs rainwater acting as a buffer for rainwater drainage.
- A green roof is a living system, which increases the biodiversity in urban areas.
- A green roof provides insulation. A green roof keeps the building cool in summer and warm in winter. This can reduce energy costs significantly.
- Vegetation, including green roofs, limits the Urban Heat effect creating a greener and hence more livable appearance and atmosphere, and helps enable more optimal use of urban space.
The prime reason for green roofs being so good lies in the air purifying properties of plants. Plants convert CO2 to oxygen. Moreover, they actively remove particulate matter compounds, thus combating air pollution. In turn, good air quality benefits good health as it helps prevent allergies and respiratory problems. In the Netherlands also, attention is increasingly focusing on air quality. It is becoming more and more difficult for urban areas to meet European air quality standards.
Improving rainwater drainage
In the Netherlands rainfall seems to be becoming ever heavier. During major peaks, the rainwater cannot always be adequately drained, particularly in urban areas. This causes considerable social and economic damage. Green roofs can contribute significantly to the absorption of excess rainwater. Together with several others, Jeroen Mentens studied the extent to which green roofs absorb rainwater compared to a traditional roof. His research revealed that rainwater does not start to drain from a green roof for several hours and that the roof itself actually processes 54% of the rainwater that falls on it. Thanks to green roofs, therefore, far less rain water actually flows onto the street and into the sewer system. This not only helps prevent flooding, it also prevent unnecessary water pollution and drainage. Clean rainwater remains clean rainwater. One square metre of green roof generates an average annual saving of €8 on sewage costs.
NB: as green roofs retain so much water it is essential that a roof be surveyed beforehand to check it could bear the weight of the volume of water. Not al roofs are suitable for intensive vegetation. However, most roofs are suitable for extensive vegetation, such as succulents. This research conducted by TU Eindhoven explains this in more detail.
In many towns and cities there is too little biodiveristy. Not only is there relatively little variety of vegetation but as a result of this, there is also little natural variety in insects and birds, for example. Research conducted by the city of Toronto demonstrated that green roofs enlarge a habitat for flora and fauna, creating a buffer between the city and the surrounding countryside. This supports the ecosystems in the city and is good for the biodiversity. Of course, this effect can only really be achieved if large numbers of green roofs are installed. In 2010 Toronto became the first major city to adopt an active green roof policy. Research conducted there did indeed reveal a sustained increase in biodiversity. Even to the extent that migratory birds no longer avoid the city, they use it. To provide more biodiversity, a roof should not only be green, the vegetation used should also be varied and natural habitats should be created. Such as shelters and breeding places.
Savings on energy costs
Research has revealed that green roofs are an ideal way of insulating buildings. A green roof will keep the cold out in winter reducing the heating requirement. The effect is possibly even larger in summer as the layers of vegetation on the roof reflect the heat. Green roofs consequently reduce the cost of air conditioning and fans. In an experiment to compare them with ordinary flat roofs without vegetation, the temperatures measured varied by as much as 21⁰C. This obviously affects the climate control within the building significantly. Besides environmental benefits, it also generates financial gain. A survey conducted on behalf of the City of Toronto revealed that households could cut their fuel costs by approx 23% by installing green roofs. Translated to the Dutch situation, this would generate an average saving of €100 per household. Of course, the benefit (both financial and in terms of sustainability) are even greater in office buildings where air conditioning and heating, especially, are used more intensively. And as energy prices continue to rise, the amounts actually saved can only increase for businesses and consumers alike.
More livable cities
Stone retains heat, and in summer cities are consequently several degrees warmer than the surrounding countryside. In summer, thanks to their ability to retain water and reflect heat, green roofs actively help cool the urban climate. Furthermore, people tend to find a green environment more livable than one of concrete and brick alone. Municipalities become increasingly active in attracting new residents and businesses. A green city is an attractive city.
Green roofs in the Netherlands
The Netherlands has an estimated 200 million m2 of flat roofs. All these roofs could be made green. Ever more municipalities adopt a policy to actively promote green roofs. Not yet on such a wide scale as in some other countries, but it’s a start. Most municipalities opt for a grant scheme for both residents and the business sector which contributes to the cost of installing a green roof. Most such grant schemes specify a minimum roof size of 6 m2 to qualify. Municipalities award grants ranging from €15 to €50 per square metre, the average being around €25. Most municipalities do impose a maximum grant, usually 50% of the total cost.