Why construct a green roof?

A. Advantages for the community

1. Water management
   In urbanised areas, a large surface area is paved. As a result, precipitation has little or no ability to penetrate the soil and immediately drains into sewers via impermeable surfaces. This way, even relatively small showers can cause temporary overloading of the sewer system, resulting in localised flooding. This can be remedied by building larger sewers so that water can be drained faster and in larger quantities. However, this is an expensive solution and moves the problems to another area. Another solution is to drain all precipitation immediately (= increase retention of precipitation water). This can be done in many ways, including through the construction of cisterns or green roofs. The latter reduce the drainage of precipitation, meaning that peak flows are lower, the sewer system is not (or less) overloaded, and no (or less) serious flooding occurs. The ECCOsedum green roof system provides an annual precipitation discharge of 50%. There is no difference in water discharge between flat and sloping green roofs with a gradient up to 15°.
2. Air and water purification
   The urban environment differs from its surroundings not only climatically but also in terms of air pollution. A green roof may have a major influence on this as its complete system of vegetation, substrate, and micro-organisms not only captures, but also degrades harmful substances such as carbon monoxide, benzene, and dust particles. Green roof vegetation extracts dust and CO₂ from the air. In addition, it captures harmful substances such as nitrogen oxide, hydrogen sulphide, etc. and is able to encapsulate and immobilise noxious particles thanks to its stomata. In this way, green roofs contribute to a better environment through air purification. Furthermore, precipitation contains many harmful substances. If contaminated precipitation flows through a green roof first, part of the pollution is filtered before flowing into the waterways. Consequently, green roofs and other greenery have an important protective function.
3. Habitat development
   Buildings and other infrastructure take up space, meaning there always is habitat loss. Despite the fact that green roofs are not a full substitute for lost natural habitat, they may form an important landscaping element for both fauna and flora. Green roofs can act as temporary habitat (as stepping stones and temporary refuge), as well as replacement habitat and basal habitat. The extent to which certain plant and animal species are present depends on many factors such as the type of green roof, the manner in which the vegetation was applied and the presence of flora and fauna in the area. Anyway, blooming sedum plants are a major draw for butterflies and other insects that feed on nectar. In other words, green roofs may contribute to an increase of various plant and animal species (so-called biodiversity).

B. Benefits for the roof owner

1. Longer lifespan for roofing
   A green roof lasts longer than regular roofing (even twice as long compared to a flat roof with bitumen covering). The presence of the layered construction protects the roofing from:
   a. UV radiation: this breaks down materials due to the photochemical reactions; in case of a green roof however, the radiation is absorbed by the vegetation.
   b. heating, freezing, and temperature fluctuations of the roof: extreme and fluctuating temperatures cause friction in the roofing, as a result of which tearing occurs or seams may break (especially in the case of bitumen roofing). A green roof tempers the temperatures through the presence of the vegetation and the substrate. This causes lower maximum temperatures and higher minimum temperatures, in addition to much smaller fluctuations in temperature. The extent to which temperatures are tempered depends on the thickness of the green roof.
   c. Uneven heating of the different roof layers: this results in blistering and deformation of the seals. With green roofs, this is not possible due to the lack of rapid and large temperature fluctuations.
   d. mechanical damage: roofs may become damaged by hail. With green roofs, this is taken care of by the different layers.
2. Thermal insulation of the roof
   The lower maximum and higher minimum temperatures mentioned above are caused by the insulating effect of the green roof. As a result, the roof waterproofing not only has a more moderate temperature regime, but there are fewer heating costs in winter and less cooling required in summer, with the effect being greatest in summer.
3. Sound insulation
   A green roof absorbs sound. However, the exact magnitude of the sound absorption is difficult to measure. The overall effect also depends on the surface and the structure of the greenery.
4. Visual benefits
   In addition to less extreme temperatures, higher humidity and cleaner air, green roofs also contribute to a better living environment and health through their visual effect. A view of greenery has a positive effect on a person’s mood and therefore on a person’s physical health. Furthermore, the look of a green roof changes with the seasons, resulting in a pleasant variation of the view.
5. Environmentally friendly image
   Installing a green roof gives businesses a more eco-friendly image. Working in a green environment also has positive psychological effects and may contribute to increased job satisfaction and a higher productivity.
6. Financial benefits
   Most people assume that a green roof costs much more than a traditional roof. If only the purchase cost of green roofs is compared with that of traditional roofs, this is indeed true, but the longer life span of green roofs ensures that the price difference is very small or zero in the long run. All things considered, green roofs are more economical.

Source: pamphlet ‘Agentschap Natuur en Bos’ (www.natuurenbos.be).