Katarina Penonić, Master Engineer of Urbanism and Regional Development, B.Sc. Landscape Architect and Dejan Lazić, Senior Landscape Architect, General Manager at Green Decor doo, Belgrade. For more information www.zelenikrovovi.rs.
Increasing population growth and the zones covered with buildings in urban areas created a less comfortable living, lower quality of life for people in cities and on the larger scale affected the climate change, natural disasters, and the decreasing sustainability. As one of the ways for the modern society to overcome these problems, in the wider picture of the sustainable development, the modern concept of green building was created, which differs in many ways from the construction trend that has been present for years. One of the segments of green building are green roofs, developed by modern architecture in finding sustainable solutions for modern construction.
What are the green roofs?
Green roofs are new technology of green building represented by the vegetative layer of the roof. They consist of several layers: the protective felt layer, a drainage-storage foil, filtration felt layer, a substrate for green roofs and a final layer of vegetation. The basic division of green roofs is: the extensive and intensive green roofs. The types of vegetation, substrate and other layers may vary depending on the type of green roof.
Figure 1: Display of green roof layers in the intersection
The vegetation on a green roof cools the surface and reduces heat from the air through evapotranspiration. These two mechanisms reduce the temperature of the roof surface and the surrounding air. Area under covered with green roof can be cooler than the surrounding air, while conventional roof can increase the air temperature over 50° C.
Green roofs can be applied to different types of buildings: residential and commercial buildings, industrial, educational and public buildings, private homes and other types of buildings.
By creating a shadow below the layer of vegetation/plants the temperature is reduced. These colder surfaces reduce heat transfer to the building and its re-emission into the atmosphere. In the case studies, the maximum reduction temperature measured was 11-25° C, due to the shadows created by the trees on two buildings that had green walls and green roofs. Another study examined the influence of climbing plants on the wall temperature and measured the reduction in temperature of over 20 ° C. In addition, growing medium (substrate) on green roofs protects the layers beneath from exposure to wind and ultraviolet rays.
Reduction of surface temperature helps to keep the building cooler by reducing the flow of heat through the roof and inside the building. In addition, the lower temperature reduces the flow of warm air above the roof, which can help in lowering the temperature of the surrounding air. A higher percentage of roofs under the vegetation, apart from the positive impact on the building itself, can helpwith the reduction of ”urban heat islands”, which are common in densely built settlements.
Green roofs benefits
- Economic benefits through the reduction of energy costs – saving money for energy that is consumed in cooling systems in summer and winter heating systems
- Economic benefits that are manifested through reduced costs for the evacuation of rainwater
- Increased value of the property
- Creating of a new natural habitat for flora and fauna
- Reduced amount of rainwater returning to the process of water circulation in nature
- Reduced impact of extreme temperature values and urban heat islands
- Absorbing negative radiation which improves of the microclimate
- Absorbing the harmful effects of pollutants
- Use of solar energy in the most rational way
- New areas for rest, relaxation and interaction of residents
- Improving the quality of life
Reducing the use of electricity: Green roofs can save the energy needed for heating and cooling of buildings in such a way that when they are wet green roofs absorb and store large amounts of heat, which reduces temperature fluctuations. When dry, the green roof layers act as insulators reducing the flow of heat through the roof, and thus reduce the energy needed for cooling the interior of the building. During winter, these insulating effects mean less heat emission from the building which reduces the need for heating. In the summer, green roof vegetation reduces the temperature of the roof and the surrounding air and thus reduces the need for energy for cooling.
Reducing air pollution and the greenhouse effect: Vegetation reduces air pollution and greenhouse gases through dry deposition, confiscation and storage of carbon. Reduced demand for electricity in the buildings with green roofs also reduce air pollution and greenhouse gases associated with energy production. In addition, since ground-level ozone is much easier formed with high temperature, green roofs affect the slower formation of ground-level ozone by reducing the air temperature.
Green roofs can remove particles and gaseous pollutants, including nitric oxide, sulfur dioxide, carbon monoxide, ground-level ozone. Studied showed that 93 square metres of green roof can remove about 40 kilograms of particles from the air in one year, creating oxygen and removing carbon dioxide from the atmosphere. For a comparison, in one year of driving, 15 cars produce approximately 40 kg of particles.
Management of atmospheric water and water quality: Green roofs reduce and slow down rainwater drainage in urban areas. Plants and substrate of green roofs, as well as the standard green areas, absorb water that would otherwise be drained. The amount of absorption depends primarily on the depth of the substrate as well as the inclination of the roof. Studies show that extensive green roof can store between 50 and 100% of rainfall, depending on the depth of the substrate, the density of vegetation and intensity and frequency of local rainfall. Intensive green roof, with a deeper substrate layer, can store more precipitation in similar conditions, compared to the extensive green roof.
Improving the resident’shealth and comfort: By reducing heat transfer through the roof, green roofs impact on improving comfort and reducing heat strokes associated with heat waves. Improving the comfort of residents in the facility may have apositive impact on the health, particularly in buildings without air-conditioners.
Improving the quality of life: Green roofs can provide many benefits as well as the rest urban vegetation. People in the higher buildings can enjoy in the view of the lower buildings with roof gardens. When it comes to intensive green roofs, they represent new additional green space for rest and relaxation.
An example of a green roof built in Belgrade
On the roof terrace of the private property at Obilićev venac in Belgrade the green roof of intensive use has been made. After installation of waterproof layer, necessary layers for the green roof were made. Execution of works was carried out by a team of engineers of landscape architecture and trained technical staff of the company Green Decor d.o.o. All works were made according to the FLL standards for green roofs and with certified materials.
After waterproofing, protective 300g felt layer was set on the entire surface of the planned green roof.
After installing the protective layer of fleece 300g, began setting up of drainage-storage foil (d25mm), which is used to drain excess water to the drainage system of the facility and to keep the amount of water needed for the vegetation layer.
Filtration felt 200g layer is placed over the drainage-storage foil and under substrate, so it allows the water to pass from the substrate layer to the other layers below and also prevents the penetration of roots and serves as a barrier. Filter layer typically comprises one or two layers of non-woven geotextiles where one of the layers may be treated with particular inhibitor to prevent penetration of roots.
After filter layer is mounted, a layer of nutrient substrate for green roofs, which has two main functions – maintenance of the vegetation layer and thermoregulation of objects, has been filled. In this project substrate layer depth was 10 cm because the final layer of grass, according to the investor’s desire. Nutrient substrate is not a classic plot that can be used for some other types of green areas. It has the specific composition and requires improved structure and nutrients in accordance with the conditions and the choice of dendro materials. For this reason, the composition of the nutrient substrate varies from situation to situation, and it is of utmost importance to be in compliance with the conditions in which it is used to make green roof vital as long as possible and to minimize the possibility of deterioration of the investment.
The additional equipment for this green roof is automatic irrigation system, which was essential in order to maintain the lawn (which is not the case if making the extensive green roofs with Sedum). It is designed and placed in accordance with the surface covered, and it is located between the substrate and filter layer and helps to maintain the vegetation layer.
Final vegetation layer is made of grass sown by the desire of investor with all the necessary operations for the formation of lawns and adequate mixture of seeds.