Green roofs are multi-layered extensions of a building roof, partially or completely covered with vegetation and a growing medium, planted over a drainage layer. Green roofs serve several purposes for a building and the surrounding environment. Primarily, they slow down storm-water runoff by increasing evapotranspiration that also decreases the air temperature.
There are two typologies of green roofs: extensive and intensive. Extensive green roofs are basic, lightweight systems, characterized by minimum maintenance and management (automated irrigation, fertilization) after the establishment of the system. Extensive green vegetation is often established on roofs that are not accessible or with very limited access to public. These extensive green roofs are accessible for annual maintenance and partially characterized by some slope. Appropriate plants for extensive green roofs are low growing, rapidly spreading and shallow-rooting plants/hardy perennials (succulents such as sedums, herbs, wildflowers, grasses, mosses) that are able to survive with minimum nutrient uptakes and without additional nutrient supply. The selected plants for extensive green roofs are generally well adapted to alpine environments/climate and tolerate Mediterranean climate conditions (e.g. drought) and temperature fluctuations.
Intensive green roofs are more complex and heavier greening systems which are characterized by a higher installation, maintenance, management effort (regular irrigation and fertilization) which leads to higher costs compared to extensive green roofs. They are more often applied on residential buildings, hotels or underground parking garages. Intensive green vegetation is usually established on roofs that are accessible for public or recreation purposes, as well as for regular maintenance. The intensive green roof type can host different activities including gardening, relaxing, socializing and educational ones. To enable human activities on green roofs and the integration of larger plants, trees and architectural elements, rooftops need to be flat. Appropriate plants for intensive green roofs are mainly trees, shrubs and perennials. The growth media is relatively thick and notably deeper than for extensive systems with integrated low-growing plants. The growth media of intensive green roofs needs to be relatively deep and nutrient rich to support the growth of plants and bigger trees.
A growing number of cities are incorporating green roofs into their official plans, policies and operating procedures. This encompasses that cities provide detailed design guidelines and, in some cases, they set up mandatory regulations for roof greening. Different funding mechanisms can be used to encourage private and public implementation of green roofs, most often through co-financing, whether by direct financial incentives for the construction, or indirect financial incentives to split different fees.
An example of detailed design guidelines promoting green roofs is the “Guide to living terrace roofs and green roofs” developed by the Barcelona City Council. It is intended to be a tool for encouraging residents to choose the type of terrace roof or green roof by providing them a thorough explanation of technicalities as well as the social and environmental benefits.
In order to foster pilot projects, the Barcelona City Council launched a green roofing competition in June 2017, an initiative to finance green roofs on private buildings. 10 winning proposals have been chosen that will receive a grant of 75% of the cost, up to a limit of 100,000 Euros. All this will involve a total investment of 1.5 million Euros and will create new 4,000 square meters of green spaces on the city’s rooftops.
Another example of co-financing is the subsidy scheme of the Milan municipality developed as a part of the CLEVER Cities project. Through a public bid the successful applications will receive 35% subsidy plus funding towards technical support (project design costs and support through the administrative process).
Costs and benefits
The installation and maintenance costs of extensive green roofs are generally lower than those of intensive green roofs. “Installation cost is the price paid to the construction company for the work; it is the sum of the costs of all materials (including transportation), labour, and rental of the equipment needed for the installation. These costs also include those related to the preparation of the construction site (e.g., scaffolding). The cost of a traditional flat roof varies from 80 to 100 €/m2 depending on the type of roof covering and insulation layer, while those for a green roof range from 140 (extensive) to 250 €/m2 (intensive)” (Perini and Rosasco, 2019).
The main positive effects associated with green roofs are related to their water retention and delaying run-off capacity, and their cooling effect which leads to a reduction of the temperature of the roofs itself, as well as of the surrounding air. It is indicated that green roofs can retain from 40% to more than 90% of rainfall, depending on their depth and type of vegetation (Livingroofs, 2018).
As a result, green roofs contribute to mitigating negative effects in urban areas, particularly the heat island effect, just as urban green spaces and corridors. A modelling study in Madrid found that, during summer, the surface temperature of a green roof with 90 mm of growing media can be 30°C lower than that of a bare roof (Saiz et al., 2006). Summer cooling load can be reduced by over 6%, while reductions in peak hour cooling load in the upper floors can reach 25% (Saiz et al., 2006). Simulations in Athens found that the installation of a green roof on a two-storey office building reduced the cooling load by up to 58% (Spala et al, 2008, cited in Block et al., 2012).
Moreover, green roofs encourage an increase of biodiversity in the city.
Ascione et al. (2013) show that in cities with scarce rainfall, the cost of irrigating green roofs can outweigh the savings from reduced energy demand for air-conditioning.
Some of implementation requirements imply: the roof slope should be up to 30˚, otherwise different techniques are required; the selected vegetation must be adapted to the local climate conditions (Tecnalia, 2017) since their adequate maintenance determines the cooling performance (Speak et al., 2013); the amount of weight of intensive green roofs may require substantial reinforcement of an existing roof structure or inclusion of extra building structural support (GLA, 2008).
Implementation time and lifetime
Green roofs are basically installed in layers. Initially the installation is preceded by an assessment from a structural engineer to determine the weight that the roof can support. The installation of all layers takes roughly 5 to 7 days, but it will take several weeks for the plants to establish themselves.
“Roof longevity refers to the protection exerted by external layers of the roofing system on insulation and waterproofing. The protection action is against ultraviolet (UV) rays, thermal shock, and damages that may occur during punctual maintenance operations. The protection action allows an extension of the life span of the roofing systems. The life span of a conventional roof is about 20 years, while for a green roof the life span is around 40–55 years.” (Perini and Rosasco, 2019).
Source for more detailed information
Barcelona City Council “Guide to living terrace roofs and green roof”
UNaLab Technical Handbook of Nature-based Solutions https://unalab.eu/en/documents/unalab-technical-handbook-nature-based-solutions
Greenroofs.com project database https://www.greenroofs.com/projects/
European Federation of Green Roofs and Walls https://efb-greenroof.eu/
Associazione Italiana Verde Pensile https://aivep.it/
CLEVER Cities – Milano https://milanoclever.net/
Perini, K., & Rosasco, P. (2019). Selection of (Green) Roof Systems: A Sustainability-Based Multi-Criteria Analysis. buildings.
Livingroofs.org. (2018). Storm Water Run Off – Green infrastructure steeming the flow in cities. Tratto il giorno 04 2020 da livingroofs: https://livingroofs.org/storm-water-run-off/
Ascione, F., Bianco, N., de’ Rossi, F., Turni, G., & Vanoli, G. (2013). Green roofs in European climates. Are effective solutions for the energy savings in air-conditioning? Applied Energy, 845-859.
Tecnalia Research and Innovation. (2017). Nature-based solutions for local climate adaptation in the Basque Country. Ihobe, Environmental Management Agency.
Speak, A., Rothwell, J., Lindley, S., & Smith, C. (2013). Reduction of the urban cooling effects of an intensive green roof due to vegetation damage. Urban Climate, 40-55.
Greater London Authority. (2008). Living Roofs and Walls – Technical Report: Supporting London Plan Policy. London: Greater London Authority.
Saiz, S., Kennedy, C., Bass, B., & Pressnail, K. (2006). Comparative Life Cycle Assessment of Standard and Green Roofs. Environmnetal Sciences & Technology, 4312-4316.
Spala, A., Bagiorgas, H., Assimakopoulos, M., Kalavrouziotis, J., Matthopoulos, D., & Mihalakakou, G. (2008). On the green roof system. Selection, state of the art and energy potential investigation of a system installed in an office building in Athens, Greece. Renewable Energy, 173-177.