Evaluation of methods for calculating the green roof potential on already existing buildings in urban areas.

Abstract: This study evaluates and compares four green roof methods with the purpose to calculate how many green roofs that potentially can be built on already existing buildings in a city. The methods are created for the cities Braunschweig, Thessaloniki, Lisbon and Lund. All of the methods mainly use Geospatial Information Systems and remote sensing analysis as tools. In order to answer the research questions, a literature study and an interview study have been conducted. Furthermore, this study observes the criteria that have been chosen in the methods and examines the possibility to create a similar method for Malmö in Sweden. Relevant stakeholders in Malmö, with knowledge on green roof technology and urban planning, give their opinion. The future for green roofs in Malmö, and what hinders a faster expansion, is also discussed. Roof slope and available roof area are used as criteria in all of the four methods. Other criteria found useful are weather conditions like sunlight availability as well as waterproofing membrane layer and load-bearing capacity. The latter is very important to incorporate in the method since it highly affects the result. The methods evaluated follow a very similar structure. Differences depend on the initial aims, which additional tools that were used as well as available input data and which scenarios were chosen. Only one of the green roof methods include a field study. However, it is concluded by the researchers of one of the other methods that field studies and in-situ measurements should be added in order to improve accuracy. Furthermore, a sensitivity -and uncertainty analysis should be performed. It might also be convenient to use a drone in order to collect both remote images and detailed images of buildings. The people interviewed in this study believe the number of green roofs will eventually increase in Malmö. The answers also demonstrate that there should be enough input data to create a green roof method for Malmö and the respondents appear positive towards it. However, one of the most difficult parts would be to find the load-bearing capacities of all roofs. Moreover, up to date there exist no political requirements or subsidies in Sweden, compared to Germany, that requires or promote more green roofs to be built. Therefore, depending on the development of this matter, a green roof method may be of more interest in the future. However, one can also argue the opposite. If a green roof method was created and used in Malmö now, it could become an argument to implement supporting laws or subsidies to increase the number. Other things that may hinder a faster expansion of green roofs in Malmö are building permits, finances and a general lack of knowledge. Available information on green roof technology needs to be distributed continuously so that more people understand the concept better. It would be interesting to do further investigations and compare the laws and subsidies of Germany with the ones in Sweden when it comes to green roofs. It would furthermore be useful to collect and compile information about physical properties of all roofs in Malmö. Roofs can be considered as empty space for not only vegetation, but also for solar cell systems or water retention systems. Moreover, it would be interesting to follow-up if the green roof methods analysed in this study have been implemented and been useful in the chosen study areas.