The Calculation tool for Multifuntional Sustainable Roofs has been developed by the City of Rotterdam in the context of the LIFE project LIFE@URBAN ROOFS (2017 - 2022), in association with Arcadis / CE Delft (Figure 1).

The background of the tool is that cities are vulnerable to flooding after extreme rainfall and to heat island effects. Effective adaptation measures are necessary to make cities more resilient to climate change.  Governments working on the development and implementation of adaptation strategies are limited by the scarcity of urban space, the availability of public funds, and regulations and policies that may be in conflict with adaptation goals.

Including roofs and facades in climate adaptation strategies can be an effective way to address some of the above mentioned problems. Roofs on houses, office buildings, hospitals and schools can be used for green infrastructure or water storage, making at the same time cities more socially inclusive.

Indeed, multifunctional roofs can combine several types of infrastructure: green (to reduce the urban heat island effect and support biodiversity), blue (water storage), yellow (energy generation) and red (social use) (Box 1).

Besides, the development of multifunctional roofs can stimulate private investment, which is one way of tackling the lack of availability of public funds, for example by including adaptation measures in building regulations for new projects or renovations.  The development of multifunctional roofs can encourage real estate developers and building owners to invest in climate change adaptation. In such a context, local government may act as a stimulator and facilitator.

More specifically, the Calculation tool for Multifuntional Sustainable Roofs is a generic calculation tool whereby administrators, policy makers and researchers may get insight (in broad outline) into the investments required for a sustainable multifunctional roof and the benefits that such a roof entails. The calculation tool that has been developed offers insight into the relevant aspects (both qualitatively and quantitatively) when considering the decision for investment in a certain type of multifunctional roof.

Key Words: Multifunctional roofs, biodiversity, sustainable energy, social cohesion, Social Cost-Benefit Analysis Municipality of Rotterdam, Stichting Arosa, De Heuvel, De Rotterdamsche Vastgoed Maatschappij, Vestia

The relevance of the tool in setting up a more socially integrative city consists in fostering the “upgrading of the physical environment”. Indeed, the tool allows the improvement of the urban built environment (building and construction) in the direction of climate neutral and efficient buildings.

During the TRANS-URBAN-EU-CHINA project, the tool has been used to provide evidence and case studies on solutions addressing air pollution and built environment in EU countries (consolidating the project knowledge base). Drawing on the case studies, it has been possible to distinguish between the financial business case and the SCBA. The financial business case shows the financial costs and benefits of the investor over a certain period of time.

In the SCBA, the costs and benefits are for society as a whole. All costs and benefits are included, regardless of who benefits or pays for them. In addition, qualitative benefits are taken into account; these are the benefits that cannot be expressed in terms of money, but which benefit society as a whole. Examples are the reputation of the neighbourhood and the city, heat stress, social cohesion and water quality.

From the cost side, the following impacts are considered:

• The investment costs of the multifunctional roof excluding subsidies (also known as the "gross investment costs")
• The investment costs of the multifunctional roof including subsidies (also called the "net investment costs"). These costs are included in the financial business case.
• The investment costs as additional costs compared to the reference alternative, excluding subsidies.

These costs are taken into account in the social cost-benefit analysis.

• Annual management and maintenance
• Replacement costs (excluding subsidies), which is used in the financial business case.
• Additional costs for replacement (excluding subsidies). These costs are taken into account in the social cost-benefit analysis

From the benefit side, the following items are considered:

• Social cohesion. When a project leads to additional opportunities to meet other people (at ground level or on the roof), this leads to a strengthening of social cohesion. In addition, literature shows that greening leads to less crime and more social cohesion. The effect is described qualitatively in the reports.
• Awareness of climate change. A project (and its effects) that is visible to residents, can lead to additional awareness of climate and water issues.
• Cultural history. Cultural history could be enhanced when historical water systems are present and are made visible by the project. This effect is describes qualitatively

The tool has been constructed in such a way that it can be used elsewhere in a generic way. Furthermore, the tool can be used to combine different roof surfaces, by 'on' and 'unchecking' of roof surfaces. The calculation tool can therefore be used to compare different designs.

The instrument is not bound to a specific location or to Rotterdam, but can also be used for applications elsewhere. This also applies across national borders. Within the framework of the LIFE @ Urban Roofs project, a possible next step is to apply the tool to developments in other European cities. An English version of the tool is available for applications outside the Netherlands.

Additionally, the tool is not tied to a specific phase in the development of a multifunctional roof. If there is only a rough sketch for the development of a multifunctional roof, key figures and assumption (described in the tool) can be used. If there is already a detailed plan, the available information from, e.g. quotations or a concrete specification could also be entered in the tool.

Despite the efforts in calculating a comprehensive SCBA, some of the benefits are assessed qualitatively: Social cohesion, Climate awareness and Cultural history. Besides, the effect on energy outside the building (cooling environment) runs via the health effect are also assessed qualitatively.

The developed tool for the financial business case and SCBA of multifunctional roofs has been applied to five cases (Figure 2).

The extent to which the tool could be tested differs per case. The design and configuration of all five multifunctional roofs were still being developed to a greater or lesser extent at the completion of this project. The data input was only for one case (DePereklip building - Vestia) complete (Figure 3), but not yet final. For the other four cases, the input was less detailed and / or incomplete, particularly in the areas "operation and maintenance", "energy yield" and the specification of the reference alternative. Where possible assumptions were made, based on key figures, to gain some results of the financial business case and SCBA.

With respect to the costs and benefits, the following observations were made:

• Two financial benefits have been identified: energy yields and exploitation of roofs with a red function (e.g. the exploitation of a roof terrace). Although the latter benefit is included in the tool, it has not been applied in any of the cases;
• Nineteen social benefits have been investigated and included in the tool. The benefits have been identified and valued in close collaboration with the case holders. Ten of these (social) benefits are valued in quantitative resp. monetary terms, the other nine are described qualitatively.

The results show the following:

• The balance of the SCBA of the multifunctional roof is positive in the case of Vestia (Peperklip). This is not (yet) the case for the other cases. It should be noted that some of the social benefits are only valued qualitatively, and therefore are not part of the monetarized balance.
• In all five practice cases, there is (still) no conclusive financial business case for the multifunctional roof. This means that the financial benefits for the owner / operator do not outweigh the costs of the multifunctional roof, as it is now composed. Using the calculation tool optimizations and / or a different configuration can easily be calculated.
• The reference alternative (the costs for a ‘regular’ replacement of the roof) has not been fully developed for any of the cases. The introduction of a reference alternative will positively influence the financial balance. The costs of the multifunctional roof are then considered as additional costs compared to the operation, maintenance and periodic replacement of a standard roof. Nevertheless, with this addition the balance of the financial business case will probably not be positive for any of the cases.
• Energy production on the roof generates financial benefits, but the construction and maintenance of a yellow roof is a costly affair. The resulting balance of a yellow roof is not necessarily positive.
• Red roofs, such as the exploitation of a roof terrace and meeting place, could generate financial benefits as well, but have not been applied in the five cases.

In China, the demand for green roofs is rising. Guangdong has taken a leading role in green roofing. As early as 1997, Guangzhou has released an urban greening regulation stipulating the construction of green roofs on suitable large-scale public buildings, which is also the first time green roofs are required by law in China. After entering the 21th century, Guangdong keeps strengthening the development of green roofs. For example, Shenzhen is giving out subsidies to organizations, enterprises and individuals for green roofing and is determined to annually increase the area of green roof by 200,000㎡from 2014 to 2020 (Figure 4).