## 3 Costs and Benefits of Green Roof  
The economic benefits of applying green roofs are an important aspect of the usage of this technology. Cost-benefit studies can be an essential tool for city planners and public decision-makers in promoting green roofs and support for draft bills [22]. There is a wide range of costs that depend on various factors, such as the type and size of the roof, weather conditions of the location, building characteristics, etc. There are also possible barriers to the application of green roofs [40] primarily related to increasing maintenance, design, and construction cost as well as the possible structural limit exceedance, which can cause serious problems in practice. Comprehensive research provides multifunction nature of green roofs in both the private and social sectors. In the presented economic model, only the private cost/benefit effect is examined in detail. It is based on the relevant references from literature and also on the consideration of local conditions and practices. For the initial cost of extensive green roofs (EGR), the value varies in different ranges: USD 130-165/m\({}^{2}\)[13], USD 70-100/m\({}^{2}\)[51], USD 60-110/m\({}^{2}\)[14], USD 53-214/m\({}^{2}\)[22], or about USD 172 /m\({}^{2}\), proposed in [52]. The estimated range for the intensive green roof (IGR) is USD 160-540/m\({}^{2}\)[13; 22], or USD 100-240/m\({}^{2}\) suggested in [51]. In [9], it is indicated that the cost of retrofitting of the existing building (with EGR) depends in the wide range (USD 60-180/m\({}^{2}\)) on the specification but also the condition of the existing waterproofing layer. According to ref. [13], property value increase is relatively close to installation cost, i.e., it can be evaluated as an increase of 2-5% for EGR to 10-20% for IGR. So, it is estimated that this increase is USD 132-648/m\({}^{2}\), depending on the type of green roof. On the other hand, ref. [53] estimated an increase of 16.2% in rental prices in buildings with green roofs. For property value increase, the most influential parameter is an aesthetic improvement, although it depends on access and ownership regime [54], and it is taken into a calculation as a one-off benefit or annual benefit in the case of renting. Without directly estimating property value, in ref. [26], the aesthetic benefits (USD 2.6-32.2/m\({}^{2}\)) and the benefit of acoustic improvement (USD 43/m\({}^{2}\)) in the building are separately analyzed and quantified, added up to USD 72.2/m\({}^{2}\)/40 years. In the economic analysis conducted in this paper, these types of benefits are considered to be incorporated into property value increase. These benefits are conservatively estimated for the apartment below the green roof, and for the other apartments in the considered building, this value is further reduced by 50%. For IGR, operating and maintenance cost is one of the main concerns of potential investors, while EGR has minimum operating requirements. These costs have notably dispersed value range in the literature overview. In refs. [13; 27], this value ranges from USD 0.7 to 13.5/m\({}^{2}\) per year for different types of green roofs. In ref. [52], this value is set to USD 2.9/m\({}^{2}\), while in ref. [16] this value is universally proposed as USD 5/m\({}^{2}\) per year. For markets and industries that are not yet developed, the higher values are more suitable. Given that green roofs in Serbia are not yet sufficiently represented, the higher value is adopted in the economic model. The lifespan of the green roof can vary from 40 to 55 years [13; 15]. Therefore, in addition to maintenance costs, replacement after the projected lifetime could be included in the analysis. However, the conventional flat roof is usually replaced or reconstructed in 20 years, or twice as often as green roofs. Consequently, the membrane longevity benefit is proposed as much as USD 160/m\({}^{2}\) in ref. [13], although this value is significantly lower (USD 20-43/m\({}^{2}\)) in refs. [55; 56; 57].