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Green roofs (GRs) contribute to urban greening and adaptation to climate change and belong to a small group of measures that can be implemented even in densely built-up areas. Although the implementation of GRs has increased significantly in recent years, their implementation potential has not, for various reasons, been fulfilled in many places. Investors often lack information about the real value and feasibility of GRs. In this context, an economic assessment of GRs can address a wide range of costs and benefits and provide a clear economic argument for urban planning and decision-making. Cost–benefit analysis (CBA) is typically used to raise awareness of the positive impacts of GRs, but there is no unified approach, and its application differs across countries. As a range of approaches has been used, studies' results cannot easily be compared. The current study aims to provide comparable information on the economic value of three already implemented GR projects using two alternative CBA approaches, namely those used in Czechia and Portugal. The results indicate that the monetarily expressed GR benefits are four to six times greater than the costs depending on the case study. Thus, applying different approaches to the same case study, even when the same data are input, can yield a difference in results of 13–106 percent. The economic value of GRs is affected by specific CBA assumptions regarding the economic assessment of aesthetics and other benefits and costs of GRs. By using case studies in Czechia we thus show that different methodologies lead to different results, explain the contrasting results in the literature and prove the need to develop a standard economic assessment method for GRs. [Display omitted] • Investors lack information on the economic value of real green roof projects. • Studies present contrasting results in regard to the economic value of green roofs. • This paper makes an economic assessment of green roofs using cost-benefit analyses. • Results differ by 13–106 percent depending on the cost-benefit analysis approach applied. • Findings reinforce the need to develop a standard assessment method for green roofs. [ABSTRACT FROM AUTHOR]