Cost-benefit analysis of green roofs in urban areas: case study in Helsinki

Abstract

This report presents a green roof cost-benefit analysis. Green roofs are roofs that are partially (or almost completely) covered with vegetation; between the roofing membrane and the vegetation there may be several technical layers. In this report we discuss the benefits and costs of lightweight self-sustaining vegetated roofs that do not require structural modifications from the building. The costs and benefits have been analysed in Helsinki, Finland. Green roofs offer various kinds of ecosystem services that are often scarce especially urban areas. These services accrue benefits to urbanites. However, ecosystem services do not generally have a market price, thus we had to use ecosystem valuation methods to estimate the benefits. Based on the valuation, the most significant benefits were: an increased lifespan of the roof, energy savings due to increased isolation and cooling, improved storm-water management, better air-quality and sound insulation especially in the air craft noise zones. In addition, other potentially significant benefits include aesthetic benefits, health benefits and improved biodiversity. Only a share of the green roof benefits accrues to the owner of the property while other benefits are distributed among the population of a larger area. Thus, benefits can be classified into private and public benefits. In the cost-benefit analysis we found that private benefits are in most cases not high enough to justify the expensive investment of a green roof instalment since the costs are incurred solely by the private decision makers (e.g. developers, real estate buyers). The cost estimates are based on supplier interviews and the additional costs of green roof were compared to a reference bitumen roof. The cost-benefit calculations hint that with a higher rate of implementation and realization of public benefits, the green roofs would be a good investment. However, because the private benefits are not high enough to justify a green-roof installation for a private decision-maker at the current cost level, the rate of implementation can be expected to stay low without corrective policy instruments. Policy instruments could include supportive policies that add incentives for private decision-makers to install green roofs and/or administrative orders.