In the Mediterranean area, rural abandonment and the rapid urbanisation after the 1950's have led to complex environmental problems, such as the exploitation of natural resources, the environmental pollution and the rapid increase of greenhouse gas emissions. Especially the unrestrained urban growth caused excessive pressure on the existing infrastructure, which affects buildings, public transportation, road networks, water quality, waste collection and public health. Green roofs are emerging building technologies that can help communities mitigate some of the above mentioned problems. Various studies have demonstrated that the implementation of green roof technology can moderate some of these environmental problems, through the reduction of heat flux and solar reflectivity, the minimisation of heat losses through the building's envelope, the air pollution removal, the storm water runoff reduction, the air cooling as well as the effective management of the urban heat island effect. In this line of thought, the aim of this study is to assess the green roof potential and the quantification of its benefits over Thessaloniki, Greece's second largest city. To delineate the available green roof areas, very high spatial resolution satellite image and natural colour orthoimagery were used along with a geographical object-based image analysis approach. Potential green roof extent was extracted with a mean error of 8.78 m(2), and discriminated from staircases, penthouse setbacks and terracotta tiles. Subsequently GIS were used to extrapolate the results from the building to the city scale. Carbon sequestration potential, rainwater retention and energy conservation were estimated based on coefficients adopted after a comprehensive literature review and actual dynamic energy simulations. The green roof potential in Thessaloniki approximated 17% of the gross built areas of the city, which means that there is a strong potential for green roof retrofit. Depending on the vegetation type installed in an extensive green roof, C stocks in Thessaloniki can increase by 2.5 times, while the dynamic energy simulations suggested that in certain city blocks reduced heating and cooling consumptions up to 5% and 16% respectively could be achieved. In regard to rainwater retention, implementation of green roofs in Thessaloniki is estimated to contribute in rainwater retention by almost 45%, allowing more than half of the rainwater precipitation to be reserved at the 50% of the city blocks. (C) 2015 Elsevier Ltd. All rights reserved.
