Green roofs, particularly in their extensive form, are a promising form of urban green with an important role to play in the climate proofing and ultimately in the sustainability of our cities. Despite increasingly widespread use and increasing scientific interest in extensive green roofs, their aesthetics have seen very limited study. Furthermore, several functional issues remain. Firstly, as vegetation develops spontaneously, weedy species can colonize the green roof. Secondly, extreme roof conditions can lead to the formation of gaps in the green roof vegetation. Apart from altering the functioning of the green roof, we expect these changes to also influence the perception of extensive green roofs, possibly affecting their acceptance and application. In this study, we therefore valued the visual aspects using a discrete choice experiment (DCE) approach. The novelty and proven suitability of this stated preference approach for environmental topics makes it an optimal method to investigate extensive green roofs. Through the creation and execution of an online DCE survey and subsequent data analysis using Mixed Logit models, the preferences and willingness to pay (WTP) of 151 Belgian respondents in relation to multiple extensive green roof characteristics were quantified.
Our results show that respondents were willing to pay ca. 1.10 euro (1.17 US Dollar) per square meter for a one percent decrease in gaps. Furthermore, model output indicated that a price increase of ca. 38.10 euro (40.68 US Dollar) per square meter was would be supported if this reduced the weed conspicuousness on the roof from the highest to the lowest level. Results also indicated that the current visual state of extensive green roofs in Belgium is seen as suboptimal, indicated by a significant and negative willingness to pay of ca. ‑128.57 euro (137.27 US Dollar) per square meter. From these results, we can conclude that visual aspects of green roofs are indeed important, with weedy species and gaps significantly lowering the preferences and WTP for an extensive green roof. Overall, this study offers an extensive proof of concept for the application of a relatively new valuation technique on an often overseen aspect of extensive green roofs and a foundation for further research aimed at generating practical recommendations for green roof construction and maintenance.