Green Roof Design Techniques to Improve Water Use under Mediterranean Conditions

Teresa Paço,Pedro Arsénio,Ricardo Cruz De Carvalho,Diana Martins

doi:10.3390/urbansci3010014

Abstract

Green roof typology can vary depending on buildings structure, climate conditions, substrate, and plants used. In regions with hot and dry summers, such as the Mediterranean region, irrigation plays an essential role, as the highest temperatures occur during the driest period of the year. Irrigation might reduce the heat island effect and improve the cooling of buildings during this period, however, the added cost of maintenance operations and additional energy consumption could outrun the benefits provided by the project. Moreover, in situations where water is scarce or primarily channelled to other uses (e.g., domestic, agriculture or industry) during drought occurrence, it is advisable to implement green roof projects with the lowest use of water possible. The objective of the present work is to investigate solutions to optimize water use in green roofs under Mediterranean conditions, such as those of southern Europe. Two case studies are presented for Portugal, and potential techniques to reduce irrigation requirements in green roofs were tested. These addressed the use of native plant species, including the extreme type of a non-irrigated green roof (Biocrust roof) and techniques for plant installation. Plant drought tolerance was found to be an advantage in green roofs under these climatic conditions and, for the species studied, aesthetic value could be maintained when irrigation decreased.

Highlights

Green roofs initially appeared in regions with temperate and cold climates, where they thrive without the need to irrigate [1]
Several techniques for the design of green roofs in regions with Mediterranean conditions are described and analysed, as a result from the practices developed in the frame of three research projects of the University of Lisbon, aiming to optimize water use in such structures
The techniques studied were: i) use of drought-adapted species; ii) use of construction materials with water retention capacity; iii) deficit irrigation maintaining aesthetic value; iv) mixtures of vascular plants and mosses; v) biocrust roofs; vi) pre-cultivated vegetation blankets for roofs; and vii) wall plants transplanted to roofs

Summary

Introduction

Green roofs initially appeared in regions with temperate and cold climates, where they thrive without the need to irrigate [1]. For climates with hot and dry summers, as those of Mediterranean regions, the need to irrigate is unavoidable, as in these regions, plants on rooftops endure harsh conditions regarding temperature and insolation [2]. In regions with Mediterranean conditions, this may decrease costs with air conditioning, but if on the other hand, it is necessary to irrigate, the costs might outrun the benefits [4]. Enlarging urban green roof areas can help mitigate the heat island effect [5,6,7], but if effective results are obtained only with large costly irrigation amounts, in economic terms, the results might again outrun the benefits. A different point of view is considering the mitigation of the heat island as an indispensable

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Conclusion