Green Roof Design

Created by: Stefano Cascone

In order to consider green roofs as an environmentally friendly technology, the selection of efficient and sustainable components is extremely important. Previous review papers have mainly focused on the performance and advantages of green roofs. The objective of this paper is to examine the primary layers: The waterproof and anti-root membranes; the protection, filter, and drainage layers; the substrate; and the vegetation. First, the history, modern applications, benefits and classification are analyzed in order to present a well-defined state of the art of this technology. Then, the roles, requirements, characteristics, and materials are assessed for each green roof layers. This technology was compared to a conventional roof technology, Mediterranean climate conditions and their influence on green roof design were assessed, also comparing them with Tropical area and focusing on irrigation systems, examples about the commercial materials and products available in the market were provided and innovative materials coming from recycled sources were analyzed. Future research should evaluate new materials for green roof technologies, in order to enhance their performance and increase their sustainability. The information provided in this review paper will be useful to develop Mediterranean green roof guidelines for selecting suitable components and materials during the design and installation phases.

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1. Introduction

Currently, climate change and the scarcity of natural energy resources are topics of interest in many countries [1]. Furthermore, cities continue to grow and expand their peripheries to accommodate increases in rural migration to urban areas. According to a recent report of the United Nations, urbanization is forecasted to attain 83% by 2030 in developed countries [2]. This results in several environmental issues on a global scale, such as increased greenhouse gas emissions.
Due to this worldwide urbanization, the demand for new buildings, land, water, and energy have drastically increased over the last four decades. According to United Nations Environmental Program, the construction and maintenance of buildings account for about 40% of the global primary energy requirement and buildings account for 33% of the global greenhouse gas emissions [3]. Therefore, the building sector is of particular interest in the reduction of energy use, in order to limit global warming and mitigate the impacts of climate change [4,5].
The effect of building envelope technologies on the design and construction of sustainable buildings and urban spaces is undeniable [6,7]. Implementing various sustainable approaches and designing more environmentally friendly components for buildings leads to the realization of low-energy buildings [8]. In addition, roofs are important components of buildings, accounting for nearly 20–25% of the overall urban surface area [9]. Therefore, efficiently designed and integrated green roofs have great potential to affect the building and urban environments, replacing the lost green spaces and habitats in modern cities. Specifically, green roofs are engineered roofing systems, planted with different kind of plants on the top of a growth medium [10].
In recent years, the number of studies carried out on green roofs has considerably increased and several review papers have been published, in an attempt to summarize and organize the scientific knowledge on this topic. One of the first reviews was carried out in 2010 by Berndtsson [11], which addressed the role of green roofs in urban drainage, considering the management of both water quantity and quality. Factors which affect the influence of a vegetative roof on runoff water quality were discussed in general terms, followed by a review of data regarding the concentrations of phosphorus, nitrogen, and heavy metals in the runoff, pH, and the first-flush effect. Likewise, Akther et al. [12] statistically synthesized the effects of the influential factors, including design and hydrologic variables, on green roof performance, to explore their effects in different climatic zones. Castleton et al. [13] reviewed the current literature and highlighted the situations in which the greatest building energy savings can be made. Similarly, Saadatian et al. [14] focused on energy-related topics. Berardi et al. [15] presented a state-of-the-art of green roofs emphasizing current implementations, technologies, and benefits. The authors reviewed the benefits related to the reduction of building energy consumption, mitigation of the urban heat island effect, improvement of air pollution, water management, an increase of sound insulation, and ecological preservation. In 2015, Hashemi et al. [16] provided an overview of the effects of the application of the green roof strategy on the quality of runoff water and the reduction of energy consumption. Shafique et al. [17] included in their review the history, components, and multiple benefits (environmental, social, and economic) associated with green roof technology. In addition, the authors also emphasized its performance in reducing stormwater and energy costs, improving air quality, and ecological benefits. Recently, Cascone et al. [18] carried out a comprehensive review of the cooling effect, due to the evapotranspiration process, as most of the benefits of green roofs are related to this phenomenon. These previous studies were mainly focused on reviewing the performance and benefits, without providing a description of their technology and materials.
Vijayaraghavan [19] analyzed desirable characteristics for the growth substrate and vegetation and suggested a methodology for constructing green roofs. Dvorak and Volder [20] conducted a review in order to investigate what is known about the application of plants on vegetative roofs across North America and their ecological implications. However, these review papers addressed mainly the roles and the performance of vegetation and substrate, providing little information with both of the materials and of the other primary layers, such as the waterproof and anti-root membranes, and the protection, filter, and drainage layers. In addition, the most used international guidelines are the German FLL 2018 [21], concerning the planning, construction and maintenance of green roofs. However, these guidelines are mainly developed for Northern Europe, characterized by cold and rainy days during most of the year. Mediterranean area, characterized by hot and sunny days, has requirements that are not fulfilled by the green roof designed, according to the German FLL standard. This is mainly due to the absence of water for extended periods. Actually, several regional guidelines exist. For example, in Italy, the guideline is the standard UNI 11235:2015 [22]. However, this standard is written in Italian and, therefore, it is not suitable as an international guideline for the Southern Europe countries.
Differently from both the review carried out by Vijayaraghavan [19] and the international FLL guidelines [21], the novelty of this paper consists in comparing it to a conventional roof technology, in terms of both materials and thermal and economic performance, in assessing the Mediterranean climate conditions and their influence on green roof design, also comparing it with Tropical area and focusing on irrigation systems, in providing examples about the commercial materials and products available in the market and in analyzing innovative materials coming from recycled sources, as possible components. All these aspects related to green roof materials and technology are not fully described neither by previous articles nor by international guidelines. In addition, for each layer, the roles, requirements, performance, and materials are assessed. The information provided in this review paper will be useful for both researchers and designers to develop Mediterranean guidelines for selecting suitable components and materials during the design and installation phases.
First, the history and modern applications are discussed, in order to present a state of the art of this technology and their benefits and classification into extensive and intensive are described.

2. History and Modern Applications

Existing literature shows that covering the building rooftop with soil, wetting the soil, and shading the surface of the wet soil have been used for centuries as passive cooling practices in different countries with confirmed benefits in different climatic conditions and building characteristics [23].
One of the most famous ancient green roofs dates to the fifth century when the Hanging Gardens of Babylon was constructed that is admitted as the earliest examples of greenery systems [24]. Living roofs were also utilized in the ziggurats of ancient Mesopotamia. Like Babylon, the Roman and Greek architecture also employed these systems at their own eras. For example, the Mysteries Villa represents such integration and offers an example of space that enhances human activities while improving the aesthetic value and roof life. In the Mediterranean region, different plants notably vines were utilized to prevent the building envelope from excessive sunlight in the summertime and to provide cooler and comfortable indoor conditions to occupants. Green roofs have also been presented in vernacular architecture in different countries. For example, the usage of the plants climbing the building greatly expanded in the UK and Central and Northern Europe (especially in Norway) during 17th and 18th centuries to increase the thermal insulation [25]. After many centuries of rare utilization in European cities, during the modern age green roofs have been rediscovered in the twentieth century by the Swiss architect Le Corbusier who included them in the five points of modern architecture [26]. Around the same time, American organic architects proposed vegetative roofs as a method to integrate buildings and nature.
Modern green roofs, therefore, may acquire their concept from ancient technique; however technological advances have made this technology far more efficient, practical and beneficial than their ancient counterparts. An intensive implementation started from Germany in the early 1960s when there were energy crises arose [27]. Several investigations have been carried out with emphasis on biodiversity, substrate, roof construction and design guidelines. Green roofs gained popularity also in Austria, Switzerland and United Kingdom (UK) in the same years, however, Germany is regarded as the world leader in the employment of this strategy, because green roofs on the large scale were being developed, designed and implemented [28]. In this respect, the first comprehensive program was put into practice from the early 20th century by retrofitting the houses with greenery surfaces. Nowadays, as Table 1 shows, research and application at the building in Germany are very popular and green roof coverage increases by approximately eight million square meters per year, which is remarkable.

3. Technology Classification

Green roofs are broadly classified into intensive and extensive roofs, though some authors include a semi-intensive classification, based on the depth of the substrate layer, maintenance, cost, vegetation type, construction material, and irrigation [44].
Intensive green roofs are generally roof gardens designed with a considerable substrate depth—more than 15–20 cm—a wide variety of plants (similar to ground-level landscapes), high water retention capacity (over 50%), high capital costs ($25 per square foot), and heavy weight (180–500 kg/m2). Typically, this type is installed when the slope is less than 10°. Due to the increased soil depth, the plant selection can be more diverse, including small trees, shrubs, and bushes [24,45]. Therefore, it requires a high level of maintenance, in the form of fertilizing, weeding, and watering. One of the main advantages of an intensive roofing system is the creation of a natural environment with improved biodiversity, providing a recreation space, as they are normally designed for the use of humans for entertainment [46]. Intensive roofs encompass a comparatively better potential than extensive green roofs in terms of stormwater management, decreasing runoff by 85% when compared to traditional roofs [47]. Likewise, intensive green roof runoff has three times less lead contamination, 1.5 times less zinc contamination, 2.5 times less cadmium contamination, and three times less copper contamination [47]. On the other hand, their greater weight may require additional structural reinforcement, and drainage and irrigation must generally be utilized, increasing the technical complexity and associated costs [27].
Extensive green roofs are characterized by a shallower depth of substrate layer (less than 15 cm) and have a lower weight in comparison to intensive ones. Owing to the thin substrate layer, extensive roofs can utilize only limited types of plants, including grasses, mosses, and a few succulents. The main advantages of extensive roofing systems are the low capital cost and maintenance and water requirements, compared to intensive roofs [11]. These roofs are usually very lightweight and useful, especially where no additional structural support is desired. Furthermore, an extensive roof can be installed on a larger slope, their construction process is technically simple, and it is appropriate for large-sized rooftops. However, both the energy performance and storm water management potentials of extensive green roofs are relatively low [48].
Of the two types, extensive roofs are most common around the world, due to their low weight, not requiring irrigation, and having less capital and maintenance costs [49].