Life cycle assessment of layers of green roofs

doi:10.1016/j.jclepro.2014.11.070

Journal of Cleaner Production

Volume 90, 1 March 2015, Pages 153-162

https://doi.org/10.1016/j.jclepro.2014.11.070 Get rights and content

Abstract

Recent years have seen an increase in the amount of green roofs in urban areas across the world. The research to date has tended to focus on the general advantages of such roofs while less attention has been paid to environmental impacts of the layers used in the construction of green roofs. In this paper, the environmental performance of two complete lightweight green roof systems were analyzed with the aim of determining the potential environmental impact of the different layers of the systems. The results show that among the materials we analyzed, the water retention, drainage and substrate layers contained the components that had the greatest negative environmental impact. More specifically, our results show that when the required performance is not impaired 1) Rockwool, virgin HIPS and expanded clay should be avoided in order to produce environmentally responsible roof systems; 2) simple roof systems may be recommendable whenever feasible and 3) recycled and local materials are better than virgin and those requiring long distance transport; and 4) the use of compost on the roofs may be recommendable if taking into account that using organic waste in landfill is a worse scenario than composting it.

Introduction

Green or vegetated roofs are roofs that have plants growing on top of them. At minimum they may consist of the roofing membrane and moss-lichen vegetation, but often a number of different kinds of layers are used to prevent plant roots from penetrating into the building structure, to hold or drain water, to retain nutrients, and to provide substrate for the plants, for example. Green roofs are considered as a solution to many urban issues including urban heat island mitigation, noise and air pollution reduction, storm-water management and support of biodiversity (Oberndorfer et al., 2007, Teemusk and Mander., 2009).

Several papers quantify or discuss the ecosystem services that green roofs can provide (Colla et al., 2009, Baumann, 2006, Schrader and Böning, 2006, Dunnett and Kingsbury, 2004, Renterghem and Botteldooren, 2008). For example, Kosareo and Ries (2007) compared the thermal transmittance and water run off quality of a 1115 m² conventional stone ballasted roof and a shallow substrate (150 mm), and a deep substrate (1200 mm) roof, in Pittsburgh, PA, United States, to find out the most environmentally friendly option among them. The results, based on life cycle analysis, showed that green roofs are an environmentally preferable choice as compared to conventional roofs, as they reduce the energy consumption of buildings and the water run-off. Saiz et al. (2006) conducted a comparative life cycle energy assessment of three kinds of roofs on an eight-story apartment building in Madrid, Spain: a common flat gray gravel roof,¹ a white-painted roof² and a green roof.³ They found over 1% reduction in the annual energy consumption and 6% reduction in summer cooling load of the building with the help of green roof.

Even if the above studies suggest that green roofs may offer a set of benefits, the net environmental benefit is always dependent on the use of natural resources in the construction of the green roofs, and a variety of choices exists as regards the use of materials in the construction of green roofs, upward from the roofing membrane. Up to date, however, there has been little consideration of the environmental impact of the different layers used in the build-up of green roofs; see however Bianchini and Hewage (2012). A green roof may consist of a few or several of these layers: a root barrier, a protection layer (against damage during installation), a drainage/water retention layer, a filter layer, the substrate and the vegetation (Fig. 1).

The roof slope, exposure, local weather conditions, and available substrate sources set limits for the planning of green roofs and the choice of materials to be used. For example, on steep roofs, pre-grown vegetation mats may be necessary in order to avoid erosion of the substrate layer before the plants are established. Yet, there is room for choices, and currently there is an increasing concern on selecting more eco-friendly layers. Therefore, a careful evaluation of options for different layers and their potential environmental impact is needed. The aim of this paper is to evaluate alternative materials for each layer of green roofs in order to identify their potential environmental impact; Life Cycle Assessment (LCA) was the method used in this study.

Section snippets

Material and methods

To determine the environmental impact of green roof's layers during its service life, life cycle assessment methodology was used with the aid of SimaPro software. Life Cycle Assessment includes different stages 1) goal and scope, 2) life cycle inventory, 3) life cycle impact assessment, and 4) interpretation (ISO 14040, 2006). LCA is based on the inventory of inputs (for example raw materials, transportation, production process and energy for production) and outputs (e.g. emissions to soil,

Results

In this section, we identify the key factors in the LCA, and discuss the limitations, accuracy and level of confidence of our results.

Discussion

Based on our calculations simple green roof systems, without several artificial layer materials, are an environmentally responsible option. We suggest simply leaving away rockwool, the egg carton-like plastic layers, and expanded clay when possible, or exploring for and developing alternative materials, in order to have a minimum environmental impact (while having the same green roof functions). For example, in Switzerland, efficient green roof systems without several plastic layers are used

Conclusion

We analyzed the environmental performance of two complete lightweight green roof systems with the aim of determining the potential environmental impact of the different layers of them. These systems represent the currently commonly available commercial layouts of green roofs, and allowed revealing the relative and absolute effects of different layers of the systems. We estimated the greatest environmental burdens to be due to rockwool, the plastic drainage layers and expanded clay. On the basis

Acknowledgements

The authors acknowledge gratefully the financial support from Kiinko Real Estate Education and Research foundation and HENVI Helsinki University Center for Environment. Our special thanks go to colleagues who reviewed the early versions of the manuscript, especially to Sini Veuro.

References (27)

E. Aguilera et al.
The potential of organic fertilizers and water management to reduce N₂O emissions in Mediterranean climate cropping systems. A review
Agric. Ecosyst. Environ.
(2013)
F. Bianchini et al.
How “green” are the green roofs? Lifecycle analysis of green roof materials
Build. Environ.
(2012)
G.A. Blengini
Using LCA to evaluate impacts and resources conservation potential of composting: a case study of the Asti District in Italy
Resour. Conserv. Recycl.
(2008)
L. Kosareo et al.
Comparative environmental life cycle assessment of green roofs
Build. Environ.
(2007)
C.H.,J. Molineux et al.
Characterising alternative recycled waste materials for use as green roof growing media in the U.K
Ecol. Eng.
(2009)
A. Nagase et al.
Amount of water runoff from different vegetation types on extensive green roofs: effects of plant species, diversity and plant structure
Landsc. Urban Plan.
(2012)
Schrader et al.
Soil formation on green roofs and its contribution to urban biodiversity with emphasis on Collembolans
Pedobiologia
(2006)
A. Teemusk et al.
Greenroof potential to reduce temperature fluctuations of a roof membrane: a case study from Estonia
J. Build. Environ.
(2009)
A.C. Woolridge et al.
Life cycle assessment for reuse/recycling of donated waste textiles compared to use of virgin material: an UK energy saving perspective
Resour. Conserv. Recycl.
(2006)
J. Yang et al.
Quantifying air pollution removal by green roofs in Chicago
Atmos. Environ.
(2008)

N. Baumann

Ground-nesting birds on green roofs in Switzerland: preliminary observations

Urban Habitats

(2006)

A. Benedette et al.

Quantity and utilization of tailoring fabric wastes output in tailoring skill acquisition centres in ebonyi and enugu states of Nigeria

Int. Res.

(2013)

J. Borken-Kleefeld et al.

Global default data for freight transport per product group

Int. J. Life Cycle Assess.

(2013)

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Life cycle assessment of layers of green roofs

Abstract

Introduction

Section snippets

Material and methods

Results

Discussion

Conclusion

Acknowledgements

Agric. Ecosyst. Environ.

Build. Environ.

Resour. Conserv. Recycl.

Build. Environ.

Ecol. Eng.

Landsc. Urban Plan.

Pedobiologia

J. Build. Environ.