Green Roof Systems Project
The principle aim of this programme is to create a long term strategic partnership between the Green Roofs Research Group at The University of Sheffield (USFD) and ZinCo GmbH (ZinCo) an SME that leads the international field in development and supply of green roof technology. The partners both aspire to increase their knowledge and capacity in relation to the development of environmentally sustainable and attractive green roof systems. This will be achieved through significant exchange of personnel between the two partners over the duration of the project and through the temporary recruitment of 3 new experienced researchers. We have prepared an innovative programme of research, knowledge transfer and networking which will ultimately, through enhancing the capacity of the partners, enhance the EU standing in the area of green roof technology particularly in relation to its international competitors.
State of the Art
Green roofs are vegetated roof surfaces. Modern green roof technologies were developed in Germany, and since that time, green roof systems have been exported across the world. Green roofs have received widespread uptake in Germany and other north-European countries, and are a common feature of built development, because of the multiple benefits that they offer to urban environments (including stormwater attenuation, biodiversity support, summer cooling of buildings, and aesthetic and amenity values).
The green roof industry has been established in Germany for 30 years. Over that time intense research has been undertaken to develop reliable and efficient systems. As a result, two main types of green roof are recognized. The ‘intensive green roof’ is the traditional roof garden, employing relatively deep layers of growing medium, and supporting a range of landscape elements, from trees and shrubs through to lawns and water bodies. In contrast, the ‘extensive green roof’ is light-weight, low-cost and low-maintenance, but is generally inaccessible to people.
Early green roof research focused on the root resistance of
waterproofing membranes. Other important
research focused on the development of substrates that were best suited to the
different types of green roofs. But the
main focus of much of the later research has been on the hydrological
performance of green roofs in relation to their runoff characteristics. This hydrological work was undertaken largely
in the context of German building regulations which describe the standards for
runoff to roof drainage outlets.
By-and-large the hydrological work has been restricted to knowledge
about the specific performance of proprietary systems, rather than the
development of generic process-based understanding and modeling tools.
The end result of this system development over several decades is that the vast majority of extensive green-roofs consist of thin, mainly mineral-based growing media (in general 60 – 80mm in depth) and are dominated by plants from the genus Sedum. Sedums are succulent plants that typically form low-growing carpets of vegetation. The development of these sedum roofs has matched very closely the requirements of building owners and the construction industry, and this type of roof has been exported from Germany to become the “international norm”. If managed well, sedum carpets are stable over time and form uniform vegetation covers. But in several respects these roofs with thin layers do not deliver optimal performance. Limitations affect rainwater-retention, rainwater-runoff-delay, thermal qualities, ecological benefits regarding flora and fauna, reduction of urban air-pollutants and having an attractive aesthetic appearance. In effect these roofs can be seen as a protection layer for the water-proofing of the roof, with little effect on run-off coefficients and some improvement regarding flora and fauna. Some research has been undertaken on the benefits of green-roofs to biodiversity – but the general conclusions have been that more than 100 mm of substrate is required in order to get a real benefit. This is often not possible for buildings with limited load bearing capacities.
For intensive green-roofs the end result of the development process has been that the systems usually need additional watering in summer and levels of maintenance similar to that of a garden on the ground. With changing climate, there are already severe restrictions in irrigating green surfaces in the hot and dry months, and these restrictions will surely increase in the future. So there is a pressing demand for good-looking green-roofs without the need for additional watering. There has been little research in this area at present, but it is an area that will become more increasingly important.
The original drivers for the development of green roof technology in Germany are still valid. However, in the last five years the context and reasons for green roof installation have changed somewhat, and some of the fundamental qualities of the now traditional sedum carpet are being called into question, and as a result and by implication, the fundamental basis of the contemporary green roof industry is also being challenged. This is of crucial importance to the green roof industry because the model light-weight sedum roof does not appear to deliver, in the long-term, the desired objectives for extensive green-roofs of being rich in species, both in flora and fauna, having an attractive appearance, and performing well in reducing stormwater runoff-peaks. Intensive green-roofs are still desired and necessary – but the demand for accessible green roof systems that are less resource and maintenance-intensive systems and which have high aesthetic value is increasing.
The implication of these new developments is a changing view of green roofs, from static systems designed to resist change, to a view of green roofs as dynamic systems that undergo succession and change in the same way as landscapes on the ground. In essence: a change from viewing green roofs as an engineering challenge to a more ecologically-influenced viewpoint of green roofs as ecosystems. This ecological view demands that extensive green roofs become biologically more diverse whilst also offering improvements in delivery of ‘ecosystem services’ such as stormwater retention, carbon sequestration, energy conservation, nutrient cycling, support for faunal diversity. Intensive green roofs must become more sustainable in reducing or eliminating the need for additional water, and in reducing their maintenance requirements, whilst remaining aesthetically attractive and green.
The emerging drivers for the evolution and development of commercial green roof products include:
Extensive green roofs
– Developing a longer water supply out of the systems in order for the vegetation to survive drought periods, whilst ensuring that under wetter conditions there shall not be an over-supply of water, which may promote unwanted changes in the vegetation –e.g. fast growing grasses becoming established which disturbs the drought resistant vegetation. This requires new focus on evapotranspiration processes and inter-event moisture balance and transfer within the system, and scope for new science. Previous work has tended to focus on storm events; but what happens between the events is also critical here. This relates to a combination of plant physiology and growing medium physical properties
– Increasing pressure to include native plant species and to widen the plant diversity of green roofs, in the context of a widespread surge in interest in the biodiversity potential of green roofs, and in the need to develop vegetations with low water demand and drought resistance characteristics.
– A new focus on stormwater attenuation for their use integrated in stormwater management systems. Much of the previous hydrological work in Germany focused on runoff characteristics to meet roof drainage outlet requirements. This data is not applicable for use in modeling green roof integration within complete stormwater management systems. There is a need for process-based understanding of performance, linked to fundamental physical properties of the system, to enable comprehensive modelling and system design.
– A renewed focus on the aesthetics and visual quality of green roofs, rather than a singular focus on their ecological benefits.
Intensive green roofs
– The need to eliminate irrigation or decrease irrigation to a minimum – roof greening that requires regular water input is contrary to the fundamental tenets of sustainable urban development and irrigation is unlikely to be an option in the future. There are two main aspects that need to be considered. Firstly, testing for suitable plants that require minimal irrigation as an alternative to current planting regimes. For example, a lawn needs up to 5-7 liters of water per m² day in summer. Secondly, development of the green roof system components to retain and deliver water. This refers not only to growing media, but also to the other components and layers.
– More complex hydrological modeling. The run-off characteristics are also important for intensive green-roofs, and the thicker substrate layers in comparison to extensive green roofs promote enhanced performance. However, the hydrological performance of intensive roofs has been little studied, for practical reasons.
– The overall aim of this programme is to further develop the strategic partnership between the University of Sheffield and ZinCo through fundamental research on new materials, substrates, plants and green roof components to maximize their efficiency under moisture stress and to promote drought tolerance without irrigation.
The partners ZinCo and University of Sheffield share the same desire to develop an integrated and ecological approach to the understanding, design and implementation of green roof systems. This ethos is reflected in our research objectives and methodology as described below.
The University of Sheffield hosts The Green Roof Centre, which undertakes fundamental research on green roof properties and performance. It has supported the installation of a wide range of green roofs in the city of Sheffield, all of which are accessible as research roofs, and all of which are of a similar age. Extensive outdoor and indoor research facilities and laboratories enable fundamental green roof research to be undertaken.
ZinCo is one of the
original German green roof companies, and as such has unrivalled experience in
product development and green roof research.
The company has testing laboratories and trial and demonstration green
roofs. Indeed, the buildings at ZinCo HQ
support green roofs of differing ages, ranging from several decades through to
relatively recent. The company also has
access to many hundreds of installed green roofs in the immediate area of Stuttgart, where they are
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