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(if any at all). Heat gain is very rarely a problem and so the thermal advantages of a green roof may be irrelevant. Further potential cost offsetting arises in the ability of green roofs to be


used for attenuation as part of a SUDS scheme. Conventional attenuation may involve extensive ground works. Where land is at a premium, this could be problematic and use developable space. Green roofs can negate the requirement for below ground attenuation measures. However, any building with a substantial external area (such as a service yard) may incorporate areas of hard-standing. Here, green roofs can only offer a partial solution. Swales, ponds and watercourses can be used to provide attenuation for ground level run-off. Their presence can usually accommodate the projected run-off without introducing a green roof.


Maintenance For the building owner, there is a potential maintenance benefit in a


green roof. A conventional membrane covering may have a 25 year lifespan. However, a green roof installation may last 40 to 50 years due to it being subject to greater thermal and environmental stability. Living roofs require increased maintenance, even if this is very slight. Generally, this may be merely a couple of return visits over the first year to check for invasive species but even this minimal input can cause concern for the cost-conscious speculative developer.


Alternative Sustainabilty Strategy Many developers have adopted a sustainability strategy based on reduced embodied energy. In a warehouse, the energy consumption in


use is relatively small. Here, it can be demonstrated that the greatest reduction in carbon emissions can be achieved by using less materials in the first place. Therefore, the modest increase in structure discussed above takes on a more onerous dimension. On, for example a unit of 2500m2


, 990 tonnes of steel will be required compared to 735 tonnes. Where rainwater is harvested, the collected run-off can be re-used for


either WC flushing or vehicle washing. However, the water from a green roof will be brown. It may be suitable for flushing but tenants may not wish to see brown water sitting in the toilet! Obviously, brown water will be unsuitable for full vehicle washing. Furthermore, if already present, alternative attenuation measures used as part of a SUDS strategy such as swales, watercourses or ponds may provide the opportunity to create wildlife habitats with greater bio-diversity than that offered by an extensive green roof.


Conclusion On a constrained urban site, a living roof can enhance bio-diversity in


an otherwise, often sterile environment. It can offer environmental and even financial benefits in the right situation. However, any benefits arising from it’s inclusion in a lightweight building on a large site can nearly all be met by other means. Desire to avoid risk combined with the potential of increased maintenance often dissuades developers from adopting a green roof strategy. However, that doesn’t alter the fact that I hope that the couple on our ‘drawing boards’ at the moment eventually see the light of day!”


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