Climate protection and adapting to climate change are amongst the most pressing environmental challenges of the 21st century. Green roof technology offers interesting approaches to reacting to both aspects. City roofs also offer space usage potential which has yet to be properly tapped into and which can be put to good use for the urban ecology. It should therefore come as no surprise that scientists around the globe are researching the many scopes of Green roof application and design.
Researchers around the world are looking for the best environmental technology to respond to the challenges of climate change and the urban heat island effect in growing cities. One focus of attention is the roof area. Apart from installing Green roofs, other “Cool roof” options are being discussed. These include using white waterproofing or painting dark roof surfaces white.
A Question of Colour
White roofs are a cost effective means of significantly reducing the urban heat island effect and building heat in the summer by reflecting the sunlight. Apart from the health aspects, the lower temperatures mean substantial energy savings on air conditioning in buildings. The study titled “Urban adaptation can roll back warming of emerging megapolitan regions” carried out by an American research group compares Green Roofs with White roofs with regard to the city climate and the indoor temperatures. Researchers looked at how Green Roofs, White roofs and a hypothetical combination of the two could reduce climate warming in various regions. To provide a more aggravated situation for the climate simulation, urban expansion in different scenarios was included together with the anticipated global warming. The prognosis was projected as far as the year 2100.
The results showed that White roofs performed slightly better than Green roofs. There were varying degrees of differences in the cooling effect depending on the region though. The best cooling values were achieved with the hypothetical combination of both techniques where more light was reflected by the White roof and the Green roof gave evaporative cooling. However, negatives also came with the positives of the White roofs in the simulation. The researchers claimed that the increased reflection from White roofs would mean higher heating costs during the winter months. Even more importantly, the White roofs could have a negative impact on the precipitation.
The researchers believe that widespread White roofs could lead to a 2–4 mm reduction in rain per day, depending on the region. Over the year, this could mean a reduction of 700–1400 mm, which would have a massive effect on the city climate and the water supply of the ecosystems. It is not surprising, therefore, that the researchers do not want to generally recommend White roofs and stress the point that the choice of environmental technique should always match the local situation. They further recommended unsealing land as a further measure to reinstate natural water cycles.
Painting a roof surface white is easier and cheaper than installing a Green roof. But it is important to point out, though, that the whiting only addresses the energy aspect of climate protection and adapting to climate change. White roofs, unlike Green roofs, do not store rainwater. There is no relief for the drainage and sewage systems during periods of heavy rainfall. The humidification of the ambient atmosphere from subsequent evaporation would also be missing. Further shortfalls include biodiversity, filtering air pollution and protecting the roof waterproofing from extreme weather conditions. Not to mention the aesthetics. The two techniques are not interchangeable.
Runoff Water Quality from Green Roofs
The quality of the runoff water from a Green roof and a conventional roof surface was compared and presented by the Universities of Manchester and Leicester (UK) in a paper titled “Metal and nutrient dynamics on an aged intensive Green Roof”.
An approximately 400 m², 43 year old roof on the Manchester University campus which carries an intensive Green roof with grass and herbaceous vegetation was chosen as the research object for heavy metal and nutrient fluxes in runoff water. A partial area of the roof only covered with paving slabs served as the non-landscaped reference area. Green roof substrate samples and dust deposit samples from both roof areas were analysed. The heavy metal and nutrient content of the rainfall was also examined. The investigation period lasted six months. The concentration of the following elements was measured in detail:
• Nutrients/Anions: chloride (Cl-), nitrate (NO3), phosphate (PO4), sulfate (SO4)
• Heavy metals: chrome (Cr), manganese (Mn), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), lead (Pb) and cadmium (Cd)
The English threshold value for the protection of surface water was used as a reference for the quality of each sample. The nutrient contents lay below the threshold values for the protection of surface water in all three samples (Green roof, bare roof and rainfall). The median values of heavy metals for zinc and copper (all samples), lead (Green roof and bare roof) and cadium (rainfall) exceeded the threshold values. If the nutrient and heavy metal input of the rainwater is included in the runoff balance of both roof types (flux model), then it becomes clear that, with the exception of lead and sulphate values, the roof runoff values are almost all lower than the rainwater values. This means that a certain amount of heavy metals and nutrients is retained by the roof and do not, therefore, contaminate the drainage system.
There is a simple explanation for the comparatively higher levels of lead found in the roof runoffs compared to those in the rainfall. Lead accumulates in the soil and does not degrade well. The researchers assume that the higher lead concentrations are due to its accumulation on the roof over decades. This was shown in the Green roof substrate and dust deposit analysis, both of which demonstrated elevated lead values. The study also showed that the research site was in an area of elevated levels of heavy metal pollution at ground-level. One cause of the lead pollution in the area noted by the researchers could be traffic emissions from leaded petrol which was in use until 1985. The Green roof was installed in 1970, which could account for some of the higher lead content of the substrate. In addition, it could not be ruled out, that lead flashings on the atrium roof contribute to the elevated lead values.
The researchers put forward various suggestions following the results of their investigation. One of these was to refrain from installing Green roofs in areas where there is high pollution (e.g. along busy streets) because the accumulation of harmful substances could lead to elevated pollutant levels in runoff over time. The study and the conclusions reached by the researchers raises the question whether the temporary storage of harmful substances in vegetated areas and its later eventual and reduced release is to be seen as a positive or negative environmental function.
It is well known that Green roofs act as environmental buffers at various levels. They also take on stormwater management functions when runoff peaks are reduced or delayed. Depending on the intensity of the rainfall however, there can still be some residual drainage. Nevertheless, there is no question that they relieve the sewage system in comparison to conventional roofs with direct runoff. Similar arguments can be used for airborne pollutants. The vegetation provides a buffer for negative effects, resulting in lower direct environmental pollution. A certain amount of the harmful substances can then be released over time while the rest remains stored in the substrate. To classify Green Roofs as “sources” of pollution because of this delayed release masks the real story. If the contribution of lead flashings on roofs to the runoff pollution is also factored in, then the real truth becomes even more blurred.
To suggest that Green Roofs should not be used in areas with high pollution is tantamount to suggesting that all ground-level vegetation should be avoided too, because it also filters out, binds and later partially releases pollutants. This is a hard to imagine scenario.
International Green Roof