It is generally accepted that there are two main types of green roof which are described by Kibert (2008) as:
- “Extensive: Extensive landscaped roofs are defined as low maintenance, drought-tolerant, self-seeding vegetated roof covers that incorporate colourful sedums, grasses, mosses, and meadow flowers that require little or no irrigation, fertilisation, or maintenance… Extensive systems can be placed on low-slope and pitched roofs with up to a 40% slope.
- Intensive: If there is adequate load-bearing capacity, it is possible to create actual roof gardens on many buildings. This type of eco-roof system may include lawns, meadows, bushes, trees, ponds, and terraced surfaces. Intensive systems are far more complex and heavy than extensive eco-roof systems and hence require far more maintenance.”
Some authors also describe a third type of green roof as 'simple intensive' which usually comprise grasses, herbaceous plants and shrubs. Simple intensive green roofs can be constructed using varying depths of substrate, thus combining elements of extensive and intensive roofs (Newton, Gedge et al. 2007).
Extensive green roofs are the most popularly deployed forms of green roofs as they are generally light weight and low cost.
The concept of an intensive roof goes back as far as the Hanging Gardens of Babylon, which formed one of the Seven Wonders of the World. Nebuchanezzar II built the gardens, essentially a series of intensive roofs, for his wife who was homesick for the plants of her native Persia. Further north, in Scandinavia, farmers created one of the first extensive roofs, by stripping the sod from surrounding grass meadows, and placing it on a roof, supported by heavy timber beams. This provided an early form of roof insulation in an unfriendly climate.
Historically green roofs were common in the UK, but they became less popular during the industrial revolution. Presently there is a resurgence in green roofs in many countries due. Germany is leading this drive, with 10% of all German roofs have been greened, 80% of which are extensive sedum roofs (CIBSE 2007). The Swiss are also pioneering the use of green roofs with 70% of flat roofed inner city buildings having roof gardens (Yuen and Nyuk Hien 2005).
 Benefits of green roofs
Vegetated roofs offer an extensive range of environmental benefits that are widely accepted across the literature:
- Decreased surface water runoff
- Decreased heating and cooling demands for the building
- Increased local biodiversity
- Increased durability and lifespan of the roof
- Improved local air quality
- Psychological benefits
- Mitigation of the Urban Heat Island effect.
The CIRIA publication, Building Greener: Guidance on the use of green roofs, green walls and complementary features on buildings (Newton, Gedge et al. 2007) provides an excellent overview to green roofs along with their benefits and design issues.
 Surface water runoff
Environmental advantages of green roofs are generally accepted to include a decrease in surface water run off volume and a reduction in peak runoff flow. Green roofs accomplish this reduction in surface water volume and peak discharge by “delaying the initial time of runoff due to the absorption of water in the green roof system; reducing the total runoff by retaining part of the rainfall; and distributing the runoff over a long time period through a relative slow release of the excess water that is temporary stored in the pores of the substrate” (Mentens, Raes et al. 2006).
The reduction of runoff has been quantified by several authors on a variety of scales and under many rainfall conditions. Mentens, Raes et al. (2006) showed that the potential regional runoff reduction by greening 10% of buildings in the Brussels area with extensive green roofs of substrate layer depth equal to 100mm to be 2.7% for the region and of 54% for individual buildings. Other figures suggest that runoff in some situations can be reduced by 100% (Wolf and Lundholm 2008).
- The effect of roof slope.
- The amount of precipitation.
- Different rainfall patterns at different locations.
- The saturation and depth of the substrate.
They also suggest that the establishment period of the green roof may effect the percentage of runoff reduction, as greater maturity may increase the hydraulic conductivity of the substrate. Their experiments with roofs of slopes ranging between 2% and 25% and varying rainfall intensities showed an average retention of 85.6%. Carter and Jackson (2007) also suggest another advantage due to the retention of rain water at roof level; this is that pollutants are not washed off impervious surfaces into the drainage systems, the benefit of which is that there is no treatment and re-release on this volume of water, as the water cycle is effectively short circuited at the roof level.
However, the reduction in rainwater runoff depends on how dry / absorbant the green roof is. If it is saturated from previous rainfall, the reduction is zero. For this reason the presence of a green roof cannot be used to reduce the capacity of the rainwater drainage system.
Green roofs have the ability to reduce both heating and cooling loads in buildings. This has positive implications in terms of their energy consumption. They achieve energy reductions by reducing the thermal fluctuation of the outer surface of the roof and by increasing the roof's thermal capacity (Niachou, Papakonstantinou et al. 2001). In addition, foliage protects the building from the wind.
The process of heat transfer into green vegetated roofs is very different to those of conventional roof surfaces. In green roofs solar radiation, external temperature and relative humidity are reduced as they pass through the vegetation layer. The plants also provide cooling by their biological processes such as evapotranspiration, which converts large amounts of solar radiation into latent heat which does not cause the temperature to rise (Takakura, Kitade et al. 2000). The remaining solar radiation is changed into thermal load which can pass through the roof and thus influence the internal climate of the building (Niachou, Papakonstantinou et al. 2001; Spala, Bagiorgas et al. 2008). Temperatures on a black flat roof can reach up to 100oC (Ramachandran, Paroli et al. 2002; Wong, Tay et al. 2003). The addition of a green roof vastly reduces the temperature fluctuations to around 20 to 25oC (Wong, Tay et al. 2003).
The majority of papers written on the cooling effect of green roofs agree that the main parameter that effects heat transfer through the roof is the leaf area index (LAI) (Barrio 1998; Takakura, Kitade et al. 2000; Theodosiou 2003; Kumar and Kaushik 2005; Sailor 2008). LAI can be defined as the upper surface leaf area per unit area of base (which in this case is the roof). It is dimensionless (m2/m2), and usually for plants has a range from 0.5 to 5 (Sailor 2008). It is also worth noting that the LAI is often directly related to the amount of evapotranspiration of the plant, the greater the LAI, the bigger the total leaf area. In addition, large values of LAI offer practically complete shading to the soil layer, protecting the roof from solar irradiation (Theodosiou 2003). There are also other factors that Sailor (2008) notes that effect heat transfer through the roof which include height, LAI, fractional coverage, albedo, and stomatal resistance. It is worth noting that excluding albedo, all other factors noted here affect the degree of evapotranspiration and shading of the roof, whilst the albedo affects the radiative heat transfer through the roof.
Barrio (1998), proposed a mathematical model to represent the dynamic thermal behaviour of green roofs, and to analyse their potential as cooling devices in summer time. Her analyses show that green roofs do not act as cooling devices but as insulators, reducing the heat flux through the roof and thus reducing heat gains. This is contradicted by more recent research (Theodosiou 2003); (Kumar and Kaushik 2005) who find in their modelling that green roofs do indeed providing cooling potential. Their model was compared to a real world green roof building and found to be very accurate with an error range of 3.3% in predicting green canopy-air temperature and 6.1% in indoor air temperature.
It should be noted that whilst green roofs offer savings in the cooling load of buildings and can also reduce winter heating load, they can in some situations increase winter heating loads. For example Sailor (2008) found that increasing the thickness of the soil layer resulted in reduced demand for both heating and cooling, with larger heating savings in a cooler climate, but increased LAI, increased the amount of winter heating required due to increased shading effects that are beneficial in summer but detrimental in winter. A year round analysis should ideally be taken to optimise the green roof for maximum benefit throughout the yearly climate cycle. Many papers that analyse green roofs through a short time period, suggest that there is cooling potential in the summer, or reduced heat energy savings in the winter through increased insulation, but few studies have looked into optimising green roofs for performance through the year.
It is widely accepted that green roofs have the ability to reduce the urban heat island UHI effect. The UHI effect is primarily caused the replacement of natural surfaces with hard impervious surfaces that are generally dark and absorb large amounts of solar radiation.
The extent to which green roofs can reduce the temperature of their surroundings and thus mitigate the UHI effect is based on climatic characteristics, the amount of vegetation and urban geometry (Alexandri and Jones 2008). Vegetation reduces the UHI effect by changing the albedos (the fraction of incoming radiation reflected by a body) of urban surfaces as well as evapotranspiration cooling. It also slows down photochemical reactions that lead to less secondary air pollutants, such as ozone (Yang, Yu et al. 2008). They also provide some shading, thus reduce the temperature of the roof.
Alexandri and Jones (2008) developed a computer model to quantify the potential reduction in temperatures in numerous cities of different climates around the world. They concluded that for hot arid climates such as Riyadh, air temperatures at roof level can be reduced by an average of 12.8oC whilst canyon temperatures can be reduced by 9.1oC. It should be noted that these results were obtained for the average hottest day of the year. Whilst their results reveal the general cooling effects of vegetation in cities, there are limitations to their methods and model. Their model assumed that all horizontal and vertical building surfaces (walls and roofs) would be covered with vegetation, something that in reality would be unfeasible. In addition, the model was two-dimensional and based on assumptions of varying canyon widths and heights that are unlikely to closely resemble those of the actual cities. This means that whilst their results are useful for showing general trends, such as where placing vegetation has the maximum effect in UHI mitigation, the actual numerical values should be viewed with caution.
The reduction of the UHI effect with green roofs is hard to predict accurately at present. This is because there are a vast number of variables involved; each city has a different climate, geometries, land uses, building types and so on, all of which require extensive research. However, it is clear from the literature that green roofs offer a large scope for vastly reducing the UHI effect. With reduced city centre temperatures follow the benefits of reduced cooling energy requirements in cities, improved air quality and a reduction in CO2 emissions at power plants (Akbari 2002). This means that improvements are made on the economic, social and environmental fronts.
Air pollutants are removed by the high surface area and roughness provided by the branches, twigs, and foliage of plants. As vegetation reduces the urban temperatures, so photochemical reactions are slowed down and this leads to less secondary air pollutants, such as ozone. (Yang, Yu et al. 2008). Yang et al attempted to quantify the air pollution removal of green roofs in Chicago by using a dry leave deposition model. Their results showed that green roofs in Chicago removed 1,675kg of pollutants, with the potential to remove 2,000 metric tonnes if all roofs tops were greened. However the cost of doing this was prohibitive at $35.2 billion. The quantities of air pollutants removed by green roofs are not well documented in the literature so validation of these results is difficult. However, there is literature on the air pollution removal of general urban vegetation which shows comparable reductions. Nowak, Crane et al. (2006) estimated that urban trees at present remove 711,000 metric tonnes of air pollution (including O3, NO2, SO2, CO2) annually in the USA. This is a considerable amount and is of particular benefit as these pollutants are removed directly from the urban environment, thus directly “cleaning up” city air. The cost to society of this carbon would have been $3.8 billion.
Niachou, Papakonstantinou et al. (2001) explain that as space at ground level becomes increasingly sparse and valuable, turning roofs green could become a significant source of urban greenery. In fact planted roofs have become the only promising and stabilising choice if we are to find space to add greenery to cities in order to mitigate dangerous and uncomfortable urban heat island effects (Kumar and Kaushik 2005). It could be argued that whilst, the installation of green roofs cannot be justified on their ability to remove air pollutants alone, when considered with their additional environmental benefits, their implementation should be encouraged.
Green roofs can provide green islands that if well planned, can cater for a variety of flora and fauna unattainable on traditional roofs. There are examples of green roofs that have created a habitat for endangered species, for example the redstart in the London area (CIBSE 2007). They can also provide islands and corridors for wildlife in areas of limited biodiversity such as towns and cities. Green roofs may function as 'stepping stone' habitats connecting isolated habitat pockets to promote urban biodiversity (Schrader and Böning 2006).
 Psychology and health
There are clear health benefits arising from green roofs ability to reduce urban air pollutants. Additionally they provide opportunities in relation to health care environments. Studies summarised in Wong, Tay et al. (2003) have referenced research that has shown patients' recovery rate can be faster where they have a view to a landscaped setting as opposed to a view of adjacent buildings. One of Ulrich's (2000) three general design guidelines for creating supportive healthcare environments is that they should “provide access to nature and other positive distractions”. Roof gardens provide an ideal way of providing this and in doing so have the ability to reducing patients' recovery times.
 Wider advantages
There are also related community wide economic and social benefits. For example with a reduction in runoff due to green roofs, flooding in an area may become less frequent with resultant cost savings, and social benefits. Whilst the direct benefits of green roofs are well documented in the literature, numerous secondary benefits which could be of much greater significance are less well assessed. Such financial and social benefits are hard to predict and quantify; however they may be greater than the benefits to their individual buildings. This however, requires a systematic, collaborative and collective approach to roof top greening across communities that without common drivers and incentives is difficult to achieve.
Other more tangible economic, social and environmental benefits come from the decreased maintenance and replacement cost savings of green roofs. This is due to the reduced temperature fluctuations on green roofs and the protection of the water proof membrane from ultraviolet (UV) radiation. The result is a prolonged lifespan, which Wong et al (2003) agues can be a minimum of threefold if installed correctly. However, more typically quoted figures are more conservative stating life extensions of the waterproofing membrane of 200% (Carter and Keeler 2008). A longer service life of roof systems means that maintenance and replacement are less frequent and so costs are reduced. Green roofs also offer other economic benefits that are associated with typical sustainable building design which include; increased property values, increased marketability of a property and business-related cost savings (Wong, Tay et al. 2003; Kibert 2008)
There is a concern that if a membrane were to fail, dealing with a leak could be an expensive and complex process. However, Modern lightweight modular green roof systems include products that are made up of 0.5 m module trays which slot together for a seamless finish and provide plant drainage. These allow the building owner to create either a sedum roof or a bespoke horticultural selection of sedum, indigenous grasses and wildflowers. The modules are installed over a geotextile filter fabric, which sits on top of the waterproofing. They interlock and need no fixing, so there is very little danger of puncturing the waterproof membrane.
With so many benefits to green roofs, it may be surprising that they have not become more widespread in the UK, as they are for instance in Switzerland and Germany. Certainly their use is being encouraged, with major cities including London and Manchester writing policies to encourage their adoption. However, there are some aspects that are less desirable than traditional flat roofs.
- Increased capital costs.
- Increased structural loads.
- Specialist contractors required.
- Maintenance requirements.
- The lack of quantifiable data on the benefits of green roofs.
- The lack of technical information about how to build them.
- Lack of incentives.
 Increased capital costs
Green roofs have higher capital costs than their traditional counterparts. This is particularly true in the UK as they are relatively uncommon at present. Capital costs for extensive green roofs are generally between 150-200% more expensive that traditional black roofs. Intensive roofs are around 200% more expensive; however this does not include the cost of the stronger structure that is likely to be required to take the increased loads. The high initial investment in green roofs is a barrier to a widespread use and much would be gained if extensive green roof systems could be installed at a lower cost (Emilsson and Rolf 2005).
Emilsson and Rolf (2005) looked into the different establishment methods of thin extensive green roofs. They looked at establishment methods for extensive green roofs in Sweden which are dominated by prefabricated mats, which is generally one of the most expensive ways of vegetating a building. They are however, low risk as they ensure instant high plant cover. The cost of the vegetation mats in Emilsson and Rolf's (2005) study was twice that of shoot establishment and close to 30% more expensive than plug plant establishment. This shows that the type of establishment can have significant impact on the capital costs. The development of reliable onsite methods that establish high plant cover could be a way of reducing capital costs and so increase the common uptake of extensive vegetated roofs.
Extensive green roofs typically add between 50 and 200kg/m2 of loading to the roof of a structure. This increase in load has to be taken by the structure of the building. Existing flat roofs however, often require no additional structural support for extensive green roof installation (Carter and Keeler 2008). In some countries this has lead to relatively common retrofitting of extensive vegetated roofs to existing structures (Kosareo and Ries 2007). However, intensive green roofs with their greater substrate depth and provision for foot traffic require stronger structures to support their increased deadweight and live weight.
Green roofs are currently not common in the UK, and thus widespread contractor knowledge is limited. Specialist contractors should be used in construction and this is probably one of the primary causes of the increased capital costs.
 Perceived requirement for increased maintenance
Maintenance of green roofs is a contentious issue. Intensive green roofs require increased regular pruning, feeding, weeding and watering as they are essentially a garden. Consequently there is no doubt that they need an increased amount of regular maintenance. Maintenance of extensive vegetated sedum roofs however is relatively low. This is about the same a traditional roof of visual inspections every six months. Thus no increased maintenance is required in the case of extensive vegetated roofs. In fact with the increased lifespan of a green roof (approximately double that of a tradition roof) the number of times the roof has to be repaired or replaced is halved. Thus actually reducing the maintenance requirements and costs.
 Green walls
Some benefits of green roofs can be extended to green walls. Increasingly popular, green walls give enormous visual pleasure, and can help to 'humanise' long stretches of wall that may otherwise be blank. They can take the eye from the mega-scale of a giant construction down to a much more intimate scale.
Image: Living wall at Cheshire Oaks M&S (Image supplied by Simmons)
Research at the University of Texas shows that people who work in offices with plants are significantly happier. And green walls have other quantifiable benefits. They help to remove dust and pollution from the air, and can make a considerable contribution to reducing the urban heat island effect. Green walls do need irrigation, not a substantial amount, as much of the water can be recycled, but the system does need to keep operating.
Although widely used in other countries, particularly in France, green walls are still a relatively new concept in the UK.
A roof can also contribute to the energy needs of a building, by using photovoltaics to generate electricity. The decision to generate electricity from a roof need not rule out the possibility of also having a green roof in an adjacent area. Indeed, research shows that the two are complementary, since the cooling effect of the planting increases the efficiency of the photovoltaics.
Green roofs can be used to for rainwater harvesting, collecting rainwater for non-potable uses. This can play a key role in sustainable urban drainage schemes. Typically, a green roof will intercept at least the first 5mm of rain in any shower, reducing run off to drains.
The volumes collected will be lower than for conventional roofs (small rain events are likely to result in no run-off), and some substrates may be more suitable than others, specifically those with a high mineral content.
Leached nutrients, vegetative matter, sediments and organic load (including bacteria) mean that the quality is lower than water harvested from a 'normal' roof. Fertilizers that may cause contamination should be avoided. The main concern however is the brownish colour that rainwater collected from a green roof tends to acquire. For this reason it is recommended that the collected water is used outside buildings, ie for irrigation, where discolouration is not so much of an issue.
This article was originally created by -- SIG Design and Technology 15:35, 3 May 2013 (BST)
It has been significantly extended and developed by --Buro Happold 14:40, 23 July 2013 (BST)
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 External references
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- Alexandri, E. and P. Jones (2008). "Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates." Building and Environment 43(4): 480-493.
- Barrio, E. P. D. (1998). "Analysis of the green roofs cooling potential in buildings." Energy and Buildings 27(2): 179-193.
- Carter, T. and C. R. Jackson (2007). Vegetated roofs for stormwater management at multiple spatial scales. 80: 84-94.
- Carter, T. and A. Keeler (2008). "Life-cycle cost-benefit analysis of extensive vegetated roof systems." Journal of Environmental Management 87(3): 350-363.
- CIBSE (2007). Green roofs. Plymouth.
- Emilsson, T. and K. Rolf (2005). "Comparison of establishment methods for extensive green roofs in southern Sweden." Urban Forestry & Urban Greening 3(2): 103-111.
- Getter, K. L. and D. B. Rowe (2006). "The role of extensive green roofs in sustainable development." Hortscience 41(5): 1276-1285.
- Getter, K. L., D. B. Rowe, et al. (2007). "Quantifying the effect of slope on extensive green roof stormwater retention." Ecological Engineering 31(4): 225-231.
- Kibert, C. (2008). Sustainable construction :green building design and delivery, Wiley.
- Kosareo, L. and R. Ries (2007). "Comparative environmental life cycle assessment of green roofs." Building and Environment 42(7): 2606-2613.
- Kumar, R. and S. C. Kaushik (2005). "Performance evaluation of green roof and shading for thermal protection of buildings." Building and Environment 40(11): 1505-1511.
- Mentens, J., D. Raes, et al. (2006). "Green roofs as a tool for solving the rainwater runoff problem in the urbanized 21st century?" Landscape and Urban Planning 77(3): 217-226.
- Newton, J., D. Gedge, et al. (2007). Building Greener: Guidance on the use of green roofs, green walls and complementary features on buildings. London, CIRIA.
- Niachou, A., K. Papakonstantinou, et al. (2001). "Analysis of the green roof thermal properties and investigation of its energy performance." Energy and Buildings 33(7): 719-729.
- Nowak, D. J., D. E. Crane, et al. (2006). "Air pollution removal by urban trees and shrubs in the United States." Urban Forestry & Urban Greening 4(3-4): 115-123.
- Ramachandran, V. S., R. M. Paroli, et al. (2002). Roofing Materials. Handbook of Thermal Analysis of Construction Materials. Norwich, NY, William Andrew Publishing: 611-632.
- Sailor, D. J. (2008). "A green roof model for building energy simulation programs." Energy and Buildings 40(8): 1466-1478.
- Schrader, S. and M. Böning (2006). "Soil formation on green roofs and its contribution to urban biodiversity with emphasis on Collembolans." Pedobiologia 50(4): 347-356.
- Spala, A., H. S. Bagiorgas, et al. (2008). "On the green roof system. Selection, state of the art and energy potential investigation of a system installed in an office building in Athens, Greece." Renewable Energy 33(1): 173-177.
- Takakura, T., S. Kitade, et al. (2000). "Cooling effect of greenery cover over a building." Energy and Buildings 31(1): 1-6.
- Theodosiou, T. G. (2003). "Summer period analysis of the performance of a planted roof as a passive cooling technique." Energy and Buildings 35(9): 909-917.
- Wolf, D. and J. T. Lundholm (2008). "Water uptake in green roof microcosms: Effects of plant species and water availability." Ecological Engineering 33(2): 179-186.
- Wong, N. H., S. F. Tay, et al. (2003). "Life cycle cost analysis of rooftop gardens in Singapore." Building and Environment 38(3): 499-509.
- Yang, J., Q. Yu, et al. (2008). "Quantifying air pollution removal by green roofs in Chicago." Atmospheric Environment 42(31): 7266-7273.
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