GREEN ROOF BENEFITS 15
Greenroofs feature vegetation on the rooftops of buildings, and they havebecome a primary element in most urban areas. The increase in thenumber of buildings in the majority of the large cities has alteredthe urban ecosystem, resulting in environmental problems. Theconstruction of buildings also degrades the natural environment, andthis changes the ecosystem of an area. However, green buildings aresustainable, and they have a variety of economic, environmental, andsocial benefits. This review will focus on the environmental aspectof green roofs’ sustainability, but it will also address some ofthe social and economic benefits of designing rooftops withvegetation cover. Most of the sources used in this review concludethat the technology employed in green roofs creates an ecosystem withproperties that are similar to the vegetation at the ground level.
Greenroofs are common in many parts of the world, and their popularity isalso increasing due to their environmental benefits.Berardiet al. (2014) evaluated different types of green roofs and theirsignificance in environmental sustainability. The construction ofbuildings and their maintenance consumes high amounts of energy, anda significant level of greenhouse gas emissions comes from thestructures as seen in the study by Berardi et al. (2014). The mannerin which buildings change the ecosystem of urban areas indicates thatenvironmental issues are likely to arise due to the presence of manystructures, and this illustrates the importance of green roofs.
Althoughthe design of green roofs is a modern concept, its variations havealways been used by different societies for centuries, as Berardi etal. (2014) explain. For instance, roof gardens were a common featureof the Roman architecture, and Northern European nations used them toenhance a building’s thermal insulation. Most urbanized areas donot have green spaces, and green roofs offer a solution to suchproblems. Berardiet al. (2014) classify green roofs into two groups, where asignificant amount of soil is necessary for an intensive green roofwhile less medium is required in an extensive living roof.
Greenroofs have various environmental benefits, and it is important toconsider some of the ways in which urban greenery improves theecosystem. Madumadhi, Radhakrishnan, and Shandhipriya assessed theindoor thermal performance of Indian residential structures that hadgreen roofs, with the area of study having a warm and humid climate.When comparing white reflective and green roofs, Madhumadhi,Radhakrishnana, and Shandhipriya, (2016) found that the latter hasvarious benefits such as insulation, shading, and evapotranspiration,and this improves a roofing system’s thermal performance. Thevegetation on a green cover provides shade to the exposed surface,hence reducing its temperature. These environmental sustainabilityaspects evidently improve the comfort of the internal spaces inbuildings located in areas with extreme temperatures.
Madhumadhi,Radhakrishnana, and Shandhipriya (2016) evaluated the energyefficiency and thermal performance of green roofs in India, andwhether they have had an impact on the urban ecosystem. The design ofa building’s roof determines the rate at which it absorbs sunlight.When a roof is flat and made up of concrete, it has the potential oftaking in most of the incident sunlight, and this heat is transmittedto the building’s interior. Besides,roofs that are artificially designed to reflect sunlight lose theirreflectivity with time due to their exposure to the elements, andthis reduces a building’s energy efficiency. Getter and Rowe (2006)also suggested that the insulation that green roofs provide on thesurface mitigates the urban heat island effect, which saves largeamounts of power.
Thevegetation on green roofs shades the surface exposed to the sun,hence lowering the temperature that the concrete absorbs. Liu andBaskaran (2003) also suggested that this is true. In the study wherethe two researchers observed an experimental Field Roof Facility,they found that the temperature fluctuations on the membrane reducedas a result of the presence of vegetation. Even if one artificiallydesigns a building’s rooftop to reflect sunlight without thepresence of vegetation, it might not be efficient. The study by Liuand Baskaran indicates that the surface temperature of an exposedmembrane rises due to the absorbed radiation, and it significantlydrops after re-radiation. With time, the efficiency of such a roofreduces because of the temperature fluctuations.
Althoughthe study conducted by Madumadhi, Radhakrishnan, and Shandhipriyasuggested that a green roof lowers the temperature absorbed by aconcrete rooftop, it would be important to understand how thisoccurs. After assessing the energy efficiency and thermal performanceof green roofs in the experimental facility, Liu and Baskaran (2003)found that vegetation decreases heat flow through insulation,evaporative cooling, and insulation. The membrane of an exposed roofexperiences positive heat flow due to the absorption of solarradiation and re-radiation during the day and night respectively. Thegreen roof abates the movement of heat between a building and theenvironment, and this regulates the room temperature when it is hot.Oberndorfer et al. (2007) found that the longevity of the membrane inan exposed roof deteriorates because it becomes brittle due to itssubjection to ultraviolet light.
Urbanenvironments have buildings and road surfaces that are impervious anddark, creating the heat island effect, as Oberndorfer et al.postulate. The conditions brought about by the lack of vegetationresult in an increase in the temperatures, especially during thenight. Getter and Rowe (2006) found that green roofs alleviate theeffects of the urban heat island because of the shade and insulationthat they provide. According to these researchers, the indoortemperatures of buildings that have green roofs are lower by 3oCto 4oC.Oberndorfer et al. (2007) theorize that green roofs alleviate theincrease in temperatures at night as a result of theevapotranspiration from the vegetation cover.
Mosturban areas have surfaces that are impervious, and this increaseswater runoff as a consequence of the decreased infiltration. As aresult, various environmental issues such as water pollution in theurban areas arise when the storm water runoff increases. Li and Yeung(2014) evaluated different types of green roofs to determine theirperformance in a maritime monsoon climate zone. The drainage layer ofa green roof affects the runoff dynamics and water retentioncapacity. Li and Yeung found that the vegetated rooftops retain 40%to 60% of the rainwater, and when the rain was lower than 10mm, therewas no runoff since the roof would keep all the water. However, thetype of medium used in making the green roof determines the amount ofwater it retains. In the findings that Oberndorfer et al. (2007)present, green roofs whose substrate was more than 10 cm keep 66% to69% of water.
Wachtel(2007) evaluated some of the characteristics of a green roof thatenable it to retain water and reduce surface runoff. The study foundthat the feature that increases the effectiveness of green roofs inthe management of stormwater is their ability to hold and releasewater at a particular rate. Besides, the medium used to make a greenroof enables it to support vegetation cover, and the resultingsurface does not leak as much as the conventional rooftops. Wachtelalso suggests that the ability of a green roof to retain water duringheavy rainfall modulates the intensity of runoff, and this conservesthe water from precipitation.
Gibbset al. (2006) similarly argue that the absence of a green roofresults in an increase in the quantity of water that enters stormdrains, resulting in water pollution and a surge in the runoff. Intheir study evaluating the impact of different fertilizer treatmentsand growing mediums on the green roof systems, Gibbs et al. (2006)found that the planting the right species and dressing them withfertilizer results in an optimal reduction of storm water runoff froma building with a green roof. Whittinghill et al. (2015) postulatethat factors such as microclimatic conditions and the plants’ wateruse efficiency influence the stormwater retention capacity.
Stormwatermanagement facilities in many urban areas might struggle to deal withthe heavy runoff that results from the nonporous surfaces ofbuildings. However, Oberndorfer et al. (2007) present findingsindicating that implementing the conventional techniques of managingstorm water in dense urban areas is challenging. The high level ofpollutants in urban areas also has a detrimental effect on theecosystem, and Oberndorfer et al. theorize that this is anotherreason as to why green roofs are important. The retention of water bygreen roofs ensures that there is a delay in the runoff and thatprecipitation returns to the atmosphere through evapotranspiration.In the review where Getter and Rowe (2006) present the sustainabilitydimensions of green roofs, they found that runoff delays occur sincethe media does not become saturated immediately after it startsraining.
Theconservation of the biodiversity is another aspect that makes greenroofs suitable especially in urban areas, as Williams, Lundholm, andMcIvor (2014) theorize in their study. While assessing thebiodiversity conservation benefits of green roofs from differentprojects, the three researchers found that vegetated rooftops providehabitats for various species of animals, and they help in theachievement of urban biodiversity conservation goals. However, whilea high number of species colonize green roofs compared to the othertypes of rooftops, they are vertically isolated from the ground-levelhabitats, and the highly mobile organisms are limited. Behm (2011)lists the design suggestions for green roofs and suggests that forthe vegetated roofs to be well maintained, they should not beaccessible. Taking this approach ensures that the green roofsconserve biodiversity.
Otherfindings from the study conducted by Williams, Lundholm, and McIvorindicate that green roofs create a natural habitat, and this issomething that cannot happen on the conventional rooftops. Williams,Lundholm, and McIvor found that different plant taxa colonize greenroofs due to the availability of media that supports vegetationcover. Li and Yeung (2014) also suggested that introducing greenroofs enriches the urban biodiversity. In their study, Li and Yeungrevealed that they had observed more than 30 species on green roofs.In addition, they also proposed that vegetated rooftops promote urbanbiodiversity by bringing together the fragmented habitats. Similarly,Getter and Rowe (2006) postulated that the inaccessibility of greenroofs to the public makes it possible for different species of birds,insects, and microorganisms to have an undisturbed habitat.
Theother environmental sustainability dimension of green roofs, whichGetter and Rowe (2006) posit in their study, is that they reduce airpollution. Air pollution is prevalent in the majority of urban areas,and it poses health risks such as respiratory problems to theresidents. In the study where Getter and Rowe evaluated the role ofgreen roofs in environmental sustainability, they found out that thevegetation on rooftops filters out the gaseous pollutants andparticulate matter. Plant tissues also absorb some of thesecontaminants, and the movement of rainwater washes away the rest ofthe particles. Oberndorfer et al. (2007) found that intensive roofgardens can act as carbon sinks and they trap different types ofcontaminants from the air as well.
Thesocial sustainability dimension of green roofs also has to beconsidered, and this will illustrate the manner in which designingrooftops that support vegetation directly affects people’s lives.There is a high level of noise in the urban environment, which is afactor that brings about negative psychological effects on the peoplethat reside in such areas. In their systematic review of theeffectiveness of green roofs as a psychological shield for theadverse impacts of noise pollution, Dzhambov and Dimitrova (2014)found that they reduce people’s negative perception of excessivesound. When noise exceeds a certain level, it negatively affects thecomfort of hearing. Dzhambov and Dimitrova postulated that cognitiveand physiological alterations, psycho-social stress, and sleepdisturbances result from noise pollution, and they adversely affectsan individual’s quality of life.
Veistenet al. (2014) provided an overview of the noise attenuation measuresassociated with green roofs, and they postulated that vegetationdiffuses, and it also absorbs sound. The noise level reduces afterthe waves strike the vegetation cover and they are reflected back.Similarly, a façade takes in sound better when there is vegetationon it, and Veisten et al. (2014) theorize that this is because of themultiple reflections of the waves. Berardi et al. (2014) conducted anempirical analysis to find out the transmission loss (TL) of soundthat results when a green roof is present. In that study, Berardi etal. found that green roofs reduce the transmission of sound at lowand mid frequencies from 5 to 13 decibels, and in high frequencies,the transmitted sound reduces from 2 to 8 decibels.
Whilegreen roofs have the potential of reducing noise pollution, it isimportant to understand why they are better than other alternativemethods. Dzhambov and Dimitrova (2014) found that when one comparesgreen roofs with materials such as metal, concrete, and plastic, theyare cheaper and more natural. Besides, vegetation has an impact onthe manner in which people perceive sound in urban settings, as itappears to alter its physical properties. Tolderlund (2010) also hada similar opinion on this issue with the suggestion that a buildingcan conform to the appropriate sound quality levels by having a greenroof as it reduces the noise levels by 40-60 decibels. Factors suchas the plant type, thickness of the vegetation, and the growingmedium influence the noise reduction capabilities of the green roof.
Theability of green roofs to affect the mental health of individuals isanother social sustainability dimension that requires investigation.Gascon et al. (2015) assessed the relationship between green spacesand mental health, and they found a positive correlation. In theirsystematic review, these researchers found that the urban environmentcontributes to the development of mental health disorders. Some ofthe mechanisms suggested to be an influence of green spaces to themental health of a person include the healthy environment associatedwith them, as well as their intrinsic qualities that promotewell-being.
Thereduction of the total area under vegetation in dense urban centersis a cause for concern, as the available green spaces are limited.Getter and Rowe (2006) found that the presence of greenery in aperson’s surroundings is beneficial to the health of thatindividual. Tolderlund (2010) also theorized that when people havevisual access to nature, their stress levels decrease. However,Getter and Rowe (2008) suggested that the aesthetic appeal that comesabout due to the presence of a green roof depends on the type ofplants that one selects. The survival of a plant species chosen for agreen roof depends on factors such as irradiance levels, rainfalldistribution, and the wind.
Greenroofs also have the social benefit of promoting urban agriculture inareas where food products have to be transported from far, asTolderlund (2010) explains. The location of many farms is far fromthe cities, and engaging in agriculture in the urban areas is quitedifficult. Berardi et al. (2014) postulate that urban agriculture isenhanced through the installation of green roofs, as they are mostlysuitable for vegetable production. Tolduerlund (2010) also suggeststhat cities can become independent of the current food distributionmodel and that restaurants can use green roofs to showcase theircommercial ventures in the kitchen gardens.
Apartfrom the environmental and social sustainability dimensions of greenroofs, several studies also indicate that incorporating vegetationcover on rooftops has economic benefits (Veisten et al., 2012Porsche and Kohler, 2013). Veisten et al. (2012) evaluated theeconomic sustainability dimension of green roofs found that theaesthetic appreciation of green roofs results in an increase in thevalue of property by 5.8 pounds per year. On the other hand, the factthat the green roof’s vegetation reduces noise levels leads to anincrease in the value of property by 51.03 pounds per annum. Porscheand Kohler also support the argument that properties with green roofshave a higher value than those without the vegetated rooftops. Theowners of buildings with green roofs charge their tenants higherrents since people can use the rooftops as recreation areas, and suchstructures have lower stormwater fees.
Althoughthe cost of installing a green roof could be more expensive, they aremore durable. Porsche and Kohler (2013) conducted a study evaluatingthe life cycle costs of different types of roofs, and their findingssupport this hypothesis. The two researchers found that in Germany,the cost of installing a green roof is twice that of a conventionalroof, and it is three times more than those that are welded orprefabricated. Francis and Lorimer (2011) hypothesize that the highcost of installing green roofs, which goes up to $150 per squaremeter, is one of the socioeconomic barriers to their utilization.However, the potential benefits of green roofs outweigh the drawbackssuch as the initial expenses that one incurs while installing them.Besides, the protection of the waterproofing membrane, which is alsoan economic benefit, make their use more attractive. Tassicker,Rahnamayiezekavat, and Sutrisna (2016) theorized that green roofshave lower maintenance costs in the long-term, and this is theprimary reason as to why they are economical.
Severalstudies illustrate that another economic benefit of green roofs isthe reduction in the energy use in a building (LaRoche & Berardi,2014 Berardi et al., 2014 Tolderlund, 2010). Berardi et al. (2014)found that the energy consumption in a building that has a green roofsignificantly reduces in cold, as well as warm climates, but theirimpact is higher in non-insulated buildings. LaRoche and Berardi(2014) also indicated that in Greece, the energy that a buildingrequires for cooling reduces by 2% to 48%, as the indoor temperaturesreduce by 4K. In the Mediterranean coastal climate, green roofs havethe potential of reducing the energy utilized in cooling by 60%.
Thereviewed literature has shed light on the advantages of green roofs,and it is apparent that more buildings have to introduce vegetationcovers to their rooftops so as to reap these benefits. Green roofshave various benefits such as insulation, shading, andevapotranspiration, and this improves a roofing system’s thermalperformance. Other studies have also shown that green roofs create anatural habitat, hence promoting urban biodiversity by bringingtogether the fragmented habitats. On the social sustainabilitydimension, it is clear that green roofs reduce people’s negativeperception of noise. The findings should be an encouragement to cityplanners to promote the installation of more green roofs. Additionalresearch should assess the types of plants that are the best forenvironmental sustainability.
Behm,M. (2011). Safe design suggestions for vegetated roofs. Journalof Construction Engineering and Management,138(8), 999-1003.
Berardi,U., GhaffarianHoseini, A., & GhaffarianHoseini, A. (2014).State-of-the-art analysis of the environmental benefits of greenroofs. AppliedEnergy,115, 411-428.
Dzhambov,A. M., & Dimitrova, D. D. (2014). Urban green spaces`effectiveness as a
psychologicalbuffer for the negative health impact of noise pollution: Asystematic review. Noiseand Health,16(70), 157.
Francis,R. A., & Lorimer, J. (2011). Urban reconciliation ecology: thepotential of living roofs and walls. Journalof Environmental Management,92(6), 1429-1437.
Gascon,M., Triguero-Mas, M., Martínez, D., Dadvand, P., Forns, J.,Plasència, A., &
Nieuwenhuijsen,M. J. (2015). Mental health benefits of long-term exposure toresidential green and blue spaces: a systematic review.International journal of environmental research and public health,12(4), 4354-4379.
Getter,K. L., & Rowe, D. B. (2008). Selectingplants for extensive green roofs in the United States.Michigan State University Extension.
Getter,K. L., & Rowe, D. B. (2006). The role of extensive green roofs insustainable development. HortScience,41(5), 1276-1285.
Gibbs,J., Luckett, K., Jost, V., Morgan, S., Yan, T., & Retzlaff, W.(2006, June). Evaluating
performanceof a green roof system with different growing mediums, Sedum speciesand fertilizer treatments.In Proc. of Midwest Regional Green Roof Symposium, Edwardsville.
Li,W. C., & Yeung, K. K. A. (2014). A comprehensive study of greenroof performance from
environmentalperspective. InternationalJournal of Sustainable Built Environment,3(1), 127-134.
Madhumathi,A., Radhakrishnan, S., & Shanthipriya, R. (2016). ThermalPerformance
Evaluationof Green Roofs in Warm Humid Climates: A Case of ResidentialBuildings in Madurai, India.In Key Engineering Materials (Vol. 692, pp. 82-93). Trans TechPublications.
Oberndorfer,E., Lundholm, J., Bass, B., Coffman, R. R., Doshi, H., Dunnett, N.,Gaffin, S.,
Köhler,M., Liu, K., & Rowe, B. (2007). Green roofs as urban ecosystems:ecological structures, functions, and services. BioScience,57(10), 823-833.
LaRoche, P., & Berardi, U. (2014). Comfortand energy savings with active green roofs. Energy and Buildings,82, 492-504.
Liu,K., & Baskaran, B. (2003, May). Thermalperformance of green roofs through field
evaluation.In Proceedings for the First North American Green Roof InfrastructureConference, Awards and Trade Show, 1-10.
Porsche,U., & Köhler, M. (2013). Lifecycle costs of green roofs.World Climate & Energy Event.
Tolderlund,L. (2010). Design guidelines and maintenance manual for green roofsin the semi- arid and arid west. GreenRoofs for Healthy Cities.
Tassicker,N., Rahnamayiezekavat, P., & Sutrisna, M. (2016). AnInsight into the Commercial Viability of Green Roofs in Australia.Sustainability,8(7), 603.
Veisten,K., Smyrnova, Y., Klæboe, R., Hornikx, M., Mosslemi, M., & Kang,J. (2012).
Valuationof green walls and green roofs as soundscape measures: Includingmonetised amenity values together with noise-attenuation values in acost-benefit analysis of a green wall affecting courtyards.Internationaljournal of environmental research and public health,9(11), 3770-3788.
Wachtel,J. (2007). StormWater management, Green Roof Style.Biocycle.
Whittinghill,L. J., Rowe, D. B., Andresen, J. A., & Cregg, B. M. (2015).Comparison of
stormwaterrunoff from sedum, native prairie, and vegetable producing greenroofs. UrbanEcosystems,18(1), 13-29.
Williams,N. S., Lundholm, J., & MacIvor, J. (2014). FORUM: Do green roofshelp urban biodiversity conservation?. Journalof Applied Ecology,51(6), 1643-1649.