Cities face a number of environmental challenges, such as the heat island effect, air pollution and stormwater runoff.
Green roofs, also known as green roofs or green roofs, offer a sustainable and effective solution to address these problems.
In this article, we will take an in-depth look at the benefits of green roofs at both urban and building user level, providing detailed information and concrete examples.
BENEFITS AT URBAN LEVEL
Reduction of the heat island effect:
Green roofs, when installed on urban building structures, play a key role in reducing the heat island effect in cities.
The benefits they provide in terms of regulating ambient temperature and trapping dust particles and pollutants are detailed below.
During the summer months, urban areas can reach temperatures 1 to 4.5°C above those in their peripheral areas, a phenomenon known as the heat island effect.
The gradient varies, depending on climate, topography and urban design.
One of the main reasons for the heat island phenomenon is the large built-up areas in cities, where solar radiation is absorbed and accumulated in the form of heat that is slowly released during the night hours.
This phenomenon can be quantified with parameters such as albedo, which refers to the amount of radiation reflected from a surface in relation to the total amount received.
It is expressed in %, being the lowest percentage of reflection, and therefore the one that leads to the greatest absorption of heat by the surface.
The albedo of a roof with an asphalt finish is approximately 8%, while that of a green roof is 25%.
On the other hand, green roofs, in addition to having a greater capacity to reflect solar radiation, also cool the air on their surface in a process known as transpiration, where the plant absorbs moisture from the earth and expels it in the form of water vapour.
These two qualities together allow the temperatures reached on a green roof to be lower than those of a conventional roof.
While an asphalt roof can reach 70°C on a summer’s day, a green roof surface does not usually exceed 26°C.
The Environment Agency of Canada conducted a study which concluded that planting at least 6% (6.5 million square metres) of Toronto’s roof surfaces could reduce the city’s summer air temperature by 1 to 2°C.
Regulation of ambient temperature
The vegetation present in green roofs acts as a natural temperature regulator, providing a cooler and more pleasant environment in urban environments.
The vegetation layer helps to mitigate the accumulation of heat in urban surfaces, contributing to reducing the ambient temperature and improving the thermal comfort of the inhabitants.
Regulation of the level of water drainage in the event of heavy rainfall:
Green roofs play a key role in the efficient management of rainwater, offering benefits for both the environment and urban sustainability.
Runoff reduction and flood prevention.
- Vegetation on roofs acts as a natural rainwater absorption and retention system, reducing the amount of runoff and preventing potential flooding in urban environments.
- By absorbing water, green roofs also contribute to the recharge of underground aquifers, promoting a more efficient and sustainable management of water resources.
The urbanisation of our municipalities leads to a loss of the permeability of the land, which prevents rainwater from filtering through street finishes, and although sewage systems are in place, they are often insufficient to cope with heavy downpours.
Green roofs have the property of retaining rainwater, reducing the number of litres to be evacuated when rainfall is occurring.
The storage capacity depends on the design of the roof, but in all cases the existence of these constructive solutions means a reduction in the amount of runoff water to be evacuated by the system and a reduction in the velocity, as the water is stored in the retaining layers and in the substrate.
According to the German standard DIN 1986, part 2, the rainwater drainage coefficient for landscaped surfaces with a minimum thickness of 10 cm is 0.3. This means that only 30% of the rainfall drains off and 70% is retained on the green roof or evaporates.
Filtering of pollutants and CO2-consuming substances:
Capture of dust particles and pollutants.
In addition to regulating temperature, green roofs function as natural filters, capturing dust particles and pollutants present in the air of cities. This air purification capacity helps to improve air quality in urban environments, creating a healthier and cleaner environment for the people living in the area.
Plants are able to absorb a certain amount of volatile toxic elements found in the air, converting them into organic matter.
Much urban pollution is made up of nitrogen compounds, which are a by-product of traffic and industry. Plants take up these compounds and use them as nutrients. A 60m2 green cover is capable of trapping and processing 15 kg of heavy metals (Darlington, 2001).
This quality is supported by several research studies. Bartfelder showed that in heavily polluted inner city areas, heavy metals are captured by leaves (Bartfelder and Kóhler 1986).
On the other hand, measurements on a Swiss federal road showed that a hedge 1 m high and 0.75 m wide reduces lead pollution by 50% through its filtering effect (mentioned by Lótsch 1981).
Another feature to be taken into account is that dust and dirt particles remain attached to the leaves and are then washed away by rainfall to the substrate of the canopies which also acts as a filter reducing the pollutant load reaching the sewers. 1m2 of vegetation cover traps 130 grams of dust per year (Darlington, 2001).
Finally, plants in their photosynthesis process are consumers ofCO2 and release O2 and although they also carry out the process of respiration in whichCO2 is produced and O2 is consumed, only 1/5 to 1/3 of the O2 produced by photosynthesis is consumed again, so the contribution of O2 is greater than the consumption.
1m2 of vegetation cover generates the oxygen required by one person in a whole year (Darlington, 2001).
Reduction of dust devil:
As already mentioned in previous points, green roofs considerably reduce the overheating of roof surfaces.
When conventional roof surfaces reach 60-70ºC there is a heating of the air close to the surface resulting in an upward movement of this air mass.
For a roof surface of 100 m2 the air velocity can reach 0.5 m/sec (Robinette 1972, p. 459).
This current causes dirt and dust particles deposited on streets, squares, courtyards and terraces to be blown into the atmosphere and form layers of dirt on residential areas.
Green roofs reduce this phenomenon to a large extent, as there is no draught on these green areas as there is no heating of the surface.
BENEFITS FOR THE USER OF THE BUILDING
Increased thermal insulation:
As we have seen above the surface temperature of a green roof in summer is significantly lower than that of a conventional roof, this is due, among other things, to the transpiration produced by the vegetation and the high percentage of reflection of the vegetation finish. This process is further enhanced by:
- The high heat storage capacity of the water in the plants and in the substrate.
- The wind protection on the roof surface, which prevents heat exchange by convection.
- The shade produced by the vegetation
These characteristics allow beneficial situations to be achieved both in summer (with lower temperatures compared to other types of construction solutions) and in winter (with higher temperatures).
Figures 2 and 3 show measurements carried out in September and January on an extensive roof in Kassel (Germany) with a 16 cm thick substrate.
If we look at the graph in figure 3 we see that for an outside midday temperature of 30°C the vegetation was at 23°C and under the substrate layer at 17,5°C.
In the winter measurements (fig. 4), for an outside temperature of -14°C, only 0°C was measured under the substrate layer.
Fig. 3: Graph of data taken in September 2000
Fig. 4: Graph of data taken in January 2001
In short, what we obtain with a green roof is a construction solution with a lower thermal transmittance than in the case of conventional roofs, which translates into greater insulation and therefore energy savings compared to the use of air-conditioning installations.
Increased acoustic insulation:
Different research in recent years has demonstrated the property of green roofs to improve sound insulation values compared to a conventional roof.
Comparing the sound transmission losses of various roofs with and without vegetation, reductions of between 5-20 dB were obtained (Lagstrom, 2004).
The vegetation finish (plants + substrate) reduces noise through absorption, producing a transformation of sound energy into movement and heat energy; and reflection, by creating a dispersion of the waves due to the irregular finish of this layer.
In the case of absorption, the effect of the substrate is greater than that of the plants. For a vertical sound incidence angle, the plant layer achieves only a negligible reduction of high frequency sound through absorption, whereas the soil layer, for a thickness of 12 cm, the reduction amounts to approximately 40 dB, and for a thickness of 20 cm to approximately 46 dB.
Measurements on a landscaped flat roof of a hospital in Karlsruhe (Germany) showed that traffic noise was reduced by about 2-3 dB in the immediate vicinity of the facades as a result of absorption and reflection.
Protection of the waterproof sheeting:
The durability of the waterproof sheeting, irrespective of the traditional covering layer on the roof, is limited, as it has to withstand weathering, temperature changes and chemical decomposition processes.
A vegetated finish would reduce the high temperature changes they have to withstand and therefore reduce movement due to expansion and contraction. On the other hand, the substrate protects the film from ultraviolet rays and possible biological attacks.
In Germany, according to a report on building damage by the Federal Ministry for Spatial Planning, Building and Urban Development, it was found that while 80% of flat roofs show initial damage after 5 years of construction, a green roof, with the right choice of waterproofing and good execution of the joints, has an almost endless service life.
Contribution to the service life of roofs
- Green roofs act as a protective layer that helps to preserve the waterproofing membranes of roofs, prolonging their useful life and preventing leaks or damage from external agents.
- The presence of vegetation on the structure helps to reduce the exposure of materials to direct solar radiation and inclement weather, which in turn reduces wear and tear on surfaces.
Creating sustainable and healthy environments
In addition to protecting structures, green roofs promote the creation of sustainable and healthy environments in urban settings. Other relevant aspects related to this benefit are presented below:
- The presence of vegetation on roofs favours urban biodiversity by providing habitats for insects, birds and other species, contributing to the conservation of local fauna.
- The green spaces generated by green roofs provide recreational and leisure areas for city dwellers, improving their quality of life and emotional well-being.
MORE BENEFITS…
Although in this article we have developed a few points, green roofs have many more advantages that you can discover by clicking here
- Biodiversity enhancement: Green roofs provide a habitat for a variety of plant and animal species, contributing to urban biodiversity.
- Aesthetic value: Green roofs create green spaces that are pleasing to the eye and improve the image of cities.
- Green spaces for leisure and recreation: Green roofs can be used as green spaces for leisure, relaxation and environmental education.
Green roof ‘Rizoma’ in Jávea
BIBLIOGRAPHY
- Akbari, Cooling Our Communities: An Overview of Heat Island Project Activities
- Earth Pledge. Green Roofs.
- Technical paper by Mª del Carmen Salas Sanjuán and José Luís Montero Pascual
- Green roofs, Planning, implementation, practical advice, Gernot Minke
- Cost-benefit analysis of green roofs in urban areas: case study in Helsinki
- Fig.1 Sustainable construction. From grey to green. Promoting green roofs in the City of Buenos Aires.
Additional resources and places of interest
Green roofs offer a wide range of benefits both at the urban level and at the level of the building user. Its
