The roof garden
Advantages - Types - Composition
1) Introduction: Green roofs
The history of green roofs dates back to the birth of the first great civilizations. We all know the most famous of the hanging gardens of Babylon from 600 BC, whose real existence historians are still studying, but it is certain that the history of this type of garden has its origins in oriental civilizations of the XNUMXth and XNUMXth centuries BC.
These renowned gardens were positioned on terraces, whose soil had to be made up of a draining layer and the total thickness was one or two meters. The peculiarity of this structure was the ability to constantly keep the vegetation green, since the climate of the place was not favorable to the growth of plants; in fact, it is assumed that the construction was designed in such a way as to make water flow inside it, through special channels. The Etruscans and Romans also used hanging gardens, especially during the construction of tombs: the part of the removed soil was used to cover the upper part with vegetation.
During the Middle Ages, hanging gardens were used especially in monasteries and castle fortresses, and had not only an aesthetic but also a functional function: in the case of monasteries, the gardens were used as vegetable gardens, while in the second case they were mounds of earth covered with vegetation, adjacent to the walls and bastions, with the function of softening the blows during battles and avoiding damage to the structures.
From the 15th century to the 19th century, the aesthetic and ornamental function prevailed again, as we can see from some examples such as the Villa D'Este in Tivoli, the Palace of Versailles in Paris and the Belvedere gardens in the Vatican.
In recent decades, there has been a significant increase in residential, industrial and rural buildings, leading to a continuous decrease in land and consequent environmental problems. Especially with the reduction of green areas, there has been a decrease in the water retention of the land and an increase in the speed of water drainage, causing significant damage in urbanized areas during rainy periods.
Furthermore, a decrease in green areas, in addition to changes in the landscape, has also led to a reduction in the ability of plants to slow down polluting dust and produce oxygen. These reasons have led to a study of alternative technological solutions, which could reconcile the need for urban greenery at the individual level, but also at the level of the collective community.
One of the solutions adopted is the creation of green areas above the roofs of buildings: these are areas called “green roofs” or “garden roofs”.
These structures are composed of cultivation and drainage layers with different depths and with a vegetal cover. Green roofs are nowadays widely used, especially thanks to the development of interest in the issue of energy saving. This technology dates back to the beginning of the 900th century, when the green roof began to be used, both in Europe and worldwide, thanks to the new and fascinating themes proposed by Walter Gropius, Frank Loyd Wright and Le Corbusier; it had a greater development starting from the last thirty years of the XNUMXth century, especially in Germany, Switzerland and Austria.
In Italy, green roofs began to be used only towards the end of the 90s, following studies on the benefits they brought to the urban ecosystem, up until the recent UNI 11235 regulation “Instructions for the design, execution, control and maintenance of green roofs”.
2) Advantages of green roofs
The advantages of green roofs are many. The aesthetic aspect is not the only positive point, but there are also favorable sides from the point of view of living comfort and quality of the environment; the hanging garden to have spaces for outdoor activities in places that are sometimes not fully exploited and are forgotten. One of the major advantages consists in energy saving and temperature management.
During the winter season, the green roof can mitigate the internal temperature (for example, with external temperatures of -10°, the roof will be around -2°); during the summer, however, it remains around 30° compared to the 80° of traditional roofs. All this leads to an improvement in the energy efficiency of the building and protection of the waterproof layers caused by temperature changes, as well as economic savings thanks to the efficient thermal insulation performance in winter and cooling in summer.
This benefit does not only bring an improvement in the climate in the internal environments, but as already mentioned, also externally, since the heat island is significantly reduced: in fact, green roofs retain from 50% to 90% of rainwater, which will then be recirculated in the environment thanks to evaporation; in this way the air becomes more humid and the sensation of dust and heat typical of heat islands decreases. Another advantage is water saving: the green roof allows the accumulation of rainwater, through the absorption and filtration of vegetation.
Water storage in panels is an excellent economic system for collecting water, which is retained in tanks, and can then be reused. The green roof contributes to the reduction of noise pollution, since this type of covering absorbs sound waves, mitigating their diffusion: the substrate absorbs low-frequency sounds and the vegetation assimilates high-frequency waves. The green roof also helps in the absorption of electrosmog: the substrate of a garden accumulates over 90% of the emissions from the mobile cellular network and transceivers.
3) Types of green roofs
There are three types of green roofs: intensive, extensive and semi-extensive or hybrid.
The" intensive green roof” is characterized by a greater weight and must have a minimum soil thickness of 30 cm. This type of roof can be built on flat roofs with a load-bearing capacity of over 150 kg, and therefore it will be possible to install furnishing elements and a greater variety of plants and trees. Naturally, it requires a large irrigation system and constant maintenance.
The "extensive green roof” is a simpler and cheaper covering, which is suitable for both flat and sloping roofs, since its load capacity is reduced and the thickness must be between 15 and 20 cm deep. In this garden, small plants can be grown and the irrigation system and maintenance are definitely less demanding.
Finally the system “semi-extensive” or “hybrid” is a combination of the two methods described above. In fact, extensive and semi-extensive techniques can be used simultaneously, using both large plants and small trees, to create modern and sustainable hanging gardens at the same time.
4) Composition of green roofs
The composition of the hanging garden is characterized by numerous layers (from bottom to top):
– the waterproofing layer It is usually a bitumen-polymer membrane, which is placed on the roof surface, following precise steps, to avoid water infiltration; first the primer, a material made up of bitumen and additives, which serves to have greater adhesion between the waterproofing surface and the cement layer;
– the protective layer serves to protect the waterproofing layer. This is done through the use of varnishes, i.e. paints based on synthetic polymers dispersed in water;
– the drainage layer it has the function of draining rainwater and irrigation, of accumulating water for the greenery and of further protection of the waterproofing layers. It consists of a layer of rigid plastic material that collects rainwater, which then rises into the overlying soil to nourish the vegetation;
- the filter layer allows the passage of water between two adjacent layers, thus slowing down the possible passage of other elements;
– the vegetation substrate: the materials used are of volcanic origin, such as pumice stone and lava, or alternatively recycled bricks are also used, which come from the demolition of buildings: these bricks are carefully selected, crushed, rounded in order to be used in the substrate;
– the vegetation: choosing the green is not so simple, since you have to consider the major conditions that determine the characteristics of a plant, such as light, water, temperature.
5) Case studies
5.1) Studio 999 – OurSecretGarden, Turin, Italy The project was created by Studio 999, in 2010: it is a hanging garden of over 150 square meters on the roof of an apartment in the San Salvario district of Turin. The materials used are simply wood and vegetation.
The terrace is reached via an internal staircase that leads to the small glass greenhouse; the floor is made of recycled planks and the large pots house plants of different species and also a small urban garden. To realize the project, the architects designed the small garden together with the condominium members of the building, which is in fact used by everyone and also allows for excellent thermal and acoustic insulation, with a consequent saving on heating and cooling.
5.2) Emilio Ambasz – ACROS, Fukuoka, Japan
The project offers an alternative and innovative solution to one of the most widespread urban problems: promoting both the need to build and the need for green areas. The building is characterized by two different profiles, one more classic, reminiscent of an office building with simple windows, the other instead has a magnificent terraced roof that reaches a height of 60 meters and contains approximately 35.000 plants of 76 different species. These are 15 terraces, open to the public, where places for meditation and relaxation have been organized, suitable for trying to isolate oneself and escape from the noise of the city. The vegetation present in the garden not only has an insulating effect, but helps to keep the temperature of the entire building and CO2 emissions low.
5.3) Renzo Piano – Academy of Sciences, San Francisco, California
The structure appears as a glass building covered with a wavy surface of 10 thousand square meters covered with greenery; the roof is in fact the peculiarity of the building, which seems to be covered with small hills, where students can go and relax after lessons.
The guideline of the project is respect and protection of the environment, in fact during the design phase studies were carried out on the essential aspects of sustainable design. For example, 95% of the materials from the previous buildings, which were demolished, were recycled, and approximately 70% of the insulating material was obtained from the recycling of old used jeans. The green roof greatly influences the rooms of the environments below and collects approximately 13 million liters of rainwater per year, then used in the museum. On the green roof stands a large skylight that opens directly onto the central square; the roof is walkable and while walking on the roof you have the impression of riding the back of a calm dragon. In addition, the garden hosts numerous species of flora of Californian origin.
5.4) Friedensreich Hundertwasser – Hundertwasser, Vienna
The building is the work of Hundertwasser, an Austrian, and dates back to 1986. The building, which houses social housing, is characterized by sinuous lines, bright colors and a large and thick vegetation that overhangs the roof. The architect Friedensreich Hundertwasser is recognized as a precursor of sustainable architecture because he worked in the 80s, in a period in which the first environmental movements were born.
Over 900 tons of soil were used on the roof; there is also a cistern that collects rainwater, which will then be used to irrigate the vegetation. The roof garden is made of anti-root leaves, to protect the underlying floors and layers of gravel, for water drainage. The designer had also created a biological system for recovering wastewater, using the characteristics of some plants such as papyrus and reed, to purify dirty and uncontaminated water and convert it into vegetal and mineral substances.
