The garden roof

1) Introduction: green roofs

The history of green roofing dates back to the time of the birth of the first great civilizations. We all know the most famous of the hanging gardens of Babylon of 600 BC, on which historians are still studying their real existence, but it is sure that the history of this kind of gardens has origins from Eastern civilizations of the tenth and ninth centuries BC. These renowned gardens were positioned on terraces, whose soil had to be formed by a draining layer and the total thickness was of one or two meters. The peculiarity of this structure was the ability to constantly keep the vegetation green, being the climate of the place not favorable to the growth of the plants; in fact it is assumed that the construction was designed in such a way as to let the water flow inside it through special channels. The Etruscans and Romans also used the hanging gardens, especially during the construction of the tombs: the part of the soil removed was used to cover the top with vegetation.

During the Middle Ages, the roof garden was used especially in the monasteries and fortresses of castles, and they had a function not only aesthetic, but also functional: in the case of monasteries, the gardens were used as vegetable gardens, while in the second case they were heaps of land covered with vegetation, adjacent to the walls and ramparts, with the function of attenuating blows during battles and avoiding damage to 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 buildings for residential, industrial and rural use, leading to a continuous decrease in land and consequent environmental problems. Above all, with the reduction of green areas, there has been a decrease in the water tightness of the soil and an increase in the rate of drainage of the water, causing significant damage in urbanized areas during the rainy periods. In addition, a decrease in green areas, in addition to changes in the landscape, has also led to a reduction in the ability of plants to curb 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 collective community level.

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 made up of layers of cultivation and drainage with different depths and with a plant cover. Green roofs are nowadays widely used, above all thanks to the development of interest in energy saving. This technology dates back to the beginning of the twentieth century, when the green roof began to be used, both at European and global level, 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 1900s, especially in Germany, Switzerland and Austria.
In Italy, green roofs began to be used only in the late 90s, following studies on the advantages brought to the urban ecosystem, up to the recent UNI 11235 standard "Instructions for design, execution, control and maintenance of green roofs ".

2) Advantages of green roofs

The advantages of green roofs are manifold. The aesthetic aspect is not the only positive point, but there are also favorable sides from the point of view of living comfort and the quality of the environment; The hanging garden to have spaces for outdoor activities in places that are sometimes not fully exploited and forgotten. One of the greatest advantages consists in saving energy and managing temperatures. During the winter season the green cover can mitigate the internal temperature (for example with external temperatures of -10 °, the coverage will be around -2 °); during the summer, on the other hand, it remains around 30 ° in comparison with the 80 ° of traditional roofs. All this leads to an improvement in the building's energy efficiency and protection of the waterproof layers caused by temperature changes, as well as savings from an economic point of view thanks to the efficient thermal insulation performance in winter and cooling in summer. This benefit does not bring about a climatic improvement not only indoors, but as already mentioned, even externally, given that the heat island is significantly reduced: in fact the green roofs retain 50 to 90% of the rainwater, which will then be put back into circulation in the environment thanks to evaporation; in this way the air becomes more humid and the sensation of dust and heat typical of the heat islands decreases. Another advantage is water saving: the green roof allows the accumulation of rainwater, through the absorption and filtration of the vegetation.

The accumulation of water in panels is an excellent economic system for collecting water, which is retained in tanks, and therefore can be reused.The garden roof contributes to the reduction of sound pollution, since this type of cover absorbs sound waves, mitigating their diffusion: the substrate absorbs sounds at low frequencies and vegetation assimilates waves at high frequencies. The green roof also helps in the absorption of electrosmog: the substrate of a garden accumulates over 90% of the emissions of the cellular mobile network and two-way radios.