The roof garden (or green roof) is a green space that does not have direct contact with the natural soil; the plants, in these cases, live on a substrate reconstructed by man which provides them with anchorage and nourishment.
Leaving aside in this discussion the ancient history and the myth that surrounded these buildings (Babylonian hanging gardens), the present function of the hanging garden will be analyzed from an architectonic, urban, and environmental point of view.
The spread of green roofs is giving designers new areas to be allocated to plants that are already foreseen within the overall architectural project.
The built volumes and their relationship with the plant masses are of fundamental importance in an urban environment where greenery, very often, does not compensate for the concrete of the structures, making the city environment increasingly unlivable.
The roof garden is perhaps the best way to increase the quantity and quality of greenery within our cities, without the need to use the soil.
It thus becomes one of the most effective systems for returning, at least in part, all the land that we have subtracted from the natural environment for the construction of our spaces.
Today, technology provides methods and materials that make the roof garden accessible from an economic point of view and applicable in almost all situations, giving the possibility of installing them even on terraces and attics not designed for the purpose, from a design point of view.
The issues touched upon by the law are:
• the Green Belt, a natural green belt that should be maintained around the urban center (also including cultivated land and places for recreation);
• the transformation of solar paving into hanging gardens in cities;
• the creation of urban gardens in the areas left free from buildings (residual spaces);
• the greening of the walls (both through the insertion of climbing plants and through the new vertical green techniques), to favor a lowering of the heat emanating from the buildings.
All this can be achieved by the municipalities thanks to the possibility for private individuals to sponsor these works in exchange for visibility in documents containing institutional communications.
We hope that these indications will soon become reality and will spread in our country, becoming practice, thus allowing greenery to live close to and within our architecture and for us to enjoy its beneficial effects.
The functions of the green roof
The hanging garden has not only an aesthetic-ornamental function, it is an operation of ecological architecture that brings numerous advantages to the building and the environment.
The research applied to green roofs is being further developed to investigate the characteristics of this technology, specifically as regards:
• water requirement and regulation
• ability to contain pollution
• climate mitigation
• ecological-environmental value.
Water requirements and regulation
Evaporation is closely related to the vegetation, the greater the vegetation cover, the less water will be lost (a loss that occurs directly from the growing substrate) while, as regards transpiration, this varies according to the chosen vegetation.
Some plants transpire less and others that transpire more, so you need to know the specific data of each species.
Generally speaking, it can be said that plants with low transpiration are generally of Mediterranean origin, small in size, and with a reduced leaf surface; to these are added the herbaceous dicotyledons, succulents, and succulent plants.
On the other hand, those with high transpiration are large plants or plants with an expanded leaf blade, grasses, and many perennial and seasonal herbaceous plants.
According to the foregoing, the designer will always have to provide an irrigation system in the Mediterranean environment to ensure the survival of the plants installed on the garden roof.
The system will allow the substrate to remain almost constantly in field capacity, that is, with the amount of water that remains structured in the ground after the loss of gravitational water.
Not all the water retained by the soil can be used by plants, the amount of water available to their root system is obtained from the difference between the field capacity and the residual water volume in the substrate.
Whoever designs the green roof, however, can modify the quantity of water available in all those cases in which the microclimatic conditions and the inserted vegetation should require it.
A very useful strategy to accumulate available water in the substrate consists of the use of drainage panels equipped with a water reserve.
Water has its weight, so the accumulation must be carefully evaluated according to the structural capacity of the floor.
The ability of the growing substrate and draining panels to accumulate is the basis of the regulating action, impeccably performing the functions of capturing and controlling drainage of rainwater.
A green roof system built by the law is, in fact, capable of reducing the peak flow from the roofs to benefit the urban water network used for the disposal of white water.
Due to the unbridled urbanization of recent decades, areas with paved, waterproof, and sealed surfaces have multiplied.
This situation has also affected the climate of cities, leading to an increase in temperatures and a decrease in the humidity in the lower layers of the atmosphere.
It is intuitively obvious that it is intuitively obvious but very difficult to impose without adequate incentives that it is necessary to review the urban development model followed so far, implementing urban planning aimed at limiting the paved areas to the minimum necessary and recovering all possible green surfaces.
Although the correlation between the impermeable surface of the territory and climatic changes has been scientifically proven, these are not easily measurable and quantifiable.
It is therefore difficult to find a rule that requires the greening of paved surfaces, although it is assumed that these interventions improve the climate.
It is easier to find the correlation between the greening of a sealed surface and the slowing of the outflow of precipitation water.
The simplest solution, as we will see better in the chapter on legislation, could be to make the citizen pay for the supply and disposal of water in a single solution.
It is, therefore, possible to think of applying reductions to those who make the impermeable surfaces of their building or land properties permeable (or green), thus favoring greater absorption of water in the soil and consequently a slowing of runoff.
The roof garden increases the thermal support of the roof and is an excellent insulator for the rooms under it, with a considerable economic advantage in terms of savings on energy consumption (up to 25%).
Various functions are recognized from the thermodynamic point of view to the green roof:
• increase in the thermal resistance of the roof covering;
• increase in the phase shift of the thermal wave;
• passive cooling in the summer;
• mitigation of external temperatures;
• reduction of thermal fluctuations on the extrados and intrados of the structure.
The green roof partially reduces urban pollution, this feature is not specific to the technology but is part of the ability that plants have to break down pollutants and suspended dust.
The power to reduce pollutants such as nitrogen oxides and fine dust is known, however, for green roofs, studies have only recently been carried out.
It has been shown that green roofs can play an important role in reducing pollution, however, proportionate to the biomass present on the roof.
Trees and shrubs, inserted in intensive roof gardens, are favored by the high leaf surface.
On low thicknesses, in extensive roof gardens, the natural lawn is preferable to Sedum (ground cover plant with succulent leaves) due to the greater leaf surface.
As regards the absorption of CO2, there is a positive balance between subtracted carbon and carbon produced and there is not much difference between an extensive and intensive coverage.
Green roofs also filter polluted rainwater from urban and industrial centers, which can be intercepted by the downspout and stored in cisterns, and then reused for irrigation and domestic purposes.
Another important function concerns the cooling of the surrounding air as a result of evapotranspiration during the summer.
The maximum temperatures can thus drop by a few degrees, making the outdoor space more liveable.
The spread of green roofs could have a positive effect within cities on the phenomenon of urban heat islands, helping to lower the temperature in the warmer months.
The hanging garden favors the urbanization of wild species (unfortunately increasingly rare in our cities), actively contributing to the construction of ecological corridors used by wild animals (mainly insects and birds) for crossing urban centers.
It also reduces the transmission of noise (sound absorption), improving the quality of life inside the building.
In the sunniest regions, it actively contributes to the reduction of the light intensity reflected by the wall structures (walls, floors, etc.), improving the comfort of those who use the outdoor space.
Finally, the roof garden increases the amount of greenery in the urban environment (to the benefit of the quality of life of those who enjoy the city), minimizing the environmental impact of the building.
The above is already a good reason to consider the roof garden one of the main elements of ecological architecture and urban and territorial planning but this technology has other, more intimate qualities and flexibility that lead it to intercept the themes of the edible garden. , of the ecosystemic hanging gardens, of the hanging gardens, and the therapeutic, educational, and cultural functions of greenery.
The only possibility to make the presence of man on this planet more sustainable is to invest in the education and training of the new generations. If children grow up in direct contact with nature, they will learn to respect it and better understand its dynamics (they will be the citizens of the future and they have the task of protecting history, architecture, and the environment). To carry out this task they must be educated to interpret their home, their neighborhood, and their city as an integral part of the natural system, welcoming it into the daily routine of their life.
All this will make them more tolerant of living beings in all their forms and more respectful of the environment.
The extensive and intensive roof garden
The green roof can be extensive or intensive, obviously, the functions just described are extended to both types; Instead, the technical characteristics of the stratification, the plants that can be inserted, and, above all, the maintenance it requires, change.
It is quite common to define greening as extensive when a low thickness of the stratification is realized, with relatively lower weight unloaded on the structure in which mainly ground cover plants or bushes with compact and limited development are inserted.
On the contrary, the intensive is defined as an intervention with high thickness and weight and with vegetation characterized by the insertion of the tree, shrub, and herbaceous species.
In reality, these aspects do not represent the motivation behind the definition of extensive and intensive.
It is possible to design greening that can be defined as extensive but characterized by even substantial thicknesses and weight and with vegetation that develops anything but limited, just as it is possible to design-intensive plants with reduced weight and thickness.
The only element that fully characterizes the extensive roof garden is that concerning the level of maintenance required.
An extensive roof garden is a low-maintenance work, while an intensive one is characterized by higher maintenance levels.
The boundary between the two types is usually defined in two maintenance operations per year.
Therefore, if you design and build a green roof that exceeds this limit, you pass from the concept of extensive to that of intensive, even if a low-maintenance project is set up.
The extensive green roof
In principle, as already mentioned, an extensive green roof has a lower stratification since the plants inserted in it require a free cultivation content (thickness of soil necessary for the normal development of the plant).
These types of installations are designed to perform the typical bio-architectural functions, with particular reference to the environmental, climatic, and energy-saving functions.
It is a very natural roof covering system, which improves the energy yield and environmental impact of the structure but is designed not to be used.
Unfortunately, the diffusion in southern Italy of this type of installation is limited; when one thinks of the green roofs in these areas, one imagines a garden to be enjoyed and enjoyed a status to show off with friends.
Today, due to the economic crisis, a new awareness and greater attention to environmental issues are developing; this pushes many people to evaluate more carefully the possibilities of covering their roofs with an extensive green roof. The savings obtained in terms of energy and the best living comfort of the house below are key elements that guide the common citizen towards its realization.
The personal advantage of this choice, however, must not make us forget all the other positive factors that should induce the public and the private sector to sponsor this type of creation; among these, is the reduction of the overall surface runoff that develops during the rains due to the impermeability of the soil and the possibility that nature is offered to create new strongholds within the city.
Extensive green roof structure
The extensive green roof, in northern Europe, has a very simple and light structure that is not managed by any irrigation system.
The climatic characteristics of these countries allow the survival of plants, all with reduced nutritional and energy needs, with relatively low costs for installation and maintenance.
The thickness of the stratification varies from 8 to 15 cm, the weight ranges from 90 kg / m2 to 220 kg / m2 at complete water saturation.
The plants generally used are ground cover plants (Sedum, mainly), perennials with a small bearing, and herbaceous plants.
These species have a high capacity for self-propagation and resistance to thermal and water changes, minimal needs for what concerns the nutritional requirements, and a good speed of development.
This greening is particularly used on flat roofs with limited loading possibilities, of medium and large dimensions (for example public structures, commercial and industrial warehouses), to replace flooring or gravel ballast).
Usability is generally very limited and access is allowed only to gardeners to carry out maintenance work, once or twice a year.
The construction of the extensive green roof is very simple, the surface (flat or sloping) is subjected to an inspection to determine the state of affairs and accessibility of the building site material at height.
The green area must be circumscribed by a curb with a minimum height of 15 cm, normally flat roofs are already equipped with it, for sloping roofs they must be made with lightweight materials using profiles.
The correct disposal of rainwater must be verified, therefore the slope in the flat roofs and the adequate presence and sizing of the downspouts become important factors for the success of the installation.
The green roof must be able to dispose of excess water quickly so as not to compromise the survival of the plants and not drastically increase the weight unloaded on the structure. The structural sheath present inside the roof or attic must be in perfect condition, if it has visible leaks in the underlying apartment it must be repaired or replaced.
The roof garden works on its sheath, as will be seen better later, but this does not mean that it can replace the structural sheath.
The water, inside the masonry, often moves unexpectedly so it is always advisable to make sure you work on a perfect roof from a structural point of view.
In this type of stratification, installation is simple and the planting can include the insertion of cuttings, seedlings, or pre-grown mats.
The pre-cultivated mats are generally composed of Sedum, herbaceous, and grasses that grew within non-decomposable geotextile support (with high tensile strength).
They are very suitable for roofs with a slope of up to 50 ° and are laid on the cultivation substrate adequately compacted and secured against slipping, to prevent erosion from taking away all the technology.
There is also on the market the possibility of using Sedum tiles already grown inside a container (tray) made of assembled plastic material.
The system includes the vegetation, the growing medium, the filtering fabric, and the drainage and can be installed directly on the anti-root sheath or in attics with any slope.
Some emergency irrigation must be provided to ensure the rooting of the mat or seedlings (considering about 30 l / m2), keeping the substrate constantly moist until it is completely rooting.
In Mediterranean climates, the roofs are generally horizontal and installations on sloping roofs are hardly required, so the construction of the extensive roof garden on a flat roof will be explored here.
It should also be remembered that the typical environmental conditions of these areas make it impossible to create a roof garden of any type without an adequate irrigation system.
The anti-root sheath is one of the most important elements of the roof garden, whether extensive or intensive.
From a structural point of view, the underground organs of plants (roots) can represent a danger since they are capable of causing damage to the structural waterproofing, paving the way for water infiltration and the relative weakening of the slab. Hence the correct practice of placing on the waterproofing layer (never in direct contact with it, to avoid dangerous chemical incompatibilities that can develop between the sheaths) or the existing floor special sheaths capable of interfering with the invasive expansion of the roots.
The anti-root action is carried out by special additives contained within the sheath that prevent the apical part of the roots from penetrating it.
They can consist of bitumen enriched with chemical additives (bituminous sheaths) or composed of long polymers (rubber sheaths or synthetic PVC sheaths).
In extensive greenery, very thin anti-root sheaths can also be used, to be welded cold. There are also sheaths made up of self-adhesive waterproofing membranes that are laid cold without using the flame. They consist of a sheet of high-density cross-laminated polyethylene, the lower face of which is coated with a highly adhesive distilled bitumen polymer mix (it also glues at room temperature), which in turn is added with a specific anti-root agent.
However, although these systems are efficient, they are often replaced by 4 mm reinforced bituminous membranes arranged in layers (which include in the composition of the package one or more elements with anti-root characteristics) and welded to each other.
This happens because this type of installation, certainly more structured, gives a feeling of greater safety.
The market offers different types of sheaths and almost all with characteristics suitable for the purpose for which they were designed, the important thing is to read the technical data sheet and have the sheath installed by qualified personnel (recognized by the supplier company) to obtain the warranty on the work performed.