Posted on: July 15, 2019 Posted by: admin Comments: 0

Building a rooftop garden

The rooftop garden is a real garden, called “hanging”, like the famous ancient gardens of Babylon, which are said to be wonderful. Even a green roof can be wonderful and not only aesthetically: it requalifies the environment and increases the economic value of a house. In short, it is an innovative and advantageous solution.


The roof garden built on boxes, houses or sheds, can offer a direct contact with nature, which often in the city is not possible to have, improve the climate inside the houses, but also the one immediately outside and also allows you to recover unused spaces such as the covers of the garage or the solar panels of the top floors.

What does a green roof consist of?

On a flat roof, a complex stratification of materials is created, each with a precise function:

  1. first of all, the first layer you come across is the slab; it not only has a load-bearing function, but must also provide a slope of 2% for the outflow of water
  2. secondly, there is the thermal-acoustic insulation, laid over the slab: it is a layer of glass wool, rock wool or expanded polyurethane panels, weighing about 30 kg/mc; together with a vapour barrier, it is necessary when the green area is on a roof or on a terrace of a house, to avoid condensation inside the house itself.
  3. after the thermal-acoustic insulation, we find the waterproofing: it is a bituminous sheath with the addition of resins and elastomers, reinforced with a glass felt with a thickness of 3-4 mm; the surface must be perfectly adherent to the support, torch-bonded on all its extension and not only on the joints. This sheath also prevents the roots from infiltrating the slab. The weight of the sheath varies between 10 and 20 kg/sqm.
  4. the protective layer, inserted between the waterproofing and the draining layer, is made of geotextile and must protect the insulating layers and the structure of the building itself
  5. the draining layer never measures less than 4 cm thick; it has a dual function: to remove excess water and to constitute a water reserve in periods of drought; usually the draining layer is composed of moulded plates in recycled plastic, therefore ecological
  6. above the draining layer, there is a filtering cloth, which prevents the fine materials of the substrate from descending and clogging the draining layer: it consists of a fabric membrane, in direct contact with the ground
  7. the rooting net, made of synthetic material and made up of wide meshes to which the roots can cling to give plants greater anchorage, is used for the area of the shrubs, and must be fixed to the perimeter walls, for a total weight of 25-30 kg/sqm
  8. finally, the real cultivation substrate, formed by a mixture of soil lightened with expanded clay of small grain size, lapilli and pumice, bark and peat, with these materials improve ventilation and drainage and retains moisture in the right way.


Let’s see in detail what are the advantages of a green roof, both from the environmental and collective point of view, and from the individual point of view of your home:

  • water balance
  • the green roof can retain up to 70% of the rainfall in order to return the water to the environment in the form of water vapour; in this way the flow of water to the artificial disposal channels is slowed down and a contribution is made to water management
  • cleaner air
  • the green cover can seal up to 20% of harmful dust circulating in the air, as this is fixed to the leaves covered with their characteristic layer of moisture
  • muffled noises
  • the turf is made up of materials that have the property of sound absorption; in homes the noise of 8 Db is thus reduced
  • Mitigated temperatures
  • heat loss is reduced in winter, heat dissipation is reduced in summer; the related heating/air-conditioning costs are more controlled
  • longer life of the covers
  • green protects the waterproof covering from temperature fluctuations and atmospheric agents, thus prolonging the integrity of the underlying covering over time.
  • The environment gains from the roof garden

Hanging gardens are an excellent aid to the environment, especially in the city, for some of the reasons listed above: improvement of air quality, water balance, thermal regulation of buildings. This also results in great energy savings, which in addition to lightening the wallet of the inhabitants of a house in the long term, further helps the environment in terms of consumption of non-renewable energy and reduction of discharges full of toxins and harmful gases, as well as a reduction in heat loss and the resulting greenhouse effect.

For all these reasons, interest in roof gardens is growing more and more throughout Europe, starting with Germany, and is also penetrating into Italy.


Given the growing interest in this type of roofing and in the energy saving and environmental sensitivity that derive from it, Uni, the Italian National Unification Body, has drawn up a standard called “Instructions for the design, execution and maintenance of green roofs”; this standard defines the criteria for the construction and maintenance phases of roof gardens, taking into account the climatic context, the building context and the intended use of the final product, aiming to make the construction of green roofs increasingly easy and economical, so that everyone can benefit from it, with individual and above all collective benefits.


The green roof can be of two types, different for the weight of the structure, the type of vegetation chosen and the maintenance costs, so we have the extensive coverage and the intensive coverage.


It is the simplest type, with low maintenance costs, fast and easy installation and minimum weight on the structure. The extensive roofing is the most suitable for flat surfaces of large extension, such as solar paving for example.

To cover a sloping pitched roof with the extensive roofing, instead, it is necessary to provide patented drainage systems and a system to retain the ground; this system is usually made up of a network of horizontal and vertical beams, about 10 cm long, made of recycled synthetic fibers.

As for the type of vegetation of the extensive roofing, it must be plants that can survive in drought, must have a great adaptability to the climatic conditions of the place where it is located and the ability to regenerate and self-propagate, to be able to recreate whenever the conditions can somehow prevent it.

The thickness of the soil substrate of this cover can vary between 5 and 10 cm and only welcomes herbaceous plants, such as Sedum, Festuca ovina, Sempervivum. The weight of this type can vary between 75 and 150 kg/sqm.


It is a real garden, with lawns, grasses and bushes, flowers and even trees. The intensive covering can be realized on any type of roof, from the terraces, to the garages to the inclined slopes: the only condition is that these bear the weight.

The thickness of the cultivation layer is rather reduced due to the presence of particular soils lightened, but the type of plants makes this type certainly heavier than the intensive coverage, depending on the plants chosen, the total weight can be between 150-170 kg / sqm, but can reach up to 200-300 kg / sqm for the intensive green light.

There is also another type of intensive cover, called heavy precisely, because the substrate cultivation is 50 cm and the essences used are trees and large shrubs and then the weight reaches around 600 kg / sqm. The intensive maintenance of the roof must be continuous and constant, because it must provide irrigation, pesticide treatments, clipping and fertilization, just like a real garden.


During the construction of a roof garden, whether it is an extensive roof or an intensive roof, it is possible to provide areas used only for lawn or paved. As far as the lawn is concerned, perhaps designed for children to play with, it is preferable to use lumps of ready lawn.

For the floor, on the other hand, to be used as a solarium or dining area, it will be separated from the rest of the garden by a curb of light stone, such as tuff, laid dry on sand or gravel, to make it drainable without further special systems. This frost-free solution does not even create cracks.


One of the biggest problems in a green roof is the disposal of excess water, whether it is irrigation or rainwater, which also helps to maintain the green itself.

In the lower points of the garden, there must be drainage openings, with different sizes depending on the size of the roof, at distances not exceeding 30-40 meters, between them. To avoid that the discharge nozzles are covered by fallen leaves or twigs, they must be protected with special grids or removable lids, also to make the drains easily inspectable.

As far as the dimensions of the nozzles are concerned, 8 cm in diameter are sufficient for surfaces of 50 square metres, 16 cm for 20 square metres, 25 for 500 square metres. If the nozzles are very far from each other and there is not enough slope to convey the water, it is necessary to install an additional drainage system, such as slit PVC pipes that collect and freeze water in the drainage channels. These pipes must be placed in thermally insulated places or places not very exposed to low temperatures, because in winter with the frost could block due to the transformation of water into ice.


To avoid the danger of fire and the force of the wind that can take away everything, it is advisable to lay gravel or curbs that serve precisely to hold the gardens firmly on the perimeter and that forming safety strips of mining materials, prevent the risk of fire.


A question that perhaps arises to many, concerns the possible penetration of large quantities of water or condensation of water vapor, in the houses directly below the green roof, given the amount of water to be used for irrigation of the green.

The composition of the green roof in several layers, however, allows us to see how this problem can be easily contained: the draining layer first of all stops the penetration of water, then completely prevented by the waterproofing layer, and also the thermal-acoustic insulation, placed right between the slab and the other layers, prevents the formation of water vapor in the houses below, thus ensuring an ideal temperature.

No annoying stains of humidity in the roofs of the houses below the roof garden, nor condensation. However, make sure that the companies that carry out the work are reliable and that during the work they lay the various layers of the roof in the right way and especially on the entire extension of the roof, not only in the conjunctions.


Before starting work on a roof garden, a few things must be checked:

  • first of all, the load-bearing capacity of the slab on which the garden is to be built must be adequate: at least 250 kg/sqm, which will increase depending on the roof chosen; the load-bearing capacity of the slab can be found on the project drawings or you can ask the designer directly
  • it is necessary to have the consent of the condominium, if you want to make a hanging terrace owned by a single condominium, while you have to share the costs between all the owners in the case of a roof garden on a condominium roof
  • it is necessary to verify at the technical office of the Municipality if where the house is located, there is a direct law on the matter
  • For further information before starting any work, on rules and permits, you can contact Aivep, the Italian Green Hanging Association.

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