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Why an intensive green roof

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Why an intensive green roof

We know that green roofs are first mentioned in history even before our era. Needs and trends of the 20 century’s architecture rediscovered gardens and terraces situated on the rooftops. Today, more and more countries implement and promote green roofs. In Switzerland it is required by the law that every new roof which is bigger than 500 m², must have such an ecological system, and in Germany 10% of all roofs are green at this moment.

Whether you choose a simple and less expensive solution like an extensive green roof, or you have the possibility to arrange a small park on the building, such a green oasis will bring considerable advantages.

The difference between the extensive and intensive green roofs – which can be semi intensive or extensive – is primarily the type of vegetation selected.

When choosing the type of green roof, we have to consider the following facts: ­
  • the particularities of the building
  • the use of green space thus created
  • climatic conditions of the area, the possibility to ensure an adequate maintenance
  • (technical possibilities for irrigation and long-term budget)
  • the structural strength of the building

The ultralight extensive green roof has a weight of 60-150 kg/m², determined by the thickness of the substrate. It is suitable for plant species with small root mass, resistant to extreme weather conditions, such as UV radiations, drought or frost. Succulent plants are ideal, like Sedum (fat bloom), solitary or in various combinations with perennials (field dianthus, thyme etc.) may offer a special appearance in all seasons.

This kind of rooftop garden has lower landscaping and maintenance costs, because unpretentious wild natural vegetation is provided.

The vegetation of the semi-intensive green roofs is mainly made up of grasses, perennials, shrubs and very small bushes. Using plants in semi-intensive systems don’t require special measures for maintenance, however some species may require regular watering, annual fertilizing and regular trimming. The minimum substrate thickness is 20-30 cm, just to ensure optimal conditions for the plants chosen.

In case of intensive green roofs we can already speak about gardens in the true sense of the word, they are beautiful landscapes. Once established that the structure of the building allows to implement such a project, the vegetal structure of the intensive green roof can consist of any type of grasses, annual or perennial plants, bushes, shrubs, trees (even high of 5-8 m), just respecting some special planting rules (anchoring, proper fit to protect them from the actions of wind).

This kind of vegetation needs care, watering, suitable fertilizing, and the substrate with special features must have a substantial thickness of 30-150 cm.

Landscaping an intensive green roof involves also some legal and technical conditions:
In case of new buildings, the design of structures with green roofs must be done according to regulations in force for all buildings in those classes of importance. In case of existing buildings a technical expertise is needed to assess the available safety level in case of vertical loads, seismic situation and then realizing the green roof. The expertise for earthquake safety must be done according to Code P100-3/2009.

The intensive ecological roof has the same multilayer technology which is used for all type of green roofs, the main difference are in the characteristics and quantities of materials used.

Of course, each project has its own characteristics, adaptations are made in each case, but the basic structure of the intensive green roofs is as it follows:

The first layer is for protection, a root resistant membrane. If the waterproofing has FLL certification, this layer is no longer necessary.

The second protective layer, a geomembrane made of polyester fibres or mechanically reinforced fibres, regenerated polyester and polypropylene with a rate of moisture absorption between 3 and 6 l/m², depending on the product.

The third layer composed of drainage/reservoir plates, has the same role of the gravel layer, but with superior properties (the gravel does not have water retention capacity, having high self-weight etc.). The plates are 50-80 mm high, retain moisture and direct excess water to the ”overflow” type of drain systems. Depending on the model, this layer can store 10-21 l of water per m² and the self-weight is about 2.4 kg/ m². Intensive green roofs are designed for pedestrian traffic and the drainage plates used are resistant to compression of 25 kN/ m²-35 kN/ m².

A 1-2 mm thick blanket of polyethylene fibres forms the filter layer, which prevents clogging and filling of the drainage and vegetation layer with fine particles.

The vegetation substrate being the space for roots, is crucial for survival and development.
In case of intensive green roofs, their thickness is about 20-150 cm, depending on the weather conditions and the type of vegetation. In the composition usually we can find recycled and natural materials like tile debris, perlite, pumice, volcanic clay, peat fibre and biodegradable plants, usually grained to 1-40 mm granules. The most important criteria for choosing a certain type of substrate will be the moisture retention capacity which can vary between 25 and 40%, and also the permeability and structural stability.

The vegetation cover itself is similar to a garden at ground level, and it can be very reach in different type of species. The most important criteria when choosing the vegetation is the climate and microclimate of the site and of course plants’ annual growth rate. Perennials or annuals, shrubs, especially acclimated native species of the region are recommended. In case of woody plants, coniferous and hardwood, some general characteristics must be tracked: undergrowth or bushes with a maximum height of 1.5-2.0 m, creeping or erect stems, adapted to dry (xerophile and xeromezophile species) and relatively resistant to frost. Trees must be properly anchored so that their centre of gravity to be placed as low as it is possible, and without having the risk of displacement by a strong wind (there are special anchoring devices – see the menu Elements of green roof).

The landscape project will be done in accordance with the project of the green roof structure. For example a common formula is: height growth/10=the built-in thickness of the substrate. Such an intensive green roof has a height of over 30-150 cm, moisture retention capacity of 30-150 l/m² and a surface weight in saturated state about 300-700 kg/m².

Of course these values can be influenced by the conditions of the landscaping project. There is the possibility of combining these intensive green terraces with parking places on the roof, elements of drainage and water retention can be continued under road surfaces, facilitating a judicious use of rainwater and also creating green islands on rooftops, natural shadow for the parked cars and of course a beautiful view which pleases the eyes.

The maintenance of the intensive green roofs is performed in two distinct stages: ­
  • Maintenance after planting, which can last up to 2 years and it is actually the consolidation process of the plants
  • Maintenance during the lifetime of the green roof, which requires regular inspections, removal of dried plants or weeds in case of semi- intensive green roofs

Terrace roofs of intensive garden type need a dedicated maintenance, depending on the use of the terrace and also needs of the vegetation must be taken into account for each season. Any time of the year the terrace must have a well-controlled aspect. Maintenance of this terrace gardens is the same of a garden in the ground with same structure and landscape, but with a greater attention to the humidity of the substrate, fertilizers with slower release and keeping as compact especially plants with high growth rate.

Such a hanging garden will provide environmental and financial benefits, but also a great aesthetic satisfaction. Even in urban areas we can create a piece of nature arranged by our own taste, whether we want a rustic or Japanese garden, and why not an English garden. These green spots can have the functions of a mini park where people can rest, walk, smoke 🙂 – if this is not allowed in office buildings, becoming so relaxing places, which raises the attractiveness and of course the market value of the building.

Our team of experts offers optimal solutions in order to realize intensive, semi-intensive green roofs, small parks, real oasis on the top of the buildings. Considering the particularities of the building, climate conditions, technical possibilities for the maintenance, we can help you already in the design stage of your green project. We provide very accurate landscape projects and solutions for a safe planting of shrubs, trees, even high trees, and any other similar elements of a park at ground level. Our projects take into account customer’s point of view and ideas and they offer the best efficiency/safety and reliability balance.

Whether it’s an industrial hall’s roof landscaped with extensive green vegetation, an office building with intensive green terraces, or a shopping centre with futuristic shape and pedestrian and vehicle traffic on its roof; everything must start with a thorough design.

If you want an intensive or extensive green roof, a landscape garden, a green wall located in an office building, just noise insulation, or to complete the ecological function of the green roof by installing solar panels, we can provide projects, system solutions, estimates and consultancy in the field.

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