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Why an extensive green roof

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Why an extensive green roof?

Travelling in Northern Europe, the Scandinavian countries or in Great Britain, we cannot but admire those older, newer, traditional or modern houses whose roofs appear to be one blooming garden with shrubs, even trees, or simply an inviting green meadow. Walls seem to be constructed of a mixture of colourful, flowing plants. They are what we call today green roofs, living or green terraces.

Probably the best known green roof in history was built by Babylonians. The complexity of the technique used even in our days arouses the admiration of experts. In the Orkney Islands, Scotland, archaeologists discovered ecological roofs dating from 2500 BC! These days, the tradition of green roofs is kept alive in Scandinavia, Iceland, Greenland, but more and more modern buildings adopt this solution in other countries too. Germany is the country where green roofs are widespread and benefit of reduced taxes.

In many places, using natural materials, live vegetation as raw material to build a house are forgotten and lots of false myths were born related to it. Many of these myths were launched by companies interested only in the production and sale of classical building materials and they saw a dangerous alternative in these ecological solutions.

It is rumoured that the installation of a green roof would be very expensive. Nothing could be more untrue! Costs of an extensive green roof are only higher by 2-3% of the value of the building, and the depreciation is done in 8-10 years (considering only the heating and cooling savings). It is also important the multiplication of the roof life itself through the protection offered by hydro- and thermal insulation layers.

We can also hear that ecological roofs could damage the structure of the building, would cause leaks and mold inside. False! The waterproofing of a building is done before implementing a roof garden, and any of the technologies used involves the existence of a protection layer (a membrane), which has the role to stop the roots, but also provides protection against rainfalls and temperature variations. There are already such waterproofing membranes which have a protective role against roots by their enzyme inhibition mechanism at the top of the root.

Another frightening rumour is about the cost of maintenance of these ecological roofs, especially the huge bills of water used for irrigation. No way! At the beginning, when installing an extensive green roof, indeed watering is required, especially in dry periods, but after plants have been installed and have evolved with the environment, usually rainwater provides a sufficient amount of moisture for a healthy garden. During the droughty months, a very rare irrigation (1-2 times) contributes to a decorative appearance and a better development of the vegetation.

If we are content with the ecological role and appearance of our roof gardens without the claim of having a colourful vegetation, this can exist without artificial irrigation.

The extensive green roof has multiple advantages. Roofs with extensive vegetation can very well thermally insulate, both summer and winter, so temperature variations inside the buildings are substantially reduced. In summer, flat roofs without vegetation can be up to 21°C warmer than green ones. The vegetation cover can also reflect the infrared rays. In other seasons, green roof insulates better, thanks to the release of condensation or frost that forms on the surface of vegetation in the morning (to produce 1 g of condensation water, 530 calories of heat are released).

Sound insulation is also an important aspect of an extensive green roof. In urban areas we can feel everyday how background noise is surrounding us, but in a building with rooftop vegetation, silence becomes a priceless environmental factor.

An extensive green roof reduces also electromagnetic radiation generated by the antennas of mobile telecommunication networks.

Knowing that the ambient temperature in cities may be 5-7 °C higher than in rural areas, in urban areas any patch of greenery helps reducing the Urban Heat Island effect, so certain types of green constructions are subsidized on local, national or European funds.

The roof vegetation is also capturing the carbon dioxide from the atmosphere, that is why local authorities contribute financially to the construction of green roofs, especially the extensive ones. These roofs have lower costs but an intensive ecological effect, similar with the intensive green roofs, which can be implemented with higher costs.

What is in fact an extensive green roof?
There are two major categories of green roofs and terraces: extensive and intensive, both can be of several types,depending on the vegetation planted, the different methods of maintenance and then different implementation technologies.

For conventional roofs, builders use PVC, asphalt, metal sheets and other materials, and the extensive green roof offers an environment friendly alternative for these, having a low weight, using different sedum species, grasses, which do not need a large depth of substrate. Thanks to this, the structure of an average building can support this overload. On the other hand, this type of ecological roof does not require intensive care or special, sophisticated maintenance work. Once the plantation is stabilized, it does not require irrigation, just one or two control interventions per year, enough to clarify the state of the plant blanket and the structure itself. These controls are not absolutely necessary, but in these cases unwanted plants are removed, the substrate can be fed, and so the vegetation is developed and will have a more intense decorative and neat look.

For the construction of an extensive green roof several technologies are used, according to the resistance structure of the building, roof pitch and weather conditions (more details in the section Help for design/checklist).

The green roof is a multilayer construction:

  • At the basis it is extended a protective layer for closure, a root resistant membrane, in order to withstand the aggressive action of the roots (only if the layer does not have FLL certification)
  • This is followed by a protection layer, a geomembrane made of polyester fibres or a mixture of mechanically consolidated fibres, reclaimed polyester and polypropylene, with a rate of water absorption between 3 and 6 l/m², depending on the product
  • It follows a layer of plates of drainage and moisture retention, with a similar role of the gravel layer, but with superior properties (the gravel does not have the capacity of water retention). Plates, having the form of egg casings or of three-dimensional networks with holes of water retention and drainage ducts, are high of 20-30 mm and they retain water and also direct the excess toward the leaks. Depending on the model, it stores water from 7 to 11.5 l/m², having a weight of only 1.8-2 kg/m².
  • A 1-2 mm tick veil of polyethylene fibres forms the filter layer, which prevents the clogging, protects drainage layer of fine particles from the substrate and vegetation.
  • The substrate for vegetation being the space for roots, it is crucial for survival and development. In case of extensive green roofs, this substrate has a thickness of 6-20 cm, depending on the climate conditions and the type of vegetation used. In its 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 would be the moisture storage capacity, which can vary between 25 and 40% of the volume, and also the permeability and structural stability.
  • And finally we reach the vegetation cover itself. For extensive green roofs it is recommended to use plants which are resistant to temperature variations, extreme conditions like frost, lack of rainfall, drought, wind, acid rain etc. Different species of sedum and wild grasses are ideal on the extensive green roofs, but they can also be combined with many types of flowering plants, reaching really spectacular results. There are several ways to plant or sow extensive vegetation, but the most effective method – both financially and execution time – is the use of vegetation rolls.

Sowing or dispersion of seedlings/sedum pieces which should root in the soil is apparently a cheaper method, but the irrigation, the maintenance until they reach maturity, exceeds several times the cost of vegetation roll, which provides already at the installation the covering of over 95% of the surface, being immediately resistant to the heavy rainfalls. We cannot say the same about the sowing method, where a torrential rain can compromise the entire plantation.

Once the sedum/herbs vegetation rolls are stretched, they very quickly root in the substrate (10-14 days). These rolls have a three-dimensional structure of roots and a well-drained but fertile soil, so under normal circumstances after a year of landscaping the extensive green roof reaches maturity. Once installed, they protect the entire extensive green roof stratification of all weather conditions that may arise.

Such a rooftop garden in water saturated state has a weight of 100-130 kg/m². Of course, this information is just indicative, based on the structure of the building. In case of a new building this kind of hanging garden which will beautify the environment can be taken into account from the very beginning, so many other solutions may be applied. There can be built ultralight extensive green roofs of 70 kg/m² or more sophisticated ones with a rich diversity of vegetation, but their weight can reach even 300 kg/m².

Depending on the structure, the vegetation used, the height at which is installed, the costs of a turnkey project are between 40-60 €/m²+VAT in Romania, but using the same material, such a rooftop garden in Germany or USA would cost between 70-100 €/m².

The main difference between expenses in Romania and other countries is the lower cost of human resources and rental of cranes for raising materials on the roof and definitely not the poor quality of the material used. They are directly imported from the EU or through local producers from Germany and Holland.

If you want to use all the advantages of an extensive green roof as an ecological alternative for common roofs, our team of experts is at your disposal! These roofs are very good thermal and sound insulators, can reduce electromagnetic radiation, they don’t require intensive care or special maintenance, and can be adapted to the shape of any building.

Our projects take into account customer’s pint 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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