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Light extensive green roof

13 min read
Light extensive green roof

In our days lots of roofs and terraces become ecological constructions during renovation works, when an attic is added to the house or simply because the owner would like to have a green area in his own environment. The structure of the building must be able to support heavy weights in order to bear intensive green landscapes. In case of new buildings additional cost aren’t needed for structure strengthening or other emerging works, technical expertise being performed before construction works start.

They also exist green roof systems especially developed for lightweight structures.

Special materials are used in smaller quantities to achieve lower surface weights. Water retention – to ensure the minimal needs of plants – is usually done at the drainage layer, so drainage tiles to be able to retain 10 l water/m².

In this case the substrate can have only 60 mm thickness and the water saturated weight is about 70-100 kg/m². The disadvantage is that roots have a relative tight space, so not many plants can be included in these rooftop gardens.

These lightweight structures cannot support pedestrian traffic either, which is another reason to landscape an extensive green roof. This doesn’t need special care or other sophisticated maintenance works. Succulent species are recommended, like sedum, perennials and flowers. The green surface mass will be reduced, but will offer a special, aesthetic view.

Unlike the living intensive green roof – which due to the sliding forces of the great weight can be only arranged on a flat roof, ideal the 2° slope – these ultralight and light extensive systems can be landscaped on sloping surfaces too, up to 45°. Before starting the design work, the roof angle must be determined and the optimal method selected. There are different technologies depending on the inclination of the roof’s surface.

Construction of a pitched green roof involves several aspects: in case of greater inclinations than 10° (17,6%), component layers must be stabilized, because the danger of erosion is pretty high.

This can be realized in several ways:

  • locally, with single profiles
  • unidirectional, transverse to the slope, with slats or other elements that ensure the necessary geometry in order to increase contact surface between substrate and its support
  • uniform on the surface by using profiled materials
  • stabilized vegetation carpets
  • textile mats
  • textiles for stabilizing slopes
  • honeycomb networks
  • networks and slats

The geographic location and conditions of the site are also very important. For example a north-south positioning can lead to a totally different development of green roof’s vegetation.

Thus, we can distinguish several categories of building features, depending on the inclination degree:
  1. Flat roof with 0-2° inclination
  2. Inclination of 2-5°
  3. Pitched roof, 5-25°
  4. Pitched roof of 25 to 45°
1. In order to realize a light extensive green roof on a flat surface, the following steps must be attended:
  • A basic course of membrane against root penetration, which protects the waterproofing of the roof
  • A geomembrane made of polyester fibres or a mixture of mechanically consolidated fibres, polyester and regenerated polypropylene having a 3-6 l/m² moisture absorption capacity, depending on the product
  • 50 mm high drainage course, made of expanded polystyrene water retention tiles with drainpipes embedded. It can retain 10,1 l/m² moisture at only 0,6 kg/m²
  • A 1-2 mm thick polyethylene cover which forms the filtration layer and prevents clogging or filling of drainage layer with fine particles from the substrate or the vegetation layer
  • The substrate of vegetation composed of recycled and organic materials, like split face natural tiles, perlite, pumice, volcanic clay, peat fibre, biodegradable fibres, in granule forms of 1-40 mm. The thickness of the substrate in this case is about 60 mm, because this is the minimum size for plants on the roof to develop normally. Every centimetre of the layer shows actually the weight of the substrate between 10-13 kg/m², when saturated with water.
  • The vegetation itself is mandatory extensive, with plants resistant to high temperature variations, extreme weather conditions, like frost or drought, lack of rains, winds and acid rains. Sedums, wild grasses are ideal for extensive green roof vegetation, but they can also be combined with many other species of plants and flowers in order to obtain really beautiful landscapes. There exist different planting and sowing methods (see Extensive vegetation).

2. Light extensive green roofs on surfaces with 2° to 5° inclinations are built in the same way like flat ones.
3. In case of pitched roofs with 5°-25°inclination, the landscaping technology is a bit different.

Besides the standard layers of light green roofs, a grill made of wooden slats is embedded between the drainage course and the substrate, against slipping. The role of this element is just provisional: fixing the substrate during spreading and wetting operations. The vegetation must take over the fixing function of component layers. This is another reason to use vegetation rolls, because they are rapidly rooted in the substrate (10-14 days).

It’s only a single exception between the landscape of pitched roofs with 5°-15° inclination and flat roofs: the vegetation substrate is put straight on the drainage course, without using the filtration cover. Thus the fertile layer’s stability is ensured against erosion. Water retention is also provided, which is also an important aspect because of the pronounced drying trend of pitched roofs.

4. Installing extensive green roofs on pitched surfaces with greater inclination (more than 25°) is more complex and needs some preparatory work.

After basic layers of extensive green roofs are settled, namely the membrane against root penetration (in case of pitched roofs is recommended, but not mandatory), the geomembrane and the drainage course, the further fixing of the substrate and the vegetation cover must be prepared. One of the usual methods in case of these rooftop gardens is the mounting of steel cables. These cables have high resistance to pressure and they have obstacles in order to install transversal thresholds. Cables are linked with fixing loops or in case of symmetrical roofs, they are anchored. Sills are pre-cut in T form and positioned in front of the cable obstacles. The system can be mounted in a short time, but must be adapted to the specifications of every building, especially the shape and the inclination of every roof. These will determine for example the thickness of the substrate, the distance between the slip thresholds, the adjustable thickness of the drainage course etc. The substrate is spread in the suitable thickness, up to 70 mm and after that the vegetation carpet can follow.

Our company offers turnkey projects and long-term maintenance services for intensive and extensive green roofs and terraces. We use high quality construction materials, vegetation rolls, seeds, plants especially developed for ecological constructions and other additional elements. Every landscape is designed together with the client, taking into account his own ideas too. We offer the most effective system solutions and costs.

Our team of experts is able to landscape extensive green roofs on top of industrial halls, vertical gardens in office buildings, intensive green gardens with special relaxing zones, landscape green roofs with pedestrian traffic for shopping malls and many other ecological constructions. If the structure doesn’t allow all this, we can build lightweight or ultra-light green gardens too. To increase the ecological value of the building, photovoltaic panels can be also installed.

We are happy to help you with projects, system solutions, estimates and professional consultancy!

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