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The Sky is the Limit

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Green Roof Types

There are two main utilization options for Green Roofs: one, as a wonderful roof garden with a pleasant view for the owner; the other, an undisturbed habitat for flora and fauna within grey city centres. Both variations provide a broad range of private and public benefits. Deciding on which utilization to focus is crucial and has to be considered as early as the planning stage. In addition, the maximum load bearing capacity, maintenance, plant selection, substrates and the expense budgeting must correspond with the desired Green Roof type.


Extensive Green Roofs / Low Maintenance Green Roofs:

Extensive Green Roofs are well suited to roofs with little load bearing capacity and sites which are not meant to be used as roof gardens. The mineral substrate layer, containing little nutrients, is not very deep but suitable for less demanding and low growing plant communities. Sun, wind and drought are additional stress factors for plants on buildings. Drought-tolerant plant communities, such as, those found in dry mountain environments, coasts, semi-deserts or dry meadows, are visibly adapted to the natural extremes of the local conditions and are preferred species. Mixtures of mosses, succulents, herbs and grasses create pleasant plant communities.


Intensive Green Roofs / Roof Gardens:

Lawn, perennials, bushes and trees are possible on Intensive Green Roofs. Walkways, benches, playgrounds or even ponds can be established as additional features on the roof. There are no limitations in design and individuality, but a few things have to be considered. The Intensive Green Roof system build-up and the selected plant communities have to be harmonious with one another. In addition, the amount of maintenance of Intensive Green Roofs is higher than on Extensive Green Roofs and permanent irrigation and fertilization have to be ensured.


Sustainable Land Use: New residential areas and new transportation routes are constantly being developed at the cost of the environment. Landscaped roofs can help reduce the local impact on nature. They act as “stepping stone biotopes” and give plants and animals a replacement for their natural habitat. Birds, wild bees and butterflies can all find nourishment and shelter here and create a lively ecosystem. Even endangered species can relocate to a green roof.

Natural Stormwater Management: Green roofs make a key contribution to flood protection. Depending on the type

of construction, between 50 and 90 % of the precipitation that falls on these rooftops is retained and returned directly to the water cycle via the process of evaporation. The excess rainwater is filtered and drained off with a temporal delay. This leads to reduced stress on the sewer system during the year and at peak flow periods.

City Climate: During the summer months many cities become true “heat islands”. It is the increasing numbers of houses and the waste heat from traffic, industry and heating systems that are responsible. Landscaped roofs can reduce the levels of excess energy through absorption and the evaporation of water, and can thus make the city climate more bearable. At the same time dust and toxic particles are filtered out, leading to massive improvements in the air quality.

Protection for the Waterproofing –”Naked” flat roofs stand unprotected against weather conditions, UV-rays and mechanical strains. Over the years, the roof waterproofing starts to experience material fatigue, cracking and leakages. With green roofs, a natural buffer is formed against the weather and other environmental effects. The lifespan of the waterproofing can increase by as much as double.

Reduce Energy Costs – As a natural “coat”, landscaped roofs can lead to marked improvements in a building’s energy balance. House owners choosing to incorporate green roof systems will find that they quickly pay for themselves due to the resulting reduction in heating costs. They are also good for the environment as a whole, contributing to a reduction in the household’s CO2 emissions. During the summer months, residents will also profit from the cooling effects of the green rooftop.

A Free Plot of Land – Landscaped roofs can be used for a range of purposes: from private rooftop gardens, to prestigious green areas on company buildings, to public parks. And the real highlight – rooftop gardens do not incur additional property costs. Both the panoramic views over the city’s rooftops and the special atmosphere are also part of the concept. Rooftop gardens improve living and working environments and provide natural surroundings in the immediate vicinity.

Green Roof Engineering

Why isn’t it enough to simply put soil on the roof? In nature, we find soil profiles which have a more or less thick but permeable mineral layer (subsoil) above groundwater level, with a humus level above them. Such profiles can also be realized on roofs. Providing the load reserves and structural heights allow, the build-up can be similar to natural ground. Alongside the requirements mentioned above, the waterproofing also has to be root-resistant and be well protected against mechanical damage. In addition, the materials used must be sufficiently permeable to prevent stagnant moisture. An example of a construction that can exhaust the whole spectrum of technical possibilities above the sufficiently load-bearing and root-resistant sealing of the roof area can be as follows:

Protective layer(s): If the roof structure has not already been sealed root-resistant, a separate root-resistant protective barrier must be laid first. This has to be designed as a completely waterproof sump and has to be laid upwards to the connectors and connected like a waterproofing. The second protective layer is mainly for the mechanical protection of the waterproofing or root barrier. Highly pressure-resistant synthetic fibre mats are usually used for this purpose.


Multi-functional drainage: It´s task is not only to drain off surplus water. At the same time, water also needs to be retained and used for watering the plants by means of a capillary effect. No waterlogging may be allowed, however on 0° roofs, this system can be combined with a dam-up irrigation system which saves water by supplying vegetation from below.

Filter layer: What nature achieves through grading (fine particle shifting should be avoided) is achieved by a thin filter sheet here. As well as securing the filter stability of the structure, it also has to be sturdy enough.

Substrate layer: This is where most of the water is stored. It provides space for the roots and takes over all the functions of natural ground. The substrate used here has both high water permeability and good water storage capacity.

Vegetation – Almost anything is possible. The depth of the vegetation layer has to be adapted to the plants used, as described above.

International Standards

Only a few countries have regulations and guidelines which apply to Green Roofs. The oldest regulations, which were published in the 1990’s, are the German Standard “Guidelines for the planning, execution and upkeep of Green Roof sites”, and are from The Landscaping and Landscape Development Research Society e.V. (FLL) in Bonn (Germany). The Green Roof guidelines contain the types of Green Roofs, the various vegetation types, requirements for the building technique, Green Roof procedures as well as upkeep and maintenance for Green Roofs. The Green Roof guidelines are available in English but the content applies to the middle European climate region and the common German Green Roof system build-up. Whether Green Roof guidelines exist or not, the complementary country regulations and guidelines must also be considered. These guidelines can include: building technique (e.g. load bearing capacity, wind uplift protection, fire protection, temperature, noise protection, etc); and the roof technique (waterproofing material and installation, upstands, slope, drainage, etc.). Consideration must also be given to any existing regulations and guidelines for garden and landscape architecture (e.g. soil and plants, lawn, seed mixture, upkeep and maintenance works, etc.). High buildings, according to the particular country, require various regulations regarding the security and fall protection on roofs.

Green Roof Policies

Various tools can be used to promote green roofs directly or indirectly at the municipal policy level. These include, for example, stipulations in new land-use plans or green roof statutes for whole urban areas. Direct financial subsidies and a reduction in storm water fees add to the instruments which, together with public relations, could provide ideal support. It is also important to act as a role model by making the roofs of municipal buildings green. These various instruments have been applied successfully in Germany for more than 30 years. Even if an exact transfer of the German experience is not possible due to differing laws, the current implementation of the municipal green roof initiatives on an international level is following a very similar pattern. The following shows examples of the instruments.


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