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A short guide to green roofs

Details of green roof types

When planning to construct a green roof, it is important to consider the potential future use of the green area, irrespective of whether a new building is being constructed or a new one renovated. There are three basic types of green roof systems. We differentiate between the following types of green roofs, depending on how the area is to be utilised, the technical parameters and requirements, and the form of construction (these also determine what plants and form of vegetation can be employed):

  • Extensive
  • Semi-intensive
  • Intensive


The various types of green roofs

Extensive systems represent the form closest to nature and are mainly laid down on roof areas that will not be used. As the possibilities for active maintenance are low due to poor accessibility, it is necessary to select appropriate vegetation.

Due to the extreme situation, the plants need to be low-maintenance, adaptable and self-sustaining; they also need to be resistant to drought and frost, while being able to survive occasional overirrigation and lack of nutrients.

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The semi-intensive green roof is a special type with respect to the maintenance it requires, the vegetation that can be used, the method of construction and possibilities for utilisation. With regard to requirements and characteristics, it lies somewhere between the extensive and the intensive type, and can often not be distinguished from these.
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In contrast with an extensive green roof, there are almost unlimited possibilities with respect to the plants that can be used in an intensive system. In view of the wide range of plants that can be employed, the resultant high level of maintenance required, and the fact that the space is regularly used, an extensive green roof is frequently indistinguishable from a garden or other accessible green space.

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Standard green roof construction techniques

In the long tradition of green roof systems, a layered build-up of the vegetation surface has been most effective and this is now the standard procedure. Every layer needs to fulfil a specific function and must be appropriately adjusted to suit the requirements of the location and the vegetation employed.

 Layer construction    Diagram    Key
  • Vegetation
  Das Bild   Vegetation can be planted in the form of seeds, seedlings, mature plants or vegetation mats.

  • Vegetation layer
  Das Bild   The vegetation layer plays a special role as it has various functions to fulfil in the system as whole. It must provide room for rooting and thus support the growth of the plants. It must be able to retain moisture, but  must also release moisture into the drainage layer in order to avoid excessive build-up of water. It must have the nutrient reserves required by the plants. To ensure that the vegetation layer can meet all these requirements over the long term, the substrate used needs to be both well aerated and structurally stable. If air volume in the soil falls below 15%, plant roots will die; on the other hand, the substrate should not become excessively compact on settlement.

  • Filter layer
  Das Bild   The filter layer prevents fine soil and substrate elements being washed from the vegetation layer into the drainage layer, thus impairing the water permeability of the drainage layer. The plastic membrane employed (100-150 g/m²) allows water to pass and provides good drainage.

  • Drainage layer

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  The drainage layer takes up the excess water from the vegetation layer and transfers it to the installed drainage system to prevent waterlogging. In some cases, the drainage layer can also serve as an additional rooting space, so that it also needs to be able to retain water to some extent. Depending on the slope of the roof, it is possible to use open-pore mineral materials, like lava stone and aerated slate, or even drainage mats, which can be used to reduce the thickness of the drainage layer in favour of the vegetation layer.

  • Protective layer
  Das Bild   Additional protective and separation layers can be included if required to protect the rooting guides. The purpose of such layers is to prevent mechanical damage to the roof during construction work and during maintenance of the vegetation. The protective sheets or forms can be made of plastic granulate, foam plastic or plastic matting (approx. 500g/m²).

  • Root barrier membrane
  Das Bild   The root barrier membrane prevents roots damaging the roof waterproofing and, if used on an intensive green roof, can also serve to collect rainwater. It prevents water entering the roof structure and the building. The materials used must conform to the FLL guidelines with respect to their root penetration resistance.

  • Protective membrane
  Das Bild   This membrane protects the root guidance system from possible chemical and mechanical effects of the surface of the roof waterproofing.

  • Roof waterproofing membrane
  Das Bild   If a non-root-resistant waterproofing membrane is used, an additional root membrane or root guidance system is essential.


The planting of a green roof

To ensure the success and long life of a green roof, it is important to select the right plants. As a green roof is an artificially planted area, the specific climatic conditions and other local factors, such as the urban microclimate, ratio of light and shade, wind exposure, pollutant exposure, exposure to heat and cold, roof orientation etc., must be taken into account together with the relevant parameters of the type of green roof to be constructed.

The plant groups that can be recommended for planting on green roofs are mosses, succulents, bulbs and tubers, grasses, herbs, herbaceous and woody plants. A few of the more popular and recommended plant combinations are listed below:

 For extensive green roof 
   For semi-intensive green roof 
   For intensive green roof
  • Moss and sedum
  • Sedum, moss and herbs
  • Sedum, moss and grasses
  • Grasses and herbs
  • Grasses and herbs
  • Herbaceous and woody plants
  • Shrubs and other woody plants
  • Woody plants
  • Shrubs and small trees
  • As nearly all plants can be used in this situation, there is little point in providing recommendations here.

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