For successful establishment and long lasting vegetation it is crucial to consider the local conditions. The vegetation layer and plant communities can be modified according to site conditions; whereas, the roof height and roof slope and the predominant climate are fixed. Additional considerations include: wind exposure, air pollution, variation of temperature as well as local light and moisture conditions. Even on one roof various microclimatic conditions can occur.
Desired Green Roof type
Extensive Green Roof, Semi Intensive Green Roof, Intensive Green Roof
Green Roof System build-up
A professional system build-up provides a growing environment on the roof which adequately compensates for the plant's natural environment. It ensures reliable technical and ecological functionality for decades. Green Roof system build-ups contain the following functional layers: root barrier, protection layer, drainage layer, filter layer, growing medium and plant level. In
cases of chemical incompatibility, separation or slip sheets are used to keep the waterproofing and Green Roof build-ups separated. Quality criteria and testing methods for the materials can be found in the German Standard "FLL-Green Roof Guideline".
Root barrier: The root barrier protects the roof construction from being damaged by roots. If the waterproofing is not root resistant according to the German Standard "FLL Green Roof Guideline" a separate root barrier has to be installed.
Protection Layer: A specially designed perforation resistant protection mat prevents mechanical damage of the root barrier and roof construction during the installation phase. Depending on the thickness and the material the protection layer can also retain water and nutrients.
Drainage Layer: The drainage layer allows for excess water to run-off into the water outlets. Depending on the design and the material the drainage layer has additional functions such as water storage, enlargement of the root zone, space for aeration of the system and protection for the build-up below it. Due to the weight constraints of the roof, the drainage layer is made of light-weight materials. Moulded drainage elements made of rubber or plastic are used quite often. Other drainage layers are made of gravel, lava, expanded clay or clay tiles
Filter layer: The filter layer separates the plant and substrate layers from the drainage layer below. Especially small particles, humic and organic materials, are retained by the filter sheet and are therefore available for the plants. In addition, the filter sheet ensures that the drainage layer and the water outlet are not clogged with silt. Filter layers are preferably made of geo-textiles such as fleece or other woven materials.
Growing medium: The growing medium is the basis of the Green Roof. A sufficient depth for the root zone has to be ensured as well as an adequate nutrient supply and a well balanced water-air relation. Depending on the type of Green Roof and the construction requirements, a variety of different system substrates are available.
The main criteria of the system substrates are: grain size, proportion of organic material, frost resistance, structural stability, resistance to wind erosion, water permeability, maximum water retention capacity, nutrient content, aeration and pH-value. It is also important that the presence of weeds is kept to a minimum. Together with the drainage layer, the growing medium facilitates adequate drainage of excess rainwater. Light-weight mineral materials, with high water retention capacity and good water permeability, such as lava, pumice, expanded clay, expanded schist, and clay tiles, have proven to be reliable for many years. Untreated organic material and top soil have disadvantages in terms of weight and drainage function; they are only used as additions to mineral substrates.
Plant level: Plant selection / Types of planting
Note: Multi-Layer Green Roof build-ups ensure ecological functions, such as, water retention, biological diversity and improvement of microclimate on a large scale. In special cases (e.g. on roofs with low load bearing capacities) single layer Green Roof build-ups with a height of at least 60 mm (2.5 in) can be installed. Further prerequisites for a single layer Green Roof build-up are: sufficient roof slope, system substrates with fewer small particles as well as drainage systems which prevent pooling water.
The plant selection depends on the growing medium as well as local conditions, available maintenance and the desired appearance. Low maintenance, durable and drought resistant plants are used for Extensive Green Roofs, versus, a nearly limitless plant selection for Intensive Green Roofs.
Extensive Green Roofs: Plants for Extensive Green Roofs have to survive intense solar radiation, wind exposure, drought, low nutrient supply, freezing temperatures and limited root area. Suitable plant varieties are those growing in severe locations with little moisture and nutrient supply, such as, dry mountain environments, coasts, semi-deserts or dry meadows.
The main varieties are Sedum, Sempervivum and Saxifraga; all of which belong to the succulent species. The plants are able to store high amounts of water in the leaves, are stress resistant and recover easily from periods of drought. Other varieties such as Dianthus species, Asteraceae and ornamental grasses are also suitable for these conditions. It is very important that plants which are native to the local conditions are taken into consideration, in order to support biodiversity.
Intensive Green Roofs: Having an appropriate Green Roof system build-up and sufficient growing medium (with higher root penetration volume, nutrients and water supply) growth of sophisticated plant varieties on the roof is possible. The selected plants need to be resistant to intense solar radiation and strong winds. Vegetation with various plant varieties such as perennials, herbs, grasses and trees allow for a natural character on the roof. Having a broader plant community increases the amount of maintenance required.
Different types of planting
There are four different ways of planting: seed sowing, cuttings (Sedum varieties), root ball plants and pre-cultivated vegetation mats. The German Standard "FLL Green Roof Guideline" specifies the amount of seeds, cuttings, plants or mats needed for the Green Roof. The recommended planting period is in early spring (April-June) and late summer (September). For planting during the summer it is important to provide enough water to compensate for periods of low precipitation. In late autumn cold days may lead to damage of the plants due to frost. Pre-cultivated plants should be stress resistant; if not, higher maintenance is required in terms of irrigation and fertilization.
Maintenance and support
To ensure the long term aesthetic and functionality of the Green Roof it is important to provide maintenance measures. There are three stages of maintenance concerning the vegetation:
Installation maintenance: several aspects of maintenance and service are involved throughout the first year for the successful accrual of the plants after installation. It is important to provide sufficient water supply during the dry season. Replanting is necessary if there are dying or missing plants after installation and weeds and other unwanted plants need to be removed
Development maintenance: to support the vegetation until total coverage of the roof is achieved. The development maintenance is equal to the installation maintenance but with lower intensity.
Up-keep maintenance: after the development of the plants on the Green Roof is ensured, it is crucial to maintain the roof once or twice a year. Weeds and other unwanted plants on the entire roof, at the perimeters and at the upstands need to be removed. The inspection chambers also need to be monitored. For grass and herb vegetation the organic material has to be removed once a year. Intensive Green Roofs require higher maintenance and service throughout the year.
Due to the high water retention capacity and the delay of water run-off, Green Roofs reduce the amount of water flowing into the sewerage system and sewage plants. The amount of water retained depends on the maximum water retention capacity, the permeability and the height of the Green Roof system build-up. The water run-off coefficient specifies the amount of water retained by the Green Roof in relation to the total amount of rainwater and the water run-off. The water run-off coefficient is important in the case of municipal subsidies and for the introduction of divided storm-water taxes.
The cost required for a Green Roof is dependent on various factors. The type of Green Roof (e.g. extensive vs. intensive), Green Roof system build-up and different types of planting are the main factors. The logistic aspects and transportation of materials as well as the roof slope and the entire roof size are also very important. Some communities support Green Roofs with financial subsidies.