Topic 2

The German Landscape Research, Development and Construction Society’s ‘Green Roof Guidelines’ (2018) offers various advice on the planning and implementation of green roofs.

These can be summarized in the following points, which will be elaborated upon in the following slides:

  • Strength and stability of building and roof structure
  • Size, slope, height and directional orientation of roof
  • Cost considerations of green roofs
  • Type of roof
  • Climate, microclimate and vegetation

In relation to the strength and stability of the building and roof structure, some factors to be considered include:

  • Which areas of the building receive direct sun, and which are shaded or dappled shaded?
  • The capacity of the building to deflect precipitation.
  • Storm and rainwater load and water load from adjoining structural elements.
    • What kind of drainage facilities and protection does the building have (e.g., roof outlets, gullies, guttering, facades, etc.)?
  • The wind flow conditions and the protection of wind pressures.
    • Must consider the direction and intensity of the wind in relation to height and shape of building.
  • The design loads and depth of structured layers, particularly the load bearing construction, insulation and waterproofing layer structures.
    • Must consider dangers from erosion or slipping of different layers.
  • Any additional technical installations (e.g., air conditioning units, antennas, solar panels, etc.) on the building and their emissions.
    • For example, ventilation and air condition units can cause frost or dry damage to plants; also chimney emissions can damage vegetation.
  • Fire prevention: there must be sufficient resistance to fire exposure from outside.
  • Any puddles forming on the roof

Size, slope, height and directional orientation of the roofing structure should also be considered:

  • For extensive and semi-intensive roofs, a fall or slope of at least 2% is advisable to be planned.
  • Flat roofs or sub-areas with no or too little slope (<2%) are in danger of unintended water accumulation, which can be problematic.
  • The effectiveness of thicker layers or additional drainage in standing water is limited.
  • In cases of flat roofs, it is advised to add water-displacing that will make stagnant water inaccessible to plants.
  • Nonetheless, with increasing roof slope, special measures against material displacement or slipping must be observed.

One of the most important considerations regarding planning and implementation is the cost of the roof  (Li & Yeung 2014):

  • According to the Environmental Protection Agency (2009), the cost of a green roof construction depends on its components (e.g., growing medium, roofing membrane, quantity of plants, etc.).
    • Initial costs per square foot calculated at €8.40 for extensive roof and €226.85 for intensive one.
    • Maintenance costs range from €6.70 to €9.25 per square meter, with costs for extensive roofs dropping when covering entire roof.
  • The Architectural Services Department (2006) in Hong Kong calculated the costs of a green roof depending on the source of the materials.
    • Using imported material, the cost was between €75.62 and €109.23 per square meter, while when using local material, cost was reduced between €42 and €67.22.

Another important consideration in the planning and implementation of green roofs is the decision over the type of roof:

  • You have to consider whether you prefer a simple, not accessible and mainly for the environmental benefits roof, or an intensive one, with the potential for larger plants such as trees.
  • Additionally, and as it has been already suggested, the type of roof influences – and is influenced by – other considerations such as initial installation and maintenance costs, or considerations regarding slope and roof stability (Li & Yeung 2014).

A last consideration in the planning and implementation of green roofs is climate (temperature and rainfall patterns), microclimate and vegetation. Some factors to consider are:

  • the ability of plants to survive adverse growing conditions (e.g., evergreens in a limited substrate thickness),
  • the wind stability of shrubs and perennials in exposed positions,
  • the sensitivity of some species to reflected light and thermal built-up,
  • the sensitivity of plants to airborne chemical emissions and warm or cold air emissions,
  • the grasses with aggressive growth (e.g., bamboo) need extra protective membrane to protect from penetration,
  • the growth pressure of subterranean plant parts on building elements,
  • the competitive weakness of different species in thinner substrate layers.

Some more specific considerations in relation to sunlight conditions and vegetation exist for extensive roofs:

  • the effect of the wind and the intensity of insolation on water storage,
  • the demands on aeration of the substrate in dry locations,
  • the transformation towards forms of vegetations for dump locations in shady conditions or in wet areas,
  • the weaker competitive species in comparison to migratory flora,
  • the temporary or permanent reshuffling of moss species for seasonal reasons,
  • the possibility of increased non-native vegetation by immigrant species from the surrounding flora.