Green roofs

See also: blog posts about green roofs

London Roof Garden
Roof Garden Cafe on the Queen Elizabeth Hall on London’s South Bank

The landscape architecture and town planning professions should work together on greening the roofs of the future:

  • town planners need to be involved because green roofs are long-term public goods
  • landscape architects need to be involved because a thin covering of sedum is an inadequate response to a great opportunity.

Roof gardens and green roofs provide a wide range of public and private goods:

1 Amenity value

Accessible roof gardens add value to buildings. They make it easier to obtain planning permission. They enhance capital values and rental values. They make buildings easier to sell and easier to let. They provide space to enjoy the sun, to hold social events, to grow flowers and to grow food. Many companies who do conventional landscaping like, would be interested in helping out a rooftop programme. This applies to housing, offices, libraries, department stores and many other types of building. All new schools and university buildings should have roof gardens to educate future generations in their benefits.

  1. Storm Water Management

Managing stormwater with green roofs reduces flooding, reduces expenditure on drains, reduces expenditure on canalising rivers and reduces expenditure on flood defences. Compared to conventional roofs, have much lower runoff coefficients. They reduce peak flow rates, by detaining water, and reduce total rainwater runoff, by evaporating, transpiring and storing water in the plants and in the substrate. Green roofs can retain 70-80% of rainfall in summer and 25-40% of rainfall in winter. Conventional roofs and pavements evaporate 5% of the rainfall and discharge the rest. Since over 90% of the land in big cities is often impervious, green roofs make a vital contribution to storm water management at a much lower cost than underground drainage systems.

  1. Air Quality

Plants reduce carbon dioxide, heavy metals, airborne particles and volatile organic compounds from the atmosphere. A green roof coupled with something such as these cupolas for sale can allow for natural lighting and ventilation to access the building through the roof too, meaning better air quality on the interior and exterior of the building.

  1. Biodiversity and Wildlife

Conventional roofs make hardly any contribution to biodiversity and wildlife objectives. People can, however, find other ways to make their homes more green, even if they need services to help them with their current roofing scenario. Green roofs make a significant contribution, particularly if they (1) have a soil depth of >150mm (2) are designed with local substrate materials (3) are planned on a citywide basis

  1. Energy Conservation improved thermal performance

A green roof acts as an insulation barrier, reducing the needs for heating in winter and air-conditioning in summer. Photosynthesis and evapo-transpiration reduce the amount of solar energy absorbed by the roof membrane.

  1. Health

Visual and physical contact with natural greenery provides a range of health benefits, including mental benefits (such as reduction of stress) and physical benefits (including the provision of cleaner air)

  1. Urban Heat Island and Albedo Effects

Cities have large areas of hard reflective surfaces which absorb solar radiation. Reducing the quantity contributes to climate change objectives.

  1. Noise and sound Insulation

Green roof systems absorb, reflect and deflect sound waves. Soil blocks lower sound frequencies. Plants block higher sound frequencies.

There should be official support for greening existing roofs, as there is for home insulation and green energy.

Information on Green Roofs for landscape architects

Green roofs in Germany

There are three planning and fiscal mechanisms in Germany promoting green roof : i) landscape use planning and building regulations ii) separate sewarage drainage charges iii) direct financial subsidy.

  1. i) Land Use Planning: green roofs are a compensatory measure in a development planning and so are specified for low-pitched and flat roofs, based on the Baugesetzbuch BauGB ( Building Code Building Code) § 9 Abs . 1, No. 20 and No. 25 a and b) which covers development plans, which reads:

“für einzelne Flächen oder für ein Bebauungsplangebiet oder Teile davon sowie für Teile baulicher Anlagen mit Ausnahme der für landwirtschaftliche Nutzungen oder Wald festgesetzten Flächen

  1. a) das Anpflanzen von Bäumen, Sträuchern und sonstigen Bepflanzungen,
  2. b) Bindungen für Bepflanzungen und für die Erhaltung von Bäumen, Sträuchern und sonstigen Bepflanzungen sowie von Gewässern;”

(for individual areas or for a development plan area, or part thereof, and for parts of building structures with the exception of areas, fixed for agricultural uses or forest

  1. a) the planting of trees, shrubs and other plantings,
  2. b) bonds for planting and for the preservation of trees, shrubs and other plantings and to water bodies;)

Basically this part of the Building Code promotes planting.

  1. ii) In separate sewerage charges, the cost of sewage and storm water are determined separately. The retention of rainwater on green roofs has a saving and can be a financially beneficial impact for building owners, if this effect is taken into account in the building costs. The savings according to surveys by the German Roof Gardener Association ( Deutschen Dachgärtner Verbandes) on average are 0.46 euros per m² turfed roof area per annum [6] Maximum savings could be upto 1.12 euros per m² turfed roof area per annum, as the example shows Cologne. (1)

iii) direct financial subsidy: in addition, financial support schemes are used, but there are no nationwide uniform guidelines. (1) In Germany, subsidies for green roofs are an average of 10-20 euros per square metre, with a pro-rata funding the upper limit is usually at 50%.(1)

The Berln City website is quite good for urban greening and sees green roofs as being an element of SUDS and also good for biodiversity a , and significantly as part of a much wider programme of urban “greening” (i.e. a Biotopflächenfaktor ) which are responses to the following Federal and Länder legislaton:

  • Bundesnaturschutzgesetz (BNatschG – Federal Nature Conservation Act)
  • Bundes-Bodenschutzgesetz (BBodSchG – Federal Soil Conservation Act)
  • Berliner Grünanlagengesetz (GrünanlG – Berlin Park Act)
  • Baumschutzverordnung (BaumSchVO – Tree Protection Ordinance)

Berliner Bodenschutzgesetz (Bln BodSchG – Berlin Soil Conservation Act).

For an overview ref. (in English)

The history of this goes back some decades, for example in 1985 it was then a West Berlin requirement to have vegetated flat roofs.

Green roofs in Switzerland

Re Swiss roof gardens see use a translation engine from German e.g

and for one canton, Basel

For Basel the cantonal building regulation reads

for the legislation : Bau-und Planungsgesetz[1] (BPG) Vom 17.11.1999 (Stand 01.01.2013) Der Grosse Rat des Kantons Basel-Stadt,

e.g. a very simple clause: 1″clause 72 3.VI.5 Ungenutzte Flachdächer sind mit einer Vegetationsschicht zu überdecken.” (Unused flat roofs are to be covered with a layer of vegetation). “Unused” means that roofs used for example, say solar panels, can be exempted.

Green roofs in London

A recent Greater London Authority map Green Roofs in central London shows some 700 roofs and is on:

Mayor of London Living Roofs and Walls (2008)

International references on green roofs

See the website of the International Green Roof Association

And the European Federation of Green Roofs Associations