What are green roofs?
A green or living roof is a roof or deck onto which vegetation is intentionally grown or habitats for wildlife are established. There are different types of green roof and it is important to understand that each type functions and looks different. This is because each green roof is unique and provides a different type of habitat, water storage capacity and energy saving potential.
Green roofs can be extensive - implying shallow substrate depths and load nutrition, through to intensive - which can be similiar to roof gardens.
At present there are no specific British Standards relating to green roofs. However most if not all elements of green roof materials are covered by either a BSI standard or a German DIN standard, as described in the German FLL Forschüngsgesellschaft Landschaftsentwicklung Landschaftbau's (Landscape Research, Development and Construction Society) Guidelines for the planning, execution and upkeep of green roof sites. The 2011 GRO Codr provides UK specific green roof recomendations.
Historically, turf roofs were common in northern Europe, but decreased in popularity as the industrial revolution progressed.
Roof gardens became popular on multi-storey buildings in Berlin in the late nineteenth century and continued to be constructed on large city-centre structures throughout the twentieth century.
However, modern extensive green roof practice was stimulated by improvements in roofing and waterproofing technology emerging from Germany in the 1970s and 1980s. Legislation was introduced to encourage the installation of green roofs, and, by 2001, 43% of German cities provided incentives for green roof installation (Lenart 2001 Lenart, Claudia. 2001. "Garden in the Sky," Utne Reader, (104): 20 - 21).
Other countries in Europe such as Switzerland and Austria also have a long tradition in using green roofs. More recently Canada, parts of the United States of America, Japan and Singapore have experienced a growth in green roof uptake.
Grass Roof in Oswego Illinois, USA, from Greg Robbins
In terms of statute and policy, the UK lags behind continental Europe, however individual cities are championing green roofs. For example, the Living Roofs and Walls, Technical Report: Supporting London Plan Policy 2007, states that:
Policy Living Roofs and Walls
The Mayor will and boroughs should expect major developments to incorporate living roofs and walls where feasable and reflect this principle in LDF policies.
It is expected that this will include roof and wall planting that delivers as many of these objectives as possible:
- accessible roof space
- adapting to and mitigating climate change
- sustainable urban drainage
- enhancing biodiversity
- improved appearence
Boroughs should also encourage the use of living roofs in smaller developments and extensions where the oppurtunity arises.
Sheffield City Council is also leading the way with its Planning Conditions, requiring new building in excess of 1000m² or 10 dwellings to have 80% vegetated cover.
The UK has seen a rise in the interest and implementation of green roofs over the past decade, due to increasing awareness of sustainable development and the need to adapt the built environment to mitigate the effects of climate change. Other factors include the desire to preserve green spaces in the face of progressive urbanisation, and targets to conserve urban biodiversity. It is estimated that there is at least 200 million m² of roof space in the UK which could be greened with little or no modification to the roof structure. This consists of mainly flat roofs on offices, schools, hospitals, other public buildings and housing blocks.
Types of green roof
Green roofs are classified according to their depth and maintenance requirement. The following names for different green roofs have been adopted by the construction industry:
These definitions are not always clear cut and it is possible to have green roofs which have characteristics of more than one type, for example a 200mm deep substrate which still has low maintenance requirements.
For a green roof to flourish it must have:
Aeration to the plants root systems
The green roof system build-up can consist of:
Root resistant material
Moisture retention/protection layer
Most extensive roofs, and many intensive green roofs, are supplied as complete systems, which include all components for green roof construction from the insulation and waterproofing membrane to specialist soil mixes and vegetation.
These 'all-in-one' systems are useful in many applications, particularly where there is limited flexibility in terms of structural capacity as pre-made systems have a verified saturated weight. However, some suppliers of green roof systems may be able to supply and provide data for individual components which can allow for the possibility of bespoke designs.
The conditions of each individual site must be considered before construction begins, drawing on the skills of specialists such as: drainage engineers for the hydrological aspects, ecologists for the biodiversity elements and landscape architects for the overall layout. Ideally each green roof is bespoke to different site conditions and local climate. Therefore, designing green roofs to take into consideration both the client's needs and local microclimatic and bioregional factors will result in a better and more sustainable solution.