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Benefits of green roofs

Green roofs provide numerous benefits, however, it is worth remembering that a client will often have their own priorities. By considering each benefit at the earliest stages of design it will be possible to create a green roof which is multi-responsive.

number 31Reducing storm-water runoff as part of a sustainable drainage systems

Any built-up area needs to be drained to remove excess rain water. Traditionally this has been achieved by underground pipe systems that convey water away as quickly as possible to prevent localised flooding. However, as development increases, this can impact negatively on existing storm-water drains causing them to be overloaded and create the potential for flooding further downstream. The conventional method of draining the built environment also flushes pollutants directly into watercourses. The incidence of storm events is predicted to increase in intensity and number due to the effects of climate change, and the total volume of winter rainfall will rise (UKCIP02), therefore methods to mitigate storm-water runoff are increasingly being required in new developments. The Pitt Report 'Learning lessons from the 2007 floods' states that "new developments should not expect to be automatically connected to the public drainage system." The flood and water management bill will further enforce the responsibility of ground water management on Local Authorities, who will in turn apply greater sanctions on construction developers to include smart water management systems in projects.

SUDS are engineered solutions that aim to mimic natural drainage processes. They help to reduce pollution of watercourses and localised flooding, as well as providing amenity benefit and biodiversity. Green roofs are considered as one method of controlling storm-water at source (i.e. closest to the source of the precipitation) under a SUDS strategy. Once a green roof has established, both peak flow rates and total runoff volume of rainwater from the roof are significantly reduced compared to a conventional roof. Green roofs store rainwater in the plants and substrate and release water back into the atmosphere through evapotranspiration and evaporation. Green roofs are much easier to retrofit in the urban environment than many other SUDS components, so their potential for reducing storm-water problems in the UK's cities is significant. The amount of water that is stored on a green roof, and then evaporated into the atmosphere, is dependent on the growing medium: its depth, type of drainage layer and vegetation used and regional weather. The FLL Guidelines should be followed to ensure that actual runoff will be in accordance with calculated runoff. In summer, green roofs can retain 70-80% of rainfall and in winter they retain 10-35% depending on their build-up. The difference is due to a combination of more winter rainfall and less evapotranspiration by the plants because growth is not as vigorous during the winter months. Generally, the deeper the substrate the greater the average annual water retention. Where green roofs are used on a new development, it is sometimes possible for cost savings made on the drainage package to be used to offset the additional cost of the green roof installation. Green roof versus convention roof comparison Between 30 and 40% of rainfall events are of such limited duration, or are at such low volume rates, that there is no measurable runoff from green-field sites, whereas virtually all rainfall events produce some runoff from developed sites. A green roof can easily be designed to prevent runoff from all rainfall events of up to 5 mm and, as part of a SUDS strategy, should reduce the volume of surface or underground attenuation required at the site boundary. In the Environment Agency recognises the same positive effects, however, there is no commonly agreed method for measuring the amount of attenuation volume that can be offset.

number 32Roof lifespan increase

Roof surfaces are constantly under attack from ultra-violet light and temperature change. The original green roofs in Germany created in the 1880s were created when bitumen was covered with 6cm of sand, which colonised naturally with vegetation. This sand covering was to protect the bitumen from fire, however the covering was found to extend the life of the waterproof layer. Green roofs have now been shown to double if not triple the life of waterproofing membranes beneath the green roof. This is possible because the very act of placing layers of organic material and moisture holding materials on top of the waterproofing protects the roof.

NOTE: Where waterproofing material is used on the vertical faces of up-stands and parapets, the material should be double skinned or metal coping should be brought down to vegetation barrier level.

number 33Reducing energy use

The UK spends more money artificially cooling commercial buildings in the summer than it does heating them in the winter (Wildlife Trust website). Green roofs have been shown to impact positively on a building's energy consumption by improving its thermal performance, although the amount of difference this makes varies depending on daily and seasonal weather. Poorly insulated roofs, as found on many existing buildings, will lead to overheating of spaces beneath them during the summer, and excessive heating demand during the winter. By retrofitting green roofs, both air conditioning and heating usage is decreased. (Nottingham Trent University study)

During the summer, the amount of solar energy utilised for evapotranspiration from the plants and evaporation directly from the soil, reduces the temperature of the green roof and leads to a cooler microclimate which in turn leads to a reduction in artificial cooling requirements, which can be significant. During the winter months, a green roof can add to the insulating qualities of the roof. However, the thermal performance is extremely dependent on the amount of water held within the green roof substrate. In a damp winter climate, such as the UK generally experiences, a green roof will add little to the overall thermal performance of the roof. Green roofs are not assigned a fixed U-Value as they are often saturated with water.

number 34Climate change mitigation

Buildings are responsible for around 50% of the UK's CO2 emissions (CABE website). A high proportion of these emissions are from heating and cooling the internal environment. Reducing the energy consumption of the UK's buildings will reduce their contribution to climate change.

Green roofs can significantly reduce the cooling load of a building, resulting in reduced air cooling requirements and therefore reduced energy consumption and associated output of atmospheric carbon.

number 35Climate change adaptation

Certain levels of climate change are predicted to occur despite our efforts to reduce the production of carbon dioxide. Even in the most optimistic of scenarios, whereby the emission of greenhouse gases ceased immediately, the associated climatic changes would continue because of the long lifespan of the gases in the atmosphere and the thermal inertia of the oceans.

This means that summer temperatures and associated urban heat islands are expected to worsen. For this reason, adaptation to climate change must now be considered an urgent priority. Green roofs are one of the most effective ways of combating the urban heat island effect (Reducing urban heat islands: compendium of strategies. US Environmental Protection Agency) and will therefore be part of the raft of future measures designed to help cities adapt.

number 45What considerations are dependent on whether the green roof is new-build or retrofit?

Features to any Green Roof: •A building structure designed to take the loads of the proposed green roof •Waterproof membrane and root resistant membrane •Protective sheet •Suitable drainage system to allow excess water to leave the roof easily •500mm gravel or paving margin to all up-stands and roof penetrations •Up-stands of a suitable height to prevent material being blown off the roof •A 1m wide, 300mm tall un-vegetated barrier every 40m, on large areas of green roof •Growing medium with no more than 20% organic matter by volume a) New-Build Specific Green Roof System No matter what species of plants are specified, for them to flourish the green roof system •Sunlight •Moisture •Protective sheet •Drainage •Aeation to the plants root systems •Nutrients To achieve this, a successful system must replicate nature, but within a compressed space. Most green roofs will have depths of between 80mm and 200mm, only roof gardens incorporating turf, shrubs and trees would be expected to have greater depths. A common green roof build up consists of: •Roof resistant barrier •Protection layer •Drainage layer •Filter layer •Growing medium •Vegetation •Appropriate components i.e. outlet inspection chambers, 500mm vegetation barrier. The type, size, and design of each layer will depend on the proposed vegetation; as will the need for irrigation b) Retrofit Specific Structural Capacity When retro-fitting a green roof on to any building it is advisable to have a suitably qualified person undertake a structural survey of the building. Many older buildings were over designed/engineered and have spare capacity in their structure, which will allow green roofs to be fitted without additional construction. Conversely, many newer buildings and industrial units are built near to the limits of their capacity. The loading of the green roof must be established before the survey is carried out to ensure capacity. Once the building's general loading capacity is known and any strong or weak loading points identified, the green roof can be designed to suit, or the capacity of the building can be adapted. Deeper substrate depths can be placed where loading capacity is higher i.e. above supporting columns. Applied Waterproofing Even if the existing waterproofing looks to be in good condition, it should be tested for watertightness. If there are concerns over the condition of the waterproofing layer, it should be replaced or covered over with a new waterproof layer. Once the green roof is in place, the waterproofing underneath should stay in good condition for many years due to the protection offered by the green roof. If the existing in-situ waterproofing is to be used, it must be of a type which is root resistant or a separate root resistant membrane can be installed on top of it before the other green roof elements are installed. Water Escape When a green roof is being designed, there will be an anticipated level of water retention within the layers and substrate. It is crucial to the success of the roof that any excess water is drained from the roof as quickly as possible. This will prevent ponding or over-loading of the roof.

number 36Lessening the urban heat island effect

The urban heat island effect is the temperature disparity between urbanised areas and surrounding rural areas. Urban landscapes have a much higher proportion of dense, impermeable and often darker surfaces (typified by conventional roof surfaces) which tend to absorb the sun's heat.

This stored heat is re-radiated at night when warming the city more than the surrounding countryside. This can result in a temperature variation of 4 °C or higher between the city and surrounding areas. The urban heat island effect will increase as summer temperatures increase and will therefore become even more of a problem in the UK in the future. During the hot summer of 2003 night-time temperatures in London reached 8-9 °C higher than outlying rural areas on a number of occasions (London's Urban Heat Island: A summary for decision makers. www.london.gov.uk). Green roofs have very different thermal qualities to conventional roofs. They tend to reflect more of the sun's rays (have a higher albedo) and they shade and insulate the underlying structure. The evaporation of water from the soil and transpiration of water from leaves cools the roof. Diagram of thermal benefits of green roof (Shade, albedo, evapotranspiration)

number 37Increasing biodiversity

As more land is built upon, and management of urban areas intensified, ensuring that biodiversity is retained is a key requirement for local councils and public bodies under the 'Duty for Biodiversity' (Natural England Website).

Whilst green roofs do not directly replace ground-based habitats and are not part of a ground level 'green corridor', they can be thought of as green 'stepping stones' for wildlife, and, if well planned, can cater for a variety of flora and fauna unattainable on traditional roofs. Different types of green roofs and different types of substrate and vegetation will support different habitats and species. Biodiverse roofs can be designed to mimic various habitats. They can be especially important as a tool to recreate the pioneer (wasteland) communities that are sometimes lost when redevelopment takes place. Within urban areas, it is very often neglected brownfield sites that are the most biodiverse. The best designs may often be ones that supports a range of habitats for wildlife, including a range of substrates, microhabitats and substrate depths. Green roof designs should be varied in different parts of the UK to meet the objectives of local Biodiversity Action Plans. Green roofs can also be used to recreate habitat for some endangered species. For example in Deptford Creek in London, when habitat that was known to support the black redstart, which is rare in the UK, was threatened by redevelopment, suitable habitat was created on the roofs of new buildings (www.blackredstarts.org.uk). The skylark, a species listed on the UK Biodiversity Action Plan, has bred successfully on the green roof of Rolls Royce factory near Chichester. Surveys of Swiss and UK extensive green roofs have shown that they support several Red Data Book invertebrates (Ref Brenneissen and Kadas) and the conditions that allow these species to survive can be replicated. Diagram to show how Dr. Brenneissen recommends varying substrate depth to encourage invertebrates

number 38Improving air and water quality

The urban heat island effect exacerbates ground-level ozone production, which is formed by a reaction between volatile organic compounds and nitrous oxides catalysed by heat and sunlight. It is classified as a pollutant and is the principal component of smog.

During the heat wave of 2006 the European Environment Agency reported that safe levels of ozone were surpassed often and in many places throughout Europe, including the UK. Green roofs can improve local air quality through mitigation of the urban heat island effect. A living roof can also help to remove airborne particles, heavy metals and volatile organic compounds from the local atmosphere. As these contaminants are retained by the green roof itself, their infiltration of the water system through surface runoff is lessened, in turn improving local water quality.

number 39Reduced sound transfer

Rain hammer on some deck roofed buildings including schools have been concerns for building designers for some years. Sound escaping from industrial process and other noise processes can also cause issues within local communities.

The buildup of organic material within a green roof significantly reduces the noise transfer through a roofs structure.(Living roofs and wall, technical report: Supporting London's planning Policy)

number 40Amenity space

Designing green roofs for people to access either physically or visually is a great way to get vegetation and some habitat types into the built environment. Whether designed as a roof terrace with paving and raised planters for full access by building occupants or the general public, or high level meadows of wildflower with limited access. People will feel a benefit from being able to access the area as a desirable space.

The Fukuoka City Hall, Japan is a great example. By extending the vegetation up the back of the building the cities only central park was almost doubled in sizes. The terraces on the back of the building are full accessible to the public and provide a great oasis in a city of glass and concrete. Fukuoka City Hall, Japan