Green Roofs
What are they and why are we keen to see more in Birmingham?
Read a short but comprehensive primer written by one of Birmingham Friends of the Earth's Planning Campaigners…
Energy Efficiency
Many cities of the world could be described as being ‘thermodynamically inefficient’; that is, comprised of buildings and public transportation systems which are both energy consuming and polluting. Individual factors can influence the thermodynamic performance of individual buildings such as the level of solar gain, topographical constraints and local microclimatic conditions.
But the solution might not be simply to construct more (or larger) drainage channels.
The presence of rainwater itself, particularly during extreme flooding events can often provide a ‘critical mass’ resulting in the spillover or discharge of effluent material into natural watercourses. The Nutrients and Organic matter contained can act to provide a food source, helping to sustain Bacteria and Algae and contributing towards the pollution and degradation of the watercourse itself. Significant weather events in particular can have a highly negative effect on local Biodiversity.
The key challenge to all green roof enthusiasts is to promote the unprecedented advantages of Green Roofs in regulating the environment, towards their legitimate recognition as forming part of the wider urban landscape.
Roof membrane (Indoor)*
Roof ‘Assembly’
Root Resistant Layer
Drainage Layer
Filter layer
Growing medium (Substrate/ Soil)
Vegetation
(*Local Clay sources can be used to provide a natural impermeable substrate layer).
Shallow soils and low grazing vegetation sustain Biodiversity. There is also the possibility of including wild meadow flowers. However, Extensive Green Roofs cannot usually accommodate habitats for Worms/ Insects as the humidity (owing to shallow soils) is too intense.Intensive- (Brown Roof)
Provision for micro-habitats by use of local sub-soils/ rocks.
Semi-Intensive–
Combines the above two types.
Eco-Roof Trays-
Applied to the footing of a pitch roof whereby the flat surface comprises trays capable of collecting rainwater to harvest a range of crops.
On commercial buildings, the optimum load of a Green Roof should be 100Kg/ Square Metre (Sqm), and should not exceed 120Kg/Sqm. The intelligent use of ‘light substrate materials’ has been known to reduce the load to 80Kg/ Sqm.
- Green Roofs should be designed so as to support locally endangered species. The PH balance of soils on a Green Roof should be monitored regularly to ensure that environmental conditions do not change too much.
- Variations in the substrate type and thickness across a Green Roof can support a greater range of species. The substrate type should ideally be locally-sourced. For instance, Seaside locations might provide soils which can accommodate coastal grasses, whereas volcanic regions might apply rich volcanic soils. Similarly, local plant species can be selected and their seeds harvested for planting (for example, local wild meadow-flower species).
- The greater the Biomass of plants, the greater the level of rainwater retention. A Green Roof dwelling should typically be expected to store up to 12mm of a one-in-ten year peak rainfall event.
- Green Roofs can be applied to tall buildings; however, energy efficiency will typically only work best at the upper floor levels- whereby (in the absence of green walls), the lower floors would continue to emit Heat/ C02 through its exterior walls. For instance, an 8-storey block (typically) would allow 20% greater efficiency to the upper floor, declining to 1%- 4 floors beneath.
- A Green Roof should not be constructed in the summer months, as the conditions are too hot for the vegetation to become established. Bamboo, Vineweed and Japanese Knotweed specifically should be avoided, as the roots may interfere with a building’s waterproofing.
- Bird Species should also be supported on Green Roofs by the provision of ‘perches’ and ‘bird boxes’. In particular, this would help to avoid the disturbance of the roof during its initial period of growth.
Eco-Roof Issues
As Green Roofs are still being ‘experimented’ with, the Government would be hard-pressed to enforce the maintenance of a Green Roof for the purpose of promoting species biodiversity- unless conditioned as part of the planning permission (i.e. where development is proposed in a particularly sensitive area and where the proposed Eco-Roof is a mitigating measure).
Where an Eco-roof project has not worked as planned (in terms of provision for Biodiversity, and not structural integrity), there should not be an immediate requirement to ‘fix’ the situation as the roof could still have benefits in terms of reducing emissions and also act to provide valued amenity space. Being lenient on the issue of enforcement is key to securing developer interest in Eco-roofs, as this approach could help to allay concerns arising from the development industry regarding the perceived risk of failure of a project, and the potential to be held accountable or liable to insurance claims.
Green Roofs are readily being applied in many European Cities including Emscher and Freiberg (Germany), Basil (Switzerland) and Linz (Austria) amongst many others. Eco-roof legislation is much more advanced and more widespread in many European Countries than within the United Kingdom. There exists therefore a great deal of established expertise to lean on and associated sources of best practice to replicate.
Proprietors are primarily keen to ensure the integrity of their building stock. Opportunities should be sought to sell the wider benefits of green roofs to land and property owners in real-cost ‘economic’ terms, both in terms of the medium-long savings made on applied energy efficiency measures and the social and environmental value of a green-roof building as a ‘place to be’.
Green roofs make economic as well as ecological sense!
- SUDS is less expensive to apply and maintain than conventional rainwater drainage systems.
- Green Roofs require only a limited amount of maintenance each year.
- Green Roofs cost approximately 10 Euros per Sqm to introduce.
- As a ‘Marketing tool’- Green Roofs signal ‘green intent’. With an increased emphasis on Environmental Sustainability, Green consumerism is set to increase rapidly over the next decade. This increase in demand for new green products and services could resonate throughout the commercial property and development sectors.
- Because Green Roofs reduce the ‘heat flow’ between buildings and the environment, they reduce the need for (and cost of) artificial measures.
When discussing Green Roof issues with representatives of the development sector, it is best to convey cost savings in terms of the following:
There is widespread acknowledgement that the first cost of a new project is the last cost of the previous (on-site) project.
Refers to the ‘…analysis and identification of cashflows regarding the acquisition and use of an asset.’
Where a site is not contaminated, the existing features of a Brownfield site could be seen as a resource through which to enhance the sustainable qualities of a new proposed Eco-roof as part of a development proposal. This approach could help to reduce off-site transportation costs, both in terms of the cost of bringing in new substrates and also disposing of unwanted materials. Instead, the developer would commit to recycling/ re-using demolished materials on-site. Crushed Material can form the underlying substrate of Green Roofs or be used to create landscape bunds
In respect of Infrastructure/ Highways planning- a commitment to SUDs would be starting point.
- Highway SUDS Design interventions: (Where a gradient exists)
Road priority schemes often comprise intervening raised structures that result in the narrowing of the carriageway. It is suggested that hard surfacing be replaced with grass/ pebbles, surrounded by a raised kerb (apart from on the water-collecting side).
Equally, where rainwater run-off is more frequent/ intense- a linear channel (space) between the road and pedestrian footpath could be an option.
- A Green Roof applied to a Chicken shed consequently resulted in a net increase in the number of eggs being laid.
- Green Roofs applied to an agricultural setting might contribute to reduced carbon emissions, whilst increasing net productivity.
- Different roof structures support different microclimates, species habitats and food sources.
- Eco-roofs can help to re-introduce plants, soil and water into the (hard)/ built environment.
Thermal Insulation Mechanism
Soils themselves have insulating properties. The thermal mass of soil can absorb solar energy, which is then released back into the atmosphere at night time. Plants can also assist in further shading the soil. Plants naturally undertake the process of Evapo-transpiration, which assists in cooling down further the Roof & Building temperature. The same processes occur in Winter although at a lesser rate.
Environmental Advantages of Green Roofs
Eco-roofs can help to reduce the rate of surface run-off from a development:
- Plants and soils can act to slow, filter, cleanse, evapotranspirate & infiltrate rainwater run-off. Green roofs can therefore reduce the effects of stormwater run-off by slowing down the rate of movement throughout the system.
- Where possible, consideration might also be given to raising/ exposing/ de-chanellising appropriate watercourses.
- Eco-roofs can contribute towards Sustainable Urban Drainage System (SUDS), providing a means of intercepting excess rainwater, akin to a ‘soakaway’.
- Green Roofs can be combined with Garden Ponds/ Water Butts/ Porous Pavements, SWALES and Soft Landscaping areas.
Data relating to Green Roofs applied in Vancouver, Canada, indicates an average annual roof temperature of 11-14 Degrees Celcius in Summer & 6 Degrees Celcius in Winter. This ensures that the latent heat of the roof remains at a relatively uniform state ensuring no extremes in temperature and helping to reduce the Urban Heat Island Effect.
Research to date highlights the apathy towards Green Roofs. Factors include:
1- Assumptions regarding long-term maintenance costs
2- Not truly understood
3- Assumed high costs of products (in respect of the high costs of some green technologies, such as Solar Panels)/ profit losses.
4- How Green Roofs would be received in the marketplace (whether would affect lettings)
5- Potential leakages (where wrongly applied)/ Liability issues
6- Limited public demand/ mixed attitude to green issues
7- Economic failure of previous ‘radical’ ideas- industry as ‘risk averse’
8- Shareholder/ Stakeholder concerns
9- Limited collaboration and consensus
10- Skills shortage in ‘green construction’ within the building industry
11- Client focus on bottom-line delivery
Portland (USA)
City/State Planning Department- Green Roof Policies:
1- City Public Works Code: Advocates ‘Eco-roofs’
2- City Zone Code: Where Eco-roofs are added, the building floor space is permitted to increase in size.
3- City Resolution- passed 2005. Requires all city buildings to install eco-roofs when re-roofing (if practical to apply).
4- The City Bureau of Environmental Services and office of Sustainable Development- provide Technical Assistance at no cost to the developer.
The Green Roof movement in Austria was born as a reaction to the intense 1960s/70s Industrialisation and the associated pollution which came with it. Compulsory laws were introduced in 1985 regarding the need to implement Green Roofs as part of the development process. This comprised at least 10 milestone regulations. The following examples of the applied legislation suggest that plant coverage:
1- Should reach at least 80% of the roof area
2- Should be applied to all new extensions if proposed to be over 100 Sqm.
3- Should be applied to all new Commercial/ Industrial Units proposed to be larger than 500 Sqm.
Exceptions: Where over 60% Open Green Space is proposed on-site/ or if the development is located within the immediate Urban Centre- where the environmental benefits of ‘centralised’ or intensive mixed-use development can be realised, consequently reducing the need for travel.
Financial incentives are available to all who seek them, particularly at the initial ‘concept’ stage of a project. Cross-departmental supplementary information is readily available and local authority staff are highly trained and experienced on the issue.
Green Roof Trays capable of producing crops of plants/vegetables have been applied to School Roofs which the Children take responsibility for maintaining; most of whom claim to also want one on their roof at home.
As a consequence, in 2006 over 40 ha of green roofs are now present across a spatial area of 37 Square Miles (as falling under the municipal ownership of the town).
The aerial photographs of the city taken both before and after the legislation was introduced in 1985 show a highly visible greening of the city. Linz will be the European Capital of Culture in 2009. Can Green Roofs therefore also help to foster a ‘green’ culture?
The GRP comprises Planning Officers/ Groundwork Sheffield/ University of Sheffield Lecturers/ City Architects/ Environment Agency
In June 2006, the GHP obtained £800k from EU Objective 1 Funding (over 2 years) to promote Green Roofs throughout the South Yorkshire Sub-regional area.
The GRF meets every two months. It is currently assisting in the development of draft Green Roof Strategic Planning Policies. It is looking to establish a demonstration centre- which will be readily available to prospective Green Roof Applicants.
- How can other Local Authorities achieve an increase in Green Roof development without adequate funding?
Example projects can help build momentum. Engaging local decision-makers and attempting to‘re-prioritise’ funding towards ‘Green Roofs’ would ensure a truly holistic approach to good Urban Planning.
- How can Eco-Roofs be achieved?
1- Holistic approach to the subject matter
2- Multi-disciplinary consultations
3- Apply an introductory/ user-friendly approach to the technologies available
4- Set increasing standards from which to benchmark against
5- Regulate
6- Provide Incentives
7- Share Information/ Knowledge
8- Training- Engineers/ Landscape Architects/ Designers
- How might Local Authorities be able to assist in promoting development through the preparation of their Strategic Plans?
A Green Roof Plan could help to provide a map of ecological areas- acting as a guide to the appropriate eco-roof required in a particular location. A Biodiversity Action Plan should further help to guide the choice of plant species required to support local Ecosystems.
Productive discussions with Water Utility Companies (WUCs) should also take place.
At present, WUCs are often reluctant to introduce SUDS as the Infrastructure required to support artificial drainage systems also provides ‘an economic asset base’. WUCs should therefore be encouraged to lend their experience towards the development of new SUDS affiliated businesses, (i.e. suppliers of ‘rainwater harvesting systems’).
- What are the key driver’s underpinning the need for Green Roofs?
The key drivers for achieving Green Roofs should be Climate Change and shifts in Local Government policy towards more sustainable forms of living. In Germany, National political pressure led to the acting Construction Minister Barbara Sneidder introducing legislation specifically relating to Green Roofs.
There is also a growing ‘marketable’ public interest in Green issues and Green consumerism, which should be investigated and exploited.
- How might Green Roofs be achieved on the ground?
Increased sources of guidance and a commitment to improving the depth of widespread knowledge on ‘Green Roofs’ should help.
The Greater London Assembly in conjunction with the London Biodiversity Partnership will in Autumn 2006 release new guidance entitled ‘Building Green’; which will provide detail on plant species & habitats applicable to Green Roofs.
Town Planners and Urban Designers should work together with developers from the initial concept stage of a proposal to provide sound strategic guidance and in order to disseminate site-specific information regarding what is required in respect of Green Roofs and renewable technologies.
Further sources of information are available to view at the following websites:
www.portlandonline.com/bes
www.livingroofs.org
www.greenroof.bcit.ca
www.greenspec.co.uk
www.ciria.org/buildinggreener
www.greenroof.co.uk
www.wlcf.org.uk
www.balancedvalue.com
www.dqi.org.uk
Multinoma Co.Ecobuilding
Ciria Roof Specifications
