Green Roofs, and Their Importance to the Built Environment

The recent changes in the political landscape are likely to have an impact on all planned expenditure for both new-build and refurbishment projects in the UK, nonetheless it is already clear that environmental issues will continue to play a key part in future strategic thinking. With both the Conservatives and Liberal Democrats having promoted green policy issues heavily in their manifestos and the election of the first Green Party MP to Parliament, there is clearly a strong desire across the political divide for continued action to reduce the environmental impact created by both new and existing structures.
With the continuing drive for sustainability, many bodies in both public and private sectors are seeking ways to support communities for many generations to ensure they enjoy their surroundings and appreciate where they live and work. Maintaining existing green spaces is critical to this goal, as is seeking opportunities for the creation of additional green areas to meet the commitment to improving the environment both now and for the future.
Buildings, and their efficient management, have a significant impact on our environment and with the exciting new innovations in construction technology that will benefit the occupiers and owners alike, it is vital that our buildings, both current and new, are sympathetically developed and upgraded to deliver a safer, healthier and more attractive environment. One way in which to achieve this is to look to the skylines to realize these goals.
The new Government’s focus is likely to be a balance between the desire for urban regeneration, coupled with the inevitable need for greenfield development in carefully selected areas, both of which will drive the need for innovative approaches to the design of new buildings to meet the ever more onerous principles of sustainable development. What this will most likely mean is a rising pressure for urban regeneration and the drive for increased density levels on all forms of development, which will have adverse impacts on drainage, water abstraction, biodiversity, accessible ground level green space, and local climate conditions. Green roofs can play a positive role in helping to mitigate these impacts and contribute towards a better quality urban environment.
Although green roofs in the UK date from the 1930s, a conventional building and planning culture has served to constrain their installation. However, in the past 5 years there has been a renewed interest in green roofs, and rising confluence of growers, suppliers, ecologists, architects, designers, and others with an interest in green roofs to bring about the necessary changes in order for green roofs to play an important and rightful role in the design and life of our towns, cities and villages.
This drive has brought to the attention of many in the public and private sectors that green roofs represent the way forward and that their well documented benefits can assist in the delivery of both environmental targets and biodiversity action plans. In addition, it is increasingly being recognised that green roofs can assist in reducing the energy usage of a development whilst also delivering rainwater attenuation characteristics that can be incorporated within the sustainable urban drainage scheme for the site.
For last few years it has been recognised that green roofs can deliver a positive benefit in terms of thermal insulation, through their ability to cool buildings in the summer and keep them warm during the winter, which is dependent on daily conductance of the green roof. Recent research in Germany has shown that retrofitted green roofs can contribute to savings in heating costs on existing structures of up to 2 litres of fuel oil/m2/year. The Possman Cider Cooling and Storage Facility in Frankfurt had a green roof installed and the management succeeded in recovering the cost of it’s installation in 2-3 years, through the savings achieved in heating and cooling and the reduction in industrial equipment associated with these activities. Further evidence comes from the experiences gained by a building services manager in London, who discovered that the application of a retrofitted green roof on a building which he was responsible for had significantly reduced the need to use the cooling and heating plant on the floor beneath. Since the green roof was installed, the use of both cooling and heating fans had significantly reduced and it is estimated that about 25.9 MW per year is now being saved, which at current electricity rates represents as saving of over £4,300 per year. If the insulating benefits of the green roof to the floor below had been identified and it had been installed as part of the original design for the building, it is estimated that there would have been a capital saving of £10,000, due to reduction in heating and cooling equipment needed for the floor below.
For the forward thinking Facilities Manager, a green roof could well be feasible for the refurbishment of a currently failing roof or indeed for any additional buildings being considered for the expansion of current premises.
When considering if a green roof could be successfully included, it is important that a number of preliminary questions are answered:
  • Is the site suitable for a green roof? – an understanding of the factors affecting a green roof must be understood; climate, building structure, plant requirements.
  • What performance is required from the landscape? – is it to mitigate storm water, maximise water retention, offer additional energy savings, provide recreational space, etc
  • How is the preferred green roof system to be specified? – is the type of green roof to be extensive for a low maintenance visual effect that will improve the air quality and rain water run off, intensive for recreational use or biodiversity where a particular species of wildlife is to be supported?
  • Where do you go to get meaningful advice? – with many people professing to be green roof specialists, who really are the experts with a proven track record and which system will deliver the technical credentials required?
  • How do you obtain meaningful guarantees? – a green roof needs to be able to survive for a long time, and assurances for this are crucial.
Everyone must be involved and fully support the project! Regardless of the type of roof selected everyone must be involved; client, architect, specifier and the local planning authority. The planner should determine if the chosen roof is feasible at the location and make it clear in the planning constraint the wildlife which is to be targeted for a biodiversity roof. The architect’s main objective is to specify the landscape and its support system, to ensure the structure of the building is not placed at risk. A knowledgeable planning department will pick up any anomalies to the application and insist that all factors be suitably considered. The Local Authority planner will previously have seen numerous applications which simply state that an extensive green roof is required; which can be a problem as there are at least six different variations of this type of green roof. Each variation will need a different balance of growing medium type and depth combined with a range of differing drainage characteristics, to support the proposed vegetation and other elements of the planned micro-ecology. A biodiversity roof needs to specifically state which type of wildlife is to be targeted and which particular plants and habitat items are required, otherwise you cannot be sure what you will end up with.
Essential factors to be considered for all green roofs
The first consideration for any green roof installation is the waterproofing system, which should be of high quality and robust; the capping sheet will often be a root barrier to prevent penetration, although this is not always the case, and any alternative root barrier will also need to meet the German FLL standards to ensure its integrity. It is also important to ensure that the roof drainage system is designed to be capable of coping with intense rainfall over short periods, as recommended within the current British Standards. Safe access for maintenance of the vegetation must also be considered and planned for and the roof should be leak tested before the landscaping is installed.
A variable factor to be considered for all green roofs
Increasingly, green roof systems are being installed on slopes of in excess 0f 5°, and these need to be designed and installed in a way to ensure that they cannot slip off the waterproofing. Whilst this is relatively easily achieved on slopes of up to 20°, steeper slopes require more careful consideration and additional restraint of the growing medium. The design of the structure will also be significant in determining if a green roof installation can be achieved.
Maintaining a Green Roof
Whilst many green roofs are intended to have a “natural” look, it is important to recognise that a certain minimal level of maintenance will be required on every installation, if only to control the build-up of bio-mass and keep the roof outlets free of debris. This should be identified as a key building maintenance requirement from the outset and planned accordingly. Whilst it is not uncommon for the maintenance to be undertaken by the building owner as part of the general management of the structure, it is increasingly common for the work to be undertaken by a local green roof maintenance specialist, trained to work at height and familiar with the requirements for the effective maintenance of green roofs.
There are challenging times ahead for all sectors of the Facilities Management community with many environmental benefits to be gained through the use of green roofs on both new and existing structures. The key to the success of green roofs as part of a sustainable ecological policy will continue to be in depth knowledge and understanding of every stage in their delivery and post installation maintenance. With it, the sky is literally the limit!