For a green roof to be successful, it is very important that all parties involved work together and fully understand what is to be achieved to avoid the risk of failure.
Points to consider should be:
- Why is a green roof required and what performance is expected?
- Is the site suitable?
- Who should be involved in delivering the project?
- Essential and variable factors for consideration.
There could be many reasons why a green roof is required. It may be to satisfy a planning constraint, in which case economic options will be considered; mitigate against storm water where the design will maximise rainwater attenuation; support a specific species of wildlife for a biodiversity solution; provide recreational space with public access; offer additional energy savings – photovoltaic units are up to 10% more efficient when used on a green roof; or for aesthetic reasons where the building needs to be masked into its surrounding environment.
There are some fundamental considerations when planning a green roof and many factors which will influence its success.
Location factors• Regional and local climate
• Volume of rainfall
• Amount of sunlight
• Exposure - strength of wind, amount of frost and snow
Structure factors• Areas exposed to sun, shaded areas & where they alternate
• Deflection of rainfall
• Wind flow conditions and roof exposure
• Slope of the roof
• Design loads
Plant or Wildlife Factors• Effect of wind & intensity of solar radiation
• Demands made by plants
• Sensitivity to airborne chemical contamination, and to warm or cold air
emissions from the building
• Suitability of proposed biodiversity species to be installed
Everyone must be involved and fully support the project, regardless of the type of roof selected, from the building owner and client to the architect and main contractors. 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 type of landscape and its support system to ensure the structure of the building is not placed at risk. On some projects it may also be appropriate to seek the services of an environmental consultant to advise on issues such as meeting a BREEAM or Biodiversity Action Plan requirement.
One of the major factors to consider is the waterproofing, which should be robust and of high quality; the capping sheet can often be a root barrier to prevent penetration, though this is not always the case and any root barrier should meet FLL standards to ensure its integrity. The roof should be leak tested before the landscaping is installed. The drainage should always be capable of coping with intense rainfall and incorporate a filter sheet to prevent substrate being washed away. Safe access for maintenance must be considered and planned for. The growing medium will vary to suit the landscape required, and for biodiverse systems the vegetation and habitat materials selected to suit the wildlife that is to be encouraged.