Rooftops can represent approximately 40-50% of impervious surfaces in commercial and industrial areas and up to 25% for residential areas.
Therefore, living roofs have a significant potential role to play in stormwater management (Dunnett & Clayden, 2007). Living roofs comprise a vegetation layer and growing medium over a waterproof membrane. They are aesthetically and environmentally rewarding for a wide range of commercial, residential and institutional applications. Further guidance for living roofs and walls can be found in Auckland Council Technical Report TR2013/045 Living Roof Review and Design Recommendations for Stormwater Management.
There are generally two understood types of living roofs: intensive and extensive. Intensive living roofs have a relatively deep soil media to support a wide range of plants and structures. These roofs often require additional load bearing structures. Intensive living roofs may be accessible spaces, providing gardens or parks above the street level. They can be also used to grow food if access is provided for residents.
Extensive living roofs have lightweight layers of free-draining media to support low-growing drought-resistant vegetation. Extensive living roofs have greater application to 'retrofit' existing buildings, as the additional weight loading is usually no more than 100 kg/m2. Roofs with existing concrete tiles have merit, since they are designed to accommodate additional weight. Notwithstanding this, an assessment by a Council-recognised qualified structural engineer will need to be carried out to fulfil any building consent requirements.
Extensive living roofs are designed for maximum thermal and hydrological (reduced stormwater runoff) performance, and minimum weight load. This type of roof is not normally accessible, except for maintenance. However, they may be visually accessible from adjacent buildings.
The vegetation and media on living roofs capture atmospheric deposition of contaminants, promotes evapotranspiration, cools stormwater runoff, and slows down rainfall response times, thereby reducing peak flow rates and runoff volumes. Studies of extensive living roofs have indicated they can remove between 40% to 80% of annual precipitation through evaporation, depending on the climate and growing media (Carter & Rasmussen, 2006; Van Woert et al., 2005; Deutsch et al., 2005; Hutchinson et al., 2003, cited in Wises et al., 2010).
There are many benefits of installing living roofs in addition to stormwater management, including:
- Accessible or perceived open space
- Enhanced building design, or visually mitigating less desirable building aspects
- Reduced energy costs through insulation of a building and localised cooling around air conditioner intakes
- Noise insulation
- Enhanced air quality and dust interception
- Ambient temperature moderation
- Increased service life for underlying roof materials.
Living roofs can provide suitable habitat for tolerant or mobile animal and plant species. Within the urban centre of cities, living roofs may be the only 'green space' available, providing links or stepping stones in a network of habitats. Designing living roofs so that they have varying substrate depths and drainage regimes creates a mosaic of microhabitats on and below the soil surface, and can facilitate colonisation by a more diverse flora and fauna (Brenneisen, 2006).
Living walls include plants growing directly on walls, on supporting structures, or within lightweight modular systems. Vegetation is similar to that specified for living roofs, with shallow root systems and resistance to drought (unless plants are climbing from an area at the base of the wall).
Similar to living roofs, living walls provide noise and heat insulation benefits. They may intercept direct stormwater precipitation or may have runoff from rooftops and like surfaces directed to them for irrigation. This will contribute to the reduction of peak flows and total volumes and may provide significant water quality renovation.
Challenges and solutions
The table below describes some of the common issues relating to living roofs and living walls. In all circumstances there is a potential design solution, which must be balanced against other objectives for the project.
The cost of a living roof
Consider savings from reduced energy use, extended roof life, and the cost of an equivalent stormwater management response when undertaking life cycle costing.
The challenge to plant health in a fast-draining media
There are multiple pilot projects within Auckland and ongoing trials at the University of Auckland to determine the appropriate soil media to balance water and nutrient demands with the free-draining properties of living roofs. This information is provided in Auckland Regional Council Technical Report TR2010/018 Extensive Green (Living) Roofs for Stormwater Mitigation: Performance Monitoring.
Many plants have also been trialled for Auckland conditions for a variety of elevations above street level, aspect, and media depth.
The weight of living roofs
Living roofs in Auckland have been formed with media as shallow as 70 mm thick with a high proportion of free-draining materials.
‘Filter strips’ of living roofs or larger plants may be applied over walls, beams, and columns.
The slope of roofs (>10 degrees for drainage) may not be suitable to retain media and plants
Soil retention methods may be adopted such as mounted boards, terraces and geotextile to reinforce planting media.
Construction access may be limited in retrofit situations
Plan ahead and allow time to obtain necessary permits. Plan roof construction work in parallel with other programmed construction activities. Utilise modular systems that are easily transported and in some instances pre-grown.
Performance in storm events
Living roof monitoring projects in Auckland have shown that living roofs meet WSD objectives for stormwater quantity control. This information is provided in Auckland Council Technical Report TR2010/018 Extensive Green (Living) Roofs for Stormwater Mitigation: Performance Monitoring.
Utilise safety barriers for access. Set in place irrigation systems and slow release fertilisers for plants to establish coverage quickly and to minimise ongoing maintenance requirements. Inspect the living roof to ensure it is still in place and that plants are healthy. Carry out any required maintenance identified during inspection.
Living roofs increase the lifespan of traditional roofs by a factor of two as they provide a buffer from harmful UV rays and thermal effects. Living roofs in Germany have been in place for 50+ years without the need for replacement.