Green roofs and walls

From the turf roofs of Viking dwellings in Scandinavia to the ‘hanging gardens’ of ancient Babylon, green roofs have a history reaching back thousands of years. Modern green roofs and walls are designed to support living vegetation in order to improve a building’s performance.

A green roof is a roof surface, flat or pitched, that has a growing medium over a waterproof membrane planted partially or completely with vegetation. Green walls are external or internal vertical building elements that support a cover of vegetation, rooted either in stacked pots or growing mats.

Design: RAAarchitects. Photo: Brett Boardman

Green roofs are an accepted part of modern building in Europe, and some city and national governments have even mandated their use. The Austrian city of Linz, for example, requires green roofs on all new residential and commercial buildings with rooftops larger than 100m²; and in Germany, green roof building has been encouraged by the Federal Nature Protection Act, the Building Code, and state-level nature protection statutes.

Australian examples, while less common, are increasing. In 2007, a national organisation, Green Roofs Australasia, was formed to promote green roofs, and various city councils have introduced policies and programs to promote green roofs and walls.

Interest in green roof and wall construction is increasing, encouraged by the availability of technologies that make their construction easier and more economical.

The benefits of green roofs and walls include:

• reduced heating and cooling requirements for the building
• reduced and slowed stormwater runoff
• improved sound insulation
• carbon sequestration
• capture of gaseous and particulate pollutants
• alleviation of urban heat island effects
• increased biodiversity.

Green roofs can be particularly useful in denser, more urban environments, where they can compensate for the loss of productive landscape at ground level

Green wall techniques can be used on housing of all types in urban and suburban settings, to improve the overall climate responsiveness of individual dwellings, and even to treat wastewater. The One Central Park apartment building in Sydney boasts the world’s tallest green wall.

Types of green roofs and walls

Green roofs may be ‘extensive’ and have a thin growing medium (up to 200mm deep) with ‘groundcover’ vegetation, or ‘intensive’ with soil over 200mm deep supporting vegetation up to the size of trees. In between these types, there are semi-extensive (extensive with areas of deeper soil) and semi-intensive roofs (intensive with areas of shallower soil). There are also different types of green walls, varying in method and complexity.

Earth-covered construction, where the building structure is partly or entirely covered by earth and vegetation, can be regarded as a specific type of green roof. For more information on earth-covered construction, see the case study below.

Extensive green roofs

Extensive green roofs have a shallow profile. This means less substrate volume is available to store water and support root growth, although there is still enough to reduce and slow stormwater runoff. This limits the variety of plant options to those that tolerate environmental stress, especially drought and drying winds. Many native plants from coastal and arid inland regions may be suited for use in such an environment.

Characteristics of an extensive green roof are:

• shallow growing medium — typically less than 200mm
• roof structure similar to conventional roof coverings
• weight 60–200kg/m²
• vegetation generally limited to low, shallow-rooting and groundcover plants that tolerate drought, wind exposure and temperature fluctuations
• not suitable for general access
• relatively economical
• some thermal and acoustic insulation benefits
• May be easy to retrofit on existing roofs - seek advice from a qualified professional to ensure your roof is able to take the weight
• low maintenance.

Source: Paul Downton

Intensive green roofs

Intensive green roof profiles can range from 200mm to over 1m deep. This increases the volume of growing medium available for root development and water-holding capacity, and greatly extends the variety of plants that can be grown. The additional weight demands a stronger physical roof structure than extensive green roofs, but allows for foot traffic. Intensive green roof gardens can be as richly planted and landscaped as ground-level gardens. They require the same level of maintenance as conventional gardens.

Characteristics of an intensive green roof include:

• deep growing medium — 200mm or greater
• requires stronger roof structure
• weight 180–500kg/m² or more
• wide range of plantings possible, from groundcovers to trees
• suitable for access and use as roof garden — wide scope for design and multiple uses
• relatively expensive due to structural requirements
• substantial thermal and acoustic insulation benefits
• difficult to retrofit on existing buildings
• regular maintenance required.

Source: Paul Downton

Green walls

Green walls are like vertical gardens and may be inside or outside a building. In their more elaborate form, green walls are ‘living walls’ and may even incorporate water elements, such as ponds and fish. Green walls may also be incorporated into the cooling strategy of a house, as a kind of evaporative airconditioner, and may even be designed as part of a water treatment system (refer to Passive cooling). An increasing number of proprietary green wall systems incorporate irrigation systems. Like green roofs, green walls at their simplest can be made on a low-tech do-it-yourself basis, or be quite complex and potentially expensive.

Types of green wall include:

• green façades — pots or vines on walls or trellises
• active or ‘living’ walls — using growing mats or modular vessels containing a lightweight growing medium, supported by cable or frame structures
• passive — epiphytes (plants that grow on a surface with no need to be attached to soil or growing medium).

Case study: Living green walls

The ‘living’ green walls wrapping the One Central Park apartment building in Sydney span 34 storeys and are the first of their kind in the world. They consist of vertical vegetated wall panels, combined with horizontal planter boxes with steel cables that vegetation can grow on. There is no soil in the vegetated walls, so plants grow within a felt material. The planter boxes contain some soil along with lightweight growing medium to reduce weight and improve drainage. Plant species were selected for their ability to resist wind and be easily maintained, with the majority being native to Australia. An integrated irrigation system supplies the plants with recycled water from the on-site treatment plant.

Photo: Sardaka

Buildability, availability and cost

Green roofs have higher construction costs than conventional roofs, with limited short-term investment returns. However, the long-term returns are potentially excellent. Individual properties benefit from reduced maintenance and running costs. The cost of green walls can be highly variable depending on the system, with green façades being cheaper to construct than active systems.

In addition, green roofs and walls can attract increased property values, as demonstrated in North America and the United Kingdom. Without legislation to encourage their construction, Australian uptake is likely to be driven by building rating systems that value green roofs and walls, their environmental benefits, and recognition of improved market values.

Appearance

Green roofs and external green walls can be small and incidental, or large and dramatic. They extend the scope for creating pleasant urban environments by introducing plants and greenery that are visually restful or refreshing. Proven therapeutic effects for commercial or office buildings include increased productivity and reduced absenteeism.

Green roofs can resemble anything from a lawn to a forest. Extensive green roofs with a thin layer of growing medium to support groundcover plants, are generally designed with building performance in mind rather than aesthetic concerns. Sloping and curved extensive green roofs may be seen from ground level.

Intensive roofs can support quite substantial, highly visible vegetation, cascading over the sides of the building, or shrubs and trees rising above the roofline. These landscaped ‘roof gardens’ can change a building’s roofline.

Photo: Paul Downton

Indoor greening can be an extension of the green wall concept or can include the creation of indoor planters as integral parts of the house. Green walls make it possible to have lots of greenery without using too much floor space.

Green wall systems range from arrangements of planting pots on layers of custom shelving, to sophisticated vertical layers of growing medium, geotextiles and purpose-made containers. Depending on the size of the wall, large or small plants can be used, and the result can be manicured and elegant (think of a carefully clipped hedge) or wild and funky.

Photo: The Greenwall Company

Structural capability

Green roofs are usually flat but may be curved or sloping. Supporting structures must be designed to be able to carry all the loads associated with the vegetation, its supporting medium, and the waterproofing and protective layers beneath — plus any live loads from people using the roof.

Green walls may be freestanding structures or depend on the building’s main structure for support using trellises, cables or frames.

Photo: © Peter Hughes Photography

Durability and moisture resistance

If well designed and maintained, a vegetated roof can extend the life of a conventional roof by some years. By preventing direct solar impact on waterproofing membranes, a green roof protects against damage from ultraviolet radiation and from constant heating and cooling of the membrane. Similar benefits derive from using green walls that add an extra ‘skin’ of protection to a building, as long as adequate drainage and ventilation are provided behind it.

Green roofs should be designed to last at least 50 years. Replacement of any components of green roofs can be relatively costly so key structural considerations include:

• longevity of the structure
• appropriate drainage
• waterproofing.

Thermal mass and insulation

In Australia, one of the most valuable energy benefits of green roofs is their ability to reduce summer cooling demands. Their contribution to insulating and shading buildings can help significantly in reducing energy consumption and greenhouse gas emissions. Green roofs and walls can also pre-cool intake air in air-conditioning systems, reducing systems requirements.

There is little thermal mass in the vegetative component of green roofs. Although there may be some mass in the soil, the usual growing medium is lightweight, so is consequently more useful as insulation and shading than thermal mass. Green walls have a relatively low thermal mass for the same reason. The supporting structure for extensive roofs (and green walls) is also usually lightweight, with little thermal mass, whereas the structures required for intensive roofs usually uses concrete slabs or similar structures with a higher thermal mass.

Green roofs may or may not include an insulating layer in addition to the soil and vegetation, but even without such a layer they provide significant thermal insulation and shading for the building.

It is difficult to obtain accredited insulation values for green roof construction. For specifying and code compliance purposes, thermal insulation standards should be met by conventional means with the additional insulation value of a green roof regarded as a bonus. An energy assessor may be able to give some credit for a green roof (for example in Nationwide House Energy Rating System (NatHERS) software) by reducing the reflectivity or emissivity of the roof.

Green walls can be retrofitted to existing homes to reduce the heat load on façades. In warmer weather, green walls reduce the surface temperature of a conventional wall through shading and evapotranspiration of the plants. Walls that use irrigation and hydroponic techniques provide additional cooling through evaporation. Deciduous vegetation can limit solar gain by shading in the summer while allowing daylight in during winter.

Sound insulation

In busy urban settings, the acoustically absorbent nature of soil and vegetation of green roofs can insulate against the noise of heavy vehicles like trains, trams, buses and trucks. One office building under the flight path of San Francisco’s International Airport, planted with a mixture of indigenous grasses and wildflowers, helped to achieve reductions in noise levels. Tests have indicated that 5 inches of growing medium can reduce noise by 40 decibels.

Fire and pest resistance

The soil used in green roof construction adds to its fire resistance. The different kinds of vegetation that might be found on a green roof range from shallow-rooted succulents, which burn poorly and offer good fire resistance, to more substantial plants on intensive roofs that can include shrubs and trees. Although dry vegetation can present a hazard, the amount of dry vegetation on an extensive roof is unlikely to support more than low-intensity fires, assuming general site preparation actions are taken as recommended by fire authorities in your area. There are no relevant Australian codes as yet, but as an example, German building codes require a 600mm fire break in the planting every 40m. Fire-activated sprinkler irrigation can further reduce risk.

Animal pests have not been identified as a problem for green roofs or walls, perhaps because they represent a deliberate effort to incorporate living material into a building, and create habitat in which there is greater biodiversity and less imbalance between humans and other fauna.

Photo: Paul Downton

Toxicity and breathability

Vegetation in urban areas, including that on green roofs and walls, can filter out fine airborne particles that then wash off into the soil. Further, foliage can absorb many gaseous pollutants. Studies have shown that green roofs can trap up to 95% of heavy metals in the local atmosphere.

If growing vegetables for consumption in inner urban areas, it may be a good idea to have soil and plants regularly tested for heavy metal or other pollution.

Indoor green walls can reduce humidity and oxygenate the air. Depending on the plant species, they further improve indoor air quality by acting as filters, trapping dust and absorbing pollutants such as volatile organic compounds that may be given off by carpets, paints, adhesives and sealants.

Environmental impacts

Green roofs and walls can contribute to sustainable development objectives, including:

• stormwater management – green roofs can reduce the costs of dealing with an increase in peak rainfall events associated with climate change by retaining stormwater and slowing rainfall runoff. Research at the University of Queensland has shown runoff reductions of up to 42% achieved with only 100mm of soil planted with moderate growth turf; as the soil depth and vegetation water use is increased, so does retention capacity
• pollution reduction – retention and binding of contaminants (for example bird droppings or atmospheric pollution) can help remove harmful pollution from runoff into aquatic ecosystems
• heat island effect reduction – the insulating and low thermal absorption properties of green roofs and walls reduce the urban heat island effect. Researchers at the Welsh School of Architecture, Cardiff University, concluded that green roofs and walls could cool the local climate around a building in a city by 3.6 to 11.3°C, and the hotter the climate, the greater the cooling effect
• conservation and increase in biodiversity – green roofs and walls can create habitat to increase biodiversity and attract wildlife including rare or migratory birds. Encouraging birds, butterflies and bees has been a significant aspect of some suburban green roofs overseas. For example, in the United States, a large vehicle manufacturer has a bee farm on the 4ha green roof above one of its factories
• greywater treatment – domestic greywater treatment has been achieved in Australia using a green wall consisting of a series of 3 planter troughs that act as filters, removing nutrients, polluting compounds and organic matter from the water
• local food production – the potential for food production on green roofs is being investigated in Australia. Led by Central Queensland University, research uses urban organic wastes via vermiculture for production of vegetables and development of urban rooftop ‘microfarms’
• increase in solar panel efficiency – by lowering ambient roof temperatures, green roofs enable solar panels mounted over them to operate more efficiently, with energy outputs up to 15% more than from panels on asphalt or gravel covered roofs which heat up
• electromagnetic radiation reduction (EMR) – EMR can be reduced by more than 99% with a 100mm substrate depth.

Design and detailing

When designing a green roof or green wall, things to consider in design drawings include:

• structure
• membranes
• mats
• drainage and irrigation
• trellises
• micro-climate and roof orientation
• plant selection
• integration with building functions

There are several Australian guides that cover design and construction of green roofs. The Royal Institution of Chartered Surveyors (RICS) has published an Australian guidance note, which explains the various steps that need to be considered for green roofs or walls to be installed on new buildings, or to be retrofitted on existing buildings (refer to References and additional reading).

Selecting the correct growing medium for the climate and appropriate plants is essential, particularly for extensive roofs. Plant selection for green roofs requires careful consideration, as different conditions apply to vegetation on the roof than on ground level. Long-term growth and maintenance requirements, including watering needs, also need to be considered. Both green roofs and green walls need to allow for irrigation of vegetation while avoiding soil erosion, and to provide reservoirs of water to carry plants through periods of low water availability.

The structural and waterproofing elements of green roofs, properly installed, require little maintenance. As with all aspects of building, good construction detailing reduces the risk of failures and facilitates access for repairs (for example, in the event of leaks).

Risk assessment

As with any building material, green walls and roofs have risks that must be considered at both concept stage, and when undertaking detailed design work. Risks include:

• drought – most green roofs and all green walls will require some irrigation, especially in the early stages of growth. Consider the availability and sources of irrigation water – will it be reliable in dry years? Water should come from on-site harvesting, not mains supply. Bore water may not be suitable for all plant species, and surface water from nearby rivers or dams may not be physically or legally accessible in times of drought.
• bushfire – the performance of green roofs and walls in bushfires is still quite poorly understood. While the soil bed could help insulate the building from radiant heat and burning embers, any combustible components of a green roof or wall, including the plants themselves, could serve as fuel during a bushfire. Those considering green roofs or walls in bushfire-prone areas, should confirm whether they are allowed within the bushfire attack level (BAL) applicable to their property under Australian Standard AS 3959. It would also be advisable to select a system that contains no combustible materials, plant species that do not burn easily, and most importantly ensure that the roof or wall is kept well irrigated. The areas surrounding the roof or wall must also be maintained and prepared as per the recommendations of the local fire authority, in line with good practice.
• high wind – wind speed increases with height above ground, and a doubling of wind speed equates to a quadrupling of its power. Vegetation on green roofs and walls on high-rise buildings and lower buildings in exposed locations (hill crests, clifftops, etc.) must be provided with means of protection. The 34-storey green façade at One Central Park required extensive engineering, modelling and wind tunnel studies during the design phase of the building to develop a system of stainless steel cables and rods capable of dealing with high wind conditions.

The build process

On top of the structural components, a green roof typically has 7 layers:

• waterproofing membrane (single-ply membrane or fluid-applied membrane; modified bitumen or plastic sheeting is most typical)
• root barrier (polyethylene sheeting, copper or copper compounds in the membrane)
• insulation (optional, however may be required to meet compliance)
• drainage layer (synthetic drainage mesh, granular aggregate)
• filter fabric (geotextile)
• growing medium, also known as planting medium or substrate (manufactured soil, crushed brick or other inorganic material which may be supplemented with organic material such as coconut fibre or coir)
• vegetation (shallow rooted on extensive roofs, deeper rooted on intensive roofs).

Green walls are constructed with plants rooted in sheets of fibrous material, which may be fixed to a wall or frame, or constructed more like vertical arrays of pots or planters. Some proprietary green wall systems come in the form of modular panels. Plants may be pre-grown in these panels or planted after the panels have been installed. Materials include steel for supporting frameworks, high-density polyethylene (HDPE) plastic for plant containers, and geotextiles. In exterior applications, irrigation may be from the top through soaker hoses or similar. Interior applications may use drip trays.

Photo: © Peter Hughes Photography

Earth-covered (also known as earth-sheltered) construction can be regarded as a type of green roof and is one of the oldest forms of construction. Often built into hillsides, modern earth-covered homes are usually purposely designed to ‘disappear’ into the environment and have a minimal visual impact. The earth is covered with vegetation and often the only sign that there is a house comes from windows.

Earth-covered homes use the enormous thermal mass of the earth to help maintain a steady and comfortable temperature. Depending on the thickness of the soil, ground temperature, structure and other building elements, earth-covered construction can provide substantial thermal and acoustic insulation. For this to work best, the walls of the building should connect directly to the earth. In practice, waterproof membranes and insulation layers may compromise some of that connectivity.

Earth-covered houses are designed to bring light into rooms as much as any normal home. The main difference is that if a house is set into a slope, most if not all of its windows are on the downslope side of the building, usually facing the solar direction. Other rooms may be day-lit through skylights or ‘borrowed’ light from sunlit rooms.

Although earth-covered homes are relatively rare in Australia, their buildability is equal to that of more conventional houses. They must be well engineered. Most of the ‘external’ walls of earth-covered houses act as retaining walls, and need to possess sufficient structural capacity and construction detailing to deal with the large sideways physical forces, and hydraulic pressure, that come from building into the earth. Their roofs are essentially green roofs, usually intensive roofs that use soil from the site itself.

The main structures of earth-covered buildings are not exposed to weather, but do have to be protected against moisture penetration for many years. The waterproof membranes of the roofs and walls are protected by the earth that covers them, adding to their longevity. Plant root barriers are a necessary part of the roof construction. You may need expert advice to ensure the insulation provided by the earth roof is taken into account for the purpose of demonstrating code compliance for building approvals.

• Baggs S, Baggs D and Baggs J (2009). Australian earth-covered and green roof building, 3rd edn. Interactive Publications, Wynnum, Queensland. 
• City of Los Angeles (2006). Green roofs – cooling Los Angeles: a resource guide [PDF].
• City of Sydney, Green roofs and walls.
• Cooperative Research Centre for Water Sensitive Cities, One Central Park green walls.
• Department of Environment and Primary Industries (2014) Growing green guide: a guide to green roofs, walls and facades. Victorian Government [PDF].
• Dunnett N and Kingsbury N (2008). Planting green roofs and living walls, 2nd edn, Timber Press, Portland, Oregon. 
• Dunnett N, Gedge D, Little J and Snodgrass E (2011). Small green roofs: low-tech options for greener living. Timber Press, Portland, Oregon. 
• Earth Pledge (2005). Green roofs: ecological design and construction. Schiffer Publishing, Atglen, Pennsylvania. 
• Green Roofs Australasia. 
• Hopkins G and Goodwin C (2011). Living architecture: green roofs and wall. CSIRO Publishing, Collingwood, Victoria. 
• Manso M and Castro-Gomes J (2015). Green wall systems: A review of their characteristics. Renewable and Sustainable Energy Reviews 41:863–871. 
• Perkins M and Joyce D (2012). Living wall and green roof plants for Australia. Rural Industries Research and Development Corporation, Canberra. 
• Royal Institution of Chartered Surveyors (2016). Green roofs and walls, 1st edn. Royal Institution of Chartered Surveyors, London.
• Snodgrass E and Snodgrass L (2006). Green roof plants: a resource and planting guide. Timber Press, Portland, Oregon. 
• Werthmann C (2007). Green roof: a case study. Princeton Architectural Press, New York. 

Learn more

• Read Shading to explore other ways to passively cool your home with shading structures and plants
• Explore Landscaping and garden design for tips on designing your garden to suit your region
• Read Reducing water use to find out how to use less water in and around your home

Authors

Original author: Paul Downton 2013

Updated: Dick Clarke 2020