2.6. Growing Substrates
The growing substrate supplies water and nutrients to plant roots, ensures gas exchange at the roots and provides anchorage to support plants. Growing substrates for green roofs are typically composed of a mix of inorganic (mineral) and organic components. They can include scoria, ash, pumice, sand, coir, pine bark, porous, chemically inert foams and even recycled materials such as crushed bricks, and roof tiles. Organic matter is usually kept to a low proportion (typically 20 per cent or less) because it has a relatively short lifespan, degrading and slumping over time, and may become water repellent and difficult to re-wet if it dries out. The physical and chemical properties of the substrate mix, together with its depth and total volume, influences what vegetation can be supported on the green roof. A substrate should:
- have a known saturated weight loading, that forms part of the structural load capacity of the roof; this is referred to as the saturated bulk density
- drain freely, to reduce water logging and prevent inundation during heavy rain, but also retain adequate water to sustain plant growth outside of heavy rain events
- be stable over time, usually achieved by using a high proportion of mineral components and a lower proportion of organic components.
Soil is generally not used on green roofs, as its properties will be less well known compared to an engineered substrate, and therefore its longevity and suitability is harder to judge. Also, silt particles from soil can clog filter sheets and cause drainage problems. Installers of green roof systems will be able to arrange the supply of a suitable substrate mix. Transport and installation of growing substrates requires consideration in the pre-construction planning process. Most can be installed by either lifting bulker bags by crane or ‘blowing’ from a truck-mounted hopper, although each project will have its own specific considerations. A range of mineral and sheet mulches can be used successfully on green roofs but need to be considered carefully. Organic mulches, particularly fine materials, are generally unsuitable for use on green roofs as they can be easily blown off the roof, degrade rapidly, block drains, or create a fire risk in hot dry conditions. The saturated bulk density of any proposed mulch layer must be included in the weight loading calculations for the green roof build-up.
A range of planting stock can be used for green roofs, including seed, cuttings, seedlings, tube stock and larger containerized plants. Depending on the time of year and rainfall, establishment irrigation could be required for up to six months. It is helpful to acquire container-grown nursery plants that have been grown in the substrate they are to be planted into; otherwise, reduce the amount of potting mix held around the roots when transplanting into the green roof. Controlled release fertilizer (CRF) can be added to the growing substrate or applied after planting is completed (top dressing). Careful consideration of rates and application methods is needed to ensure proper distribution and to limit any rapid discharge of nutrients. Trees planted on green roofs will require a deep substrate as well as anchorage to prevent wind-throw. Tree bracings must be checked regularly to ensure they are functioning as specified and not causing damage to the tree. Trees will require construction of a tree ‘pit’ in the substrate to house the tree roots: this needs to be deep and wide enough for lateral root growth to ensure tree stability.
2.8. Thermal Insulation and Leak Detection
In some cases, a green roof may include thermal insulation, typically a layer of extruded polystyrene. While this can be positioned below the roof deck, installation above the waterproofing (known as an inverted green roof) is preferred, as it further protects the membrane from condensation and physical damage. Advice should be sought from a building energy consultant to establish the value of additional insulation, considering the insulating properties of the green roof assembly, substrate and vegetation.
Leak detection is carried out when the waterproofing installation is complete. There are three methods typically used for testing the effectiveness of the waterproofing layer on green roofs:
It is advisable to perform an additional leak test prior to installation of the growing substrate if significant time has elapsed since the waterproofing was completed, or if there has been construction activity or other traffic on the roof. The repair of leaks presents a greater challenge once the growing substrate and vegetation are installed. However, EFVM® leak detection should be performed at this time (project completion). It may also be carried out prior to the expiration of membrane warranties.
2.9. Irrigation Systems
An irrigation system is highly advisable on green roofs: to extend plant species selection options, to improve plant growth rates and increase long-term vegetation success – thereby ensuring aesthetic and environmental (such as building cooling, storm-water reduction) outcomes can be achieved. Planning for irrigation on a green roof should consider the site layout and conditions (access, exposure), type of plants, climate and water supply issues (pressure, quality, etc.). Substrate properties and depth are also important as they influence water infiltration, holding capacity and drainage. In most cases, irrigation design will be heavily influenced by the nature of the water supply resource (for example, harvested vs. potable water) and a water budget should be used to guide not just irrigation but also plant selection. In larger green roof projects, irrigation is best undertaken by a specialist consultant to guide system design, component selection, installation and maintenance.
2.10. Automatic systems
If an automatic irrigation system is to be installed on a green roof, consider a system that incorporates a rain sensor that shuts off the system in the event of rainfall above a certain threshold. This removes the risk that the roof loading may be compromised if the irrigation system is running during a heavy downpour. Even automatic systems require regular physical checks and operation tests. Low irrigation It is important to understand that, while some succulent species can survive on rainfall alone, plants that experience moisture stress will decline over time. This leads to a loss of vigour, leaf shedding, canopy reductions and ultimately plant death. Plant failure means more water run-off from the roof, less transpiration cooling and more opportunities for weed invasion. As such, irrigation is advisable. However, if designing for very low water use, select plant species that are better able to tolerate the extreme moisture stress on a green roof during a typical summer. Another option is to provide supplementary irrigation on a contingent basis during the hotter periods of the year. Depending on the level of attention that can be provided, inclusion of indicator species with moderate requirements for water may be helpful to show when supplementary irrigation is necessary. This avoids compromising the performance of all the plants.
2.11. Irrigation frequency
During the establishment phase after planting, irrigation may be frequent, for example, two to three times per week. For food crops, irrigation will also be necessary during high activity phases of the growth cycle, such as when the plant moves into flowering and fruit set. The frequency of irrigation should be matched to the drainage and water-holding capacity of the mix: frequent irrigation of a very free-draining substrate is likely to waste water. The irrigation delivery method will partly determine the timing of irrigation. Watering in high daytime temperatures will transfer more heat into the building, as water heats up as it passes through the hot growing substrate and transfers some of this heat into the building when it drains onto the roof surface. For surface and subsurface irrigation, there is little to no wetting of the foliage, which lowers the risk of fungal disease. If spray irrigation is used, it should be applied very early in the morning to enable foliage to dry off throughout the day and thus reduce the likelihood of disease.
2.11.1. Moisture Sensors
Be aware that moisture sensors used to estimate moisture content in standard landscaping soils do not provide reliable information about the moisture content of free-draining, porous growing substrates used on green roof.
2.12. Wind Considerations
Even on flat roofs, wind uplift may present serious challenges for retention of substrate and plants. Wind damage can be dangerous to people and property and costly to repair. Retention systems may be required to ensure that materials cannot be blown off the roof. Wind uplift pressure is lowest on the centre of the roof and highest at the edges, around the perimeter and at corners. The higher the building, the greater is the risk. On pitched roofs, the roof peak is also subject to uplift. Minimising untethered components greatly reduces the risk of damage to the green roof by uplift. Where possible, waterproofing should be fully adhered to the roof, or mechanically fixed. For waterproofing layers that are not bonded to the roof, the green roof assembly must provide adequate ballast to prevent uplift.
Edge treatments are the most critical: the un-vegetated zone around the perimeter of a green roof may require heavy concrete paving slabs rather than loose gravel ballast. The materials used must conform to the design wind load calculated for the specific green roof location. On some green roofs, perimeter balustrades or parapet walls will provide some protection against wind flow. Other treatments, such as jute erosion control netting or coated wire retaining systems, may be useful. Plant selection may also be used to mitigate the impact of winds. If the tolerance of different species to wind exposure is understood, plantings can be planned so that the lowest-growing, most robust species are planted in the most exposed areas. Taller, less wind tolerant species are installed behind them. The resulting gentle gradation of vegetation heights interrupts, and slows, wind flow over the planting.
2.13. Slopes and Wind Protection
Landscapes on pitched roofs are subject to wind forces and gravity affecting the stability and retention of the growing substrate and plants. In many cases slippage can lead to poor plant performance and ultimately green roof failure. For roof pitches of up to 15 degrees, no additional protection is needed, unless there are strong wind issues on the site. Waterproofing must be root resistant, and covered with a protection mat. A drainage layer is not always required as the roof can drain effectively through gravity. The stability of steeply pitched green roofs is increased by maintaining vegetation cover: provision of irrigation is essential. For green roofs constructed on pitches greater than 15 degrees, basic protection can be provided through anti-erosion jute netting installed just below the substrate surface to provide some anchorage to plants (see image). This netting breaks down over time, and is used simply to help keep the substrate in place whilst the plants establish.
Greater protection can be provided on steeper sites by using a drainage layer with large cells, or cups. The growing substrate fills the cells of the drainage layer, reducing slippage and providing spaces for plant roots to grow, ensuring further stabilisation. A filter sheet must be placed beneath the drainage layer to reduce wash-through of fine particles from the substrate. For green roofs with slopes between 20 degrees and 45 degrees, ‘honeycomb’ webbing, comprising multiple, enlarged drainage cells, can be installed above the drainage and filter sheet layers (Figure 4). This holds the substrate in place, increases stability and reduces slippage. Other specialised structural elements can be incorporated to transfer shear forces into stable, reinforced abutments that form part of the roof structure, or wind barriers can be installed.
A green roof on a slope can include a drainage layer with large cells to retain substrate as well as a layer of erosion control netting laid close to the top of the substrate to help retain substrate and plants during the establishment period.
2.14. Hard Landscape Elements
Some of the functional elements that are used in green roofs include:
- non-vegetated zones
- retaining edges
- topographical construction
- planting containers
- drains and gutters
- a range of other elements, not discussed in detail here, such as: harness attachment points; controller boxes/solenoid boxes (to house irrigation components); decorative and functional landscape elements such as decking, paving, seating, shade protection, ponds and lighting.
The colour of materials should be considered as this will affect their heat gain. Consider location of shade structures as additional elements. Non-vegetated zones Non-vegetated zones are used to group roof penetrations vent pipes and other up stands and assist in lateral drainage. They are generally constructed with large diameter aggregate rock or ballast (16-32 mm size), rather than the growing substrate, and provide additional lateral drainage into the roof drains. They are usually between 300-500 mm wide and are separated from the roof perimeter ballast by metal edging installed around the planting area. Similar vegetation-free zones may be created through use of paving slabs or ballast to provide access pathways across the green roof, or as firebreaks on very large roofs.
2.15. Retaining edges
Edging can be used to define and retain planted and non-vegetated zones across a green roof. It can include concrete, stainless steel, recycled plastic or aluminium products; L-shaped edges are installed above the filter sheet and often have perforations to allow drainage through the profile. Topographical construction Blocks of polystyrene foam can be used to build up areas to create mounds or hills without the additional weight of the substrate. Topographical variations on a green roof create different growing conditions and microclimates to increase habitat opportunities for beneficial insects, as well as visual, aesthetic interest.
2.16. Planting Containers
Planters must be made from weather resistant materials, and the components must be physically and chemically compatible with each other. Common examples of materials used to build planting containers are powder-coated metal, galvanised steel, ceramic, timber, UV stable plastic and glad reinforced concrete (lightweight concrete).
2.17. Drains and Gutters
All drains must be accessible for maintenance, protected from blockage by leaf litter and substrate wash, and housed with inspection chambers, drain covers, filters or strainers. Inspections after construction, following storms and every three months are recommended. For drains located flush with the roof surface, a grille should be installed to prevent drain clogging. Flashings Roofs with a parapet that extends above the roof deck require installation of a cover (flashing) to protect the building fabric. This should be included in the waterproofing installation to ensure those membrane terminations, and any areas of membrane extending over the vertical and horizontal surfaces of the parapet, are not exposed.
2.18. Planting Containers
Planters must be made from weather resistant materials, and the components must be physically and chemically compatible with each other. Common examples of materials used to build planting containers are powder-coated metal, galvanized steel, ceramic, timber, UV stable plastic and glad reinforced concrete (lightweight concrete).
2.19. Drains and Gutters
All drains must be accessible for maintenance, protected from blockage by leaf litter and substrate wash, and housed with inspection chambers, drain covers, filters or strainers. Inspections after construction, following storms and every three months are recommended. For drains located flush with the roof surface, a grille should be installed to prevent drain clogging. Flashings Roofs with a parapet that extends above the roof deck require installation of a cover (flashing) to protect the building fabric (Figure 5). This should be included in the waterproofing installation to ensure that membrane termination, and any areas of membrane extending over the vertical and horizontal surfaces of the parapet, are not exposed.
Functional elements on a green roof can include flashing (capping) and non-vegetated zones
A well designed and maintained green roof, wall or facade can provide: • aesthetic improvements in highly visible locations • protection of building materials leading to an increased lifespan of those materials • reduced building heating and cooling costs due to increased insulation • green spaces (and sometimes usable recreation spaces) in densely built environments • increased property value • food production areas for residents or commercial tenants • rain water run-off management and water filtering/pollution reduction • habitat creation and increased biodiversity • cooling effect for the city – where there is a high density of green roofs, walls, facades and other types of greenery such as street trees • cleaner air, with less pollutants.
A development application (DA) is needed for all green roofs and many green walls. While not all of the information below may be relevant to your particular project, these are standard conditions that apply to many developments. With your DA, think about submitting: • Evidence the green roof or wall has been assessed as part of the structural certification for the building. • Evidence the green roof or green wall has been assessed as part of the waterproofing certification for the building. • A cross-sectional diagram that details all the components of the green roof or green wall: – The location of existing and proposed structures; – Drainage, irrigation and waterproofing, and overflow provisions; – Earthworks and mounding and retaining walls and planter boxes (if applicable); – The proposed growing medium, with soil types and depth; – The location, species and numbers of plants likely to be used; – Safety features such as balustrades and maintenance hooks (if applicable); – The parts of the green roof that are accessible and inaccessible; – How the green wall is attached or fastened to the wall. You will also need to submit a maintenance plan to the City. This will help ensure the green roof or wall is properly maintained (https://greenroofsaustralasia.com.au/sites/default/files/sponsors/files/Green-Roofs-DA-and-Design-Advice_CoS_0.pdf).