Green roofs are becoming more popular across the country to create healthy, environmentally responsible buildings. They improve building performance and support living vegetation on flat or sloped surfaces.
A green roof can extend the life of your roof by protecting the waterproofing layer from weather and temperature changes. They can provide sound insulation, reduce heating and cooling needs and slow down storm water runoff, mitigate the urban heat island effect, trap gaseous and particulate pollutants, and improve air quality. These buildings can support local biodiversity, create new outdoor spaces for recreation, grow food, and support residents’ physical and mental health.
Green roofs have come a long way since Viking times and the Middle Ages, when most houses in Scandinavia had a “turf roof”. Modern green roofs have flourished around the world since their massive sales in Germany in the early seventies. Last year, France passed a law requiring the roofs of new buildings in business districts to be partially covered with plants or solar panels. In 2014, the City of Sydney adopted a green roofs and walls policy. Under this policy, the City of Sydney will encourage the installation of quality green roofs and walls by supporting, promoting, researching and overseeing the installation of green roofs and addressing key barriers to implementation.
There are two types of green roofs. Extensive green roofs have a thinner growing environment, while dense green roofs have deeper, heavier soils.
Extensive green roofs are shallow and can provide the environmental benefits of green roofs, but cannot support normal foot traffic. Large area green roofs have a growth environment of less than 200mm and a roof structure similar to traditional roofing. They weigh between 60 and 200 kg/m² and are relatively economical. They can support less water and root growth, so a limited number of hardier plants can be grown. They have some thermal and acoustic benefits and are relatively easy to retrofit to existing roofs.
Intensive green roofs are the big brother of extensive green roofs. They have a larger profile and can be up to 1m deep, allowing them to support larger plants and have greater water holding capacity. They can weigh 180-500 kg/m2 or more and require a stronger physical roof structure. This makes them a more expensive option, but they are designed and used in a wider range. They have the advantage of better thermal and acoustic insulation, but are difficult to retrofit into existing buildings.
Once you have decided on the type of green roof and made sure the engineer has accounted for the saturated soil plus the total weight of all the plants, the next step is to select the material for each layer.
A typical green roof consists of seven layers on top of structural components. It starts with a waterproof membrane, which is usually followed by a root barrier, additional insulation, drainage layers, filter cloth, growing medium, and finally vegetation.
In Australia, waterproofing membranes fall into three broad categories: liquid membrane treatment, preformed sheets, including “single layer” and integrated systems.
Liquid coated membrane materials can be made from bituminous emulsions, modified asphalts, polymer cement systems, polyurethanes, polyurethane modified acrylic materials, acrylic materials, or two-component polyurethane hybrid elastomers. The material is liquid, hand-applied and suitable for small roofs or roofs with many studs, walkways, corners or steep slopes. Liquid-coated films are easy to apply, resistant to surface imperfections, easy to repair or reapply, and seamless. But they are not resistant to roots, it is difficult to ensure a uniform thickness, and they can become brittle when exposed to sunlight and heat.
Prefabricated membranes are suitable for large, flat or sloping roofs. Most of them are based on bitumen or polymer and are either loose or fully bonded to the substrate. They are uniform thickness, bituminous base systems that are fired and seam welded for added airtightness and strength, but are generally impenetrable, UV or root resistant. PVC, EPDM and oil films are root resistant, while thermosets and thermoplastics are UV resistant. Installing prefabricated membranes requires a high level of skill, and condensation or moisture can soften the adhesive or tape at seams and joints.
To create an integrated system, waterproofing additives are added to the concrete mixture. They are used on larger concrete structures such as parking lots or landscaping of car park buildings.
The root barrier layer protects the waterproof membrane from damage caused by plant roots. This may not be necessary if a waterproofing membrane with root protection is used. Typical materials used are polyethylene (plastic) sheets and polypropylene geotextiles. Make sure the root barrier sheet is compatible with the waterproofing membrane to avoid adverse reactions or chemical interactions.
Green roofs provide significant insulation, but since it is difficult to obtain approved insulation values for green roof buildings, conventional insulation may be required to meet BCA insulation standards.
Drainage layers carry away excess water and must provide a balance between water-retaining soil and stormwater drainage.
Plastic gutters, or downspouts, are a common drainage material for green roofs, installed as a continuous layer. Plastic drainage layers can be rigid, open mesh, or have a bowl-shaped design that allows water to accumulate at the bottom and use the plants later. Cup drain boards should overlap to eliminate gaps between boards, while other plastic drain boards should connect adjacent boards together. For high traffic green roofs, install a rigid drainage layer to avoid compression.
The drainage layer is covered with a geotextile filter cloth such as wool or other woven material. This layer holds the soil in place and separates the drainage layer from the growing medium, preventing the growing medium from clogging the drainage layer or storm sewer system.
Then comes the growing medium, which is artificial soil, crushed brick, or other inorganic material that can be supplemented with organic material such as coir or coir. Use a mixture of natural soils and organic or mineral supplements to help with water retention, infiltration, density, and erosion control. As a general rule, the growing medium should be 75-80% inorganic material (eg expanded clay or crushed clay) and 20-25% organic material (eg humus and clean topsoil). This will provide drainage and air permeability of the soil, as well as nutrients for plants.
The last layer is vegetation. Large green roofs require low-maintenance vegetation, and many native plants from coastal and arid inland regions are suitable. Dense, green rooftop plants can be treated in the same way as above ground gardens and require the same level of maintenance, but native plants are preferred.
Plants suitable for large green roofs include low-growing succulents and herbaceous perennials from dryland habitats. Annuals and biennials can be used successfully on green roofs and are generally among the fast growing annuals and ephemerals. The stonecrop family consists of 600 species of succulents and is a win-win. Try the local creeping boubialla (Creeping boobialla), creeping myoporum (Myoporum parvifolium), winter apple tree (Winter Apple), grass carp (Eremophila debilis) and the non-native sedum stonecrop (Sedum sexanagulare).
Intensive green roofs with a substrate depth of more than 250 mm can also support small shrubs and turf. Sports turf requires a specially designed soil or growing medium to allow efficient drainage and regular irrigation, fertilization and mowing. Overly viable species such as wheatgrass (Cynodon dactylon) and Kikuyu (Pennisetum clandestinum) should be avoided as their rhizomes can damage the waterproofing membrane. Choose local Nara turf (Zoysia macrantha), St. Augustine grass (Stenotaphrum secundatum) and tall fescue (Festuca arundinacea) instead. Vegetables should be irrigated and have a substrate depth of at least 200mm. Suitable small shrubs include Acacia amblygona, native fuchsia (Festuca arundinacea. Correa reflexa) and coastal daisy (Olearia axillaris).
Substrates deeper than 500 mm can be planted with shrubs up to two meters long, but plants with a dense upright habit should only be used where wind exposure is minimal. Suitable shrubs include pink velvet shrub (Lasiopetalum behrii), gray honey myrtle (Melaleuca incana), and lilac species and varieties.
Substrates over 1 m deep can support small trees up to 5 m high. Trees with sparse crowns, flexible trunks, and high heat tolerance are best suited to areas with strong winds. Try bottle tree (Brachychiton rupestris), stenophyll tree (Acacia stenophylla), and silver princess (Eucalyptus caesia).
The Florence Street project is a personal project of Emilio Fuscaldo, founder of Nest Architects. The affordability crisis in Australia prompted Emilio and his partners to buy 180 square meters of land in Cobb, Melbourne, on which to build an 80 square meter “beach hut” and a 50 square meter green roof. A green roof provides thermal insulation and thermoregulation, eliminating the need for air conditioning in the summer and reducing gas bills in the winter. The 200 mm thick soil layer reduces the thermal radiation of the city and promotes the diversity of flora and fauna. The water collected from the roof goes to the toilets for the roof garden and the above ground garden of the house.
The cost of a green roof is about $30,000, but Fuscaldo says it “makes a lot of sense” to include it.
“Why build high performance floors, walls and windows and then insulate the roof with low-tech glass wool batting? It’s hard, so why not.
Fuscaldo wanted to show that green roofs could be built into a modest downtown home, and he wanted to prove that they were “not just for big country houses or the top of a skyscraper”.
“As an architect, I never want to say ‘because I can’t afford it’ when a client asks us why we don’t do green roofs.” We really wanted to show that while this greatly increases the cost of the project, it can be done in others. Compromises have been made to make a house on a tight budget. “
Above: Drainage and permeability of green roof layers are important factors in green roof projects. If storm water is not properly drained, it can accumulate and create structural risks. Above: Fuscaldo says he no longer needs to water his garden thanks to a selection of hardy plant varieties.
Fuscaldo believes that the most important aspects to consider before building a green roof are the soil and the weight of the plants.
“The more they grow, the heavier they become, and the stronger the roof structure needs to be,” he said.
Succulents grown on Florence Street require little watering. Despite installing an irrigation system that Fuscaldo shut down after a year, “the plants are doing great.”
The center planting of the roof is Sir Walter Buffalo which replaces the bud grass (Dichondra repens) which dies back in summer. Plants successfully grown on the streets of Florence are sheep’s ear (Stachys bizantina), blue crayon (Senecio serpens), chicken plant (Echivera glauca), backcountry pork noodles (Carpobrotusmodeus) and blue linen lily (Dianella revolute).
“It was really trial and error with planting types, and luckily everything except the weeds worked, as they tolerated the wind, sun, erratic rainfall and soil profile very well,” Fuscaldo explained.
“These plants were also chosen because the soil is very permeable… the last thing we need is a pond.”
Top: Conceptual drawing of a green roof structure. Above: Precast concrete slabs will be used to construct the tunnel, which will be landscaped.
Misho + Associates is responsible for designing the new Sydney Zoo to be built in the west of Sydney.
This will be Australia’s first green roof habitat, with animals and nocturnal animals on display supporting the wildlife sanctuary and enhancing biodiversity. Architect Misho Vasilyevich of Misho + Associates said that environmental considerations and the location in the exhibition center led to the immediate decision to make a green roof.
The three exhibits were to provide a stable temperature and humidity controlled environment without the need for air conditioning. They need to provide total darkness and controlled artificial lighting, as well as eliminate unnecessary noise from the movement of materials.
This will be achieved by constructing a tunnel using precast concrete slabs and the top of the tunnel will be landscaped. Michaud chose Humes concrete road culverts and reclaimed wood from Tasmania for the exterior cladding. The construction will also use Xlam cross-laminated wood.
Green roofs can beautify and greatly increase the stability of a structure. Despite the higher initial cost, the insulating properties of a green roof will save you money on your energy bill. Nest Architects recommends that those looking for a green roof choose plants that will grow quickly and cover exposed dirt, and ensure that waterproofing membranes and drainage elements can withstand UV rays and perform well over the long term. The introduction of green rooms will make buildings and residents healthier.
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Post time: Jul-07-2023