Extensive Green Roofs
An extensive green roof is the most basic form of green roof. Like all green roofs, an extensive roof comprises of the following components; A water proof membrane, a root protection layer (although typically this can be combined with the membrane in an extensive roof system), a drainage layer, a filter mat, growing medium and finally, vegetation.
It is typically found to have a soil/substrate of no more than 15cm in depth. As a result of the shallow substrate depth, the range of vegetation is limited to low growing vegetation types including; grasses, moss and sedums. The composition of the growing me- dium is crucial in an extensive green roof system. It is important to avoid an overly fertile substrate as this will encourage competi- tion amongst vegetation species and may result in an uneven coverage of vegetation. The ideal scenario is to have a moderately fertile substrate that maintains a constant coverage yearlong.
An extensive green roof system is commonly used in situations where no additional structural support is desired. Typically, an exist- ing roof will be able to support an extensive system that can weigh up to 100kg/m2. However, it is essential to seek advice from a structural engineer before installing a green roof.
Extensive green roofs are not design to be accessed on a regular basis. This minimises the structural demand for the roof. How- ever it is important to have some degree of access for maintenance.
Extensive green roofs require maintenance in the first 2 years to ensure that the vegetation has stabilised and there are no compet- ing weeds. Beyond the first 2 years, maintenance is minimal and may only be required once or twice annually.
Intensive
- Intensive Green Roofs
On the other end of the spectrum, intensive green roofs are the most comprehensive roofing system. It is generally designed to maximise the environmental benefits that can be achieved from a green roof whilst also providing for public amenity. An intensive green roof can also support the production of food produce. Food production on city rooftops can drastically minimise, if not elimi- nate the need for transporting fresh produce and thus reduce carbon emissions.
The components of an intensive green roof are the same as all other green roofs, however each component requires much more consideration with regards to its form and materiality due to the sensitive relationship between vegetation types, water harvesting and growing medium.
An intensive green roof needs to balance the quantity of water harvested, the fertility of its substrate and the varieties of vegetation chosen. Other considerations that are important are the load capacity of the roof deck, accessibility and other roof top structures.
Intensive green roofs offer greater substrate depths, typically greater than 15cm. The greater soil depth allows a larger variety of vegetation to grow. However, different plant types have different requirements with regard to growing medium, nutrients and water needs, which will ultimately affect the selection of materials to be used in the green roof system. Therefore it is important to realise that extra consideration should be given to the selection of plant species when there are a large variety of plants to chose from.
Similarly to any green roof, consideration needs to be given to the structural capacity of the roof deck to compensate for the extra load. It is essential to obtain the advice from a structural engineer on each individual roof. An intensive green roof can have a saturated weight of up to 1000kg/m2
Maintenance for intensive green roof systems is the most involved and depends on the type of vegetation that is grown.
Semi Extensive Green Roofs
A semi-extensive green roof is a hybrid of the two systems (extensive and intensive). A semi-extensive roof system is appropriate when the rooftop can be viewed from adjacent buildings but the possibility for access is limited and the structural capacity of the roof deck could not support an intensive green roof.
A semi-extensive green roof allows for a slightly deeper substrate depth than a traditional extensive green roof but it still enjoys the relatively minimal maintenance that an extensive green roof system has.
Due to the relatively deeper substrate, a semi-extensive roof can support a greater variety of vegetation than an extensive green
roof, which means that the architect or landscape architect has more flexibility in the aesthetic design of the green roof.
The selection of vegetation on the roof is important as it will directly determine the level of maintenance required to keep the green roof functioning properly.
A semi-extensive green roof applies an extra load to the building, as do the other green roofs. Generally the semi-extensive green roofs can weigh up to 630kg/m2 depending on the selection of materials. Always consult with a structural engineer to assess the structural capacity of the subject roof deck.
Choosing a Green Roof Based on Structural Capabilities
When deciding to install a green roof on a building, one of the first and most important decisions is to choose the appropriate type of green roof (extensive, intensive or anywhere in between). This decision will be based on only a few pieces of information and is relatively easy to make.
The structural capability of the roof deck is the most fundamental determinant that will influence the type of green roof chosen. This factor is particularly important in a retrofit situation whereby the load capacity is predetermined unless additional structural support is allowed for. In a new development however, the extra load of a green roof can be accounted for if it is factored in at the early stages of the design process.
Typically, in a retrofit application, a roof deck would have been designed to support extra weight such as storm water and a protective layer of gravel. Therefore it is usually within its means to support an extensive green roof system. However it must be stressed that a structural engineer must be consulted on all projects before a green roof is installed.
Choosing A Roof Type Based on Functional Requirements
Another determinant that assists in selecting the correct green roof type is an understanding of the functional requirements that are desired from the green roof. If for instance, the sole objective of the green roof is to achieve environmental benefits then there is no point installing an intensive green roof. In contrast if it were desired to provide public amenity for the building occupants or to grow food produce, then an intensive green roof would be the correct system to choose.
Source:
City of Sydney Green Roofs and Walls Policy
Portable Modular Roof Gardens
Portable modular roof gardens offer a flexible and lightweight alternative to traditional green roof systems. These systems consist of pre-fabricated, self-contained modules that include all necessary components, such as waterproof membranes, drainage layers, growing medium, and vegetation, within a single unit. Portable modular systems are ideal for buildings with limited structural capacity or for temporary installations, as they can be easily relocated or replaced as needed. These gardens allow for customized layouts and designs, providing opportunities for both aesthetic appeal and functional benefits, such as urban cooling and biodiversity. Additionally, the modular approach simplifies maintenance and offers scalability, making it a versatile solution for both residential and commercial applications.
