Key considerations for green roof installations in South Africa

Submitted by: Amanda Botes, Wednesday, June 5, 2013

“People have this idea that green roofs have to exist on large public or commercial buildings; however, a green roof can exist on a dog kennel, which in its own is an ecosystem,” says Clive Greenstone, owner of Green Roof Designs at a recent lecture on Green Roofs to the Durban branch of the Botanical Society of South Africa.

In the last eight years or so, green roofs have become more popular in South Africa due to the many benefits that they offer including cooling building temperatures down and attracting biodiversity, says Greenstone. There are a number of key factors however to consider before developing a green roof on a building in the South African context. Greenstone notes the following key considerations:

Safety and structure considerations

Safety and structure are key when installing a green roof, “I can’t stress the importance of these two points,”says Greenstone. “Green roofs are predominantly retrofits, they are placed on existing buildings. You need to know the structural loading capacity that the roof can take, this can be done by a structural engineer,”says Greenstone.

Location

“The location of the building is key because sometimes buildings shade each other in the urban environment. That is going to affect your plant combinations. You may get full sun in the morning but full shade in the afternoon. Wind creates whirlwinds which can affect plants too,” explains Greenstone. You should also take into account whether the roof will be visible or not, as it will need to have a neater look if people are looking onto it.

Roof slope

Although green roofs can be installed on a gently sloping roof, Greenstone notes that “flat roofs are probably better suited for green roofs”. However Greenstone adds that water can pool and collect on a flat roof which can negatively impact on the structure of the roof and plant root systems can damage roof materials to get to the collected water.

Types of green roof systems in South Africa

There are two main types of green roof says Greenstone; a direct green roof system, where the plants are grown directly on top of a number of different layers including water proofing sheets, drainage and growing mediums and modular green roof system where plants are grown in specially designed trays.

Direct green roof system

The direct green roof system consists of a number of layers. The first layer is a horticultural infused waterproof layer that has a root retardant infused within it or alternatively a durable plastic. Greenstone recommends using a 1000?m polyethelene sheeting. The second layer is a drainage layer of synthetic or geotextile material. This layer helps to prevent oversaturation and ensures that the roots of the plants are kept well ventilated. The third layer is the growing medium and finally the last layer is the plants.

This type of green roof system accommodates low growing species and a shallow growing medium is used. The system requires very little maintenance and is cheaper than the modular system. Plants are given more space to spread out. The disadvantages of this type of system according to Greenstone is that they require an expert to do the installation and if there is a problem the whole green roof system has to be rolled back to find the source.

 An example of a direct green roof system (Image source: Clive Greenstone)

Modular green roof system

The modular green roof system consists of a number of plastic trays that are placed alongside each other.  The trays have cups in them with built in drainage systems. Greenstone custom makes trays specifically for green roofs that are UV resistant and made from a recyclable long lasting HDPE plastic. The trays are available in three different sizes depending on the load that can be accommodated on the specific roof. Understandably the shallower trays are lighter. The specialised trays allow water to be available when the plant needs it and helps to slow down runoff. The trays are also raised 3cm off the ground to allow water to flow to the lowest point on the roof so that water does not collect and lead to damage, explains Greenstone.

The trays are lightweight and can be moved around easily if there is a problem or if a change in design is required, adds Greenstone. Plants can also be changed seasonally and food can be grown in the trays. Greenstone states that although these systems are slightly more expensive than the direct system, they do not require professionals to install them. A disadvantage of the modular system is that the plants are more limited by the trays but they do grow over the trays notes Greenstone. Unlike the direct system waterproofing is not required but Greenstone recommends that either a layer of 750?m polyethelene sheeting or bidum is used as a preventative measure as the trays will move slightly with changes in heat which may cause friction and some damage.  

An example of a modular green roof system (Image source: Clive Greenstone)

What is the difference between a green roof and a rooftop garden?

“The modern day green roof has been around since the 1960’s…the 1960s was the main take off time for green roofs internationally. In Africa we are a bit lagging behind in that practice. Roof top gardens have been around for a while… but green roofs as such are really only about eight years old in South Africa”, says Greenstone. 

According to Greenstone there are a few main differences between a green roof and a rooftop garden.  Firstly a rooftop garden is intended for human use, whereas a green roof is not intended for human use. Secondly, rooftop gardens are generally planted in deeper soils (on average between 200-500mm) whilst a green roof substrate is much shallower (20mm-150mm), and can be as little as 2cm. Rooftop gardens are usually broken up into areas of lawn, taller trees and shrubs whereas on a green roof plants are planted “en masse” and a greater variety is grown. Rooftop gardens are generally more “park-like” in design, whereas on a green roof the vegetation forms an ecological protective layer. An example of a rooftop garden in Durban is on the roof of the Priority Zone Building in Monty Naicker Street, and an example of a green roof is the eThekwini Municipality’s City Engineers Building in KE Masinga Road.

Example of a rooftop garden at the Priority Zone Building in Durban (Image credit: Amanda Botes)

Example of a green roof at the Sisonke Municipality Office Ixopo (Image source: Clive Greenstone).

A useful guideline document on designing green roofs- “Guidelines for designing green roof habitats” has been developed for the eThekwini Municipality and can be accessed online.

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Amanda Botes