Defining Green Roofs
At the outset it must be clearly defined what Green Roofs are and what they are not and what advantages they have for the environment.  Green Roofs are certainly not roof top gardens where buildings are designed and engineered to be planted to large trees, shrubs and groundcovers and to have well manicured lawn areas for use by our fellow man. Green Roofs which are often referred to as extensive roof top greening abroad is the growing of suitable vegetation in a very lightweight thin layer or growing medium mainly over existing roofs to green and to protect the structures that they cover.
Due to the very nature of the roofs that are to be greened, roofs that were in most cases not engineered to carry the weight of vast quantities of soil and vegetation the weight of both the growing medium as well as the weight of the plants is of great significance. Weight must be kept to an absolute minimum.
Roofs are increasingly being greened in the industrial countries of the northern hemisphere in particular in Germany for a number of reasons mostly related to the environment and to climate change.

Some of the advantages related to the extensive greening of roofs both existing and new are;

• The roofs are protected from the extremes and harsh effects of the local climate, in particular temperature, roofs and in particular on flat roofs because they are protected from the elements last longer.

• Temperature extremes at roof level are dramatically reduced, thus decreasing the need for insulation against both heat and cold, heating and cooling costs are reduced.

• Water runoff after rain is  much reduced in particular peak runoff, much of the water is retained on the roof to provide for the growth of the plants where it is returned to the atmosphere without overloading the already badly altered natural drainage systems, less pollutants reach our watercourses and oceans.

• The greening of large expanses of roofing also helps reduce the heat sink effect experienced in all large cities thus helping to reduce the temperatures again this has the benefit in particular in warm climates of vastly reducing the air conditioning costs.

• Extensive roof greening can  provide habitat for a whole range of plants and other small wildlife from insects, to reptiles, to amphibians right up to birds and in some cases even to small mammals. For instance in parts of Germany and the Netherlands the preferred nesting place for the endangered oyster catcher and a number of other ground nesting birds is to be found on green roofs often high up on city roofs.

Considerations before considering to undertake the greening of a roof
Before considering to green a roof it is wise to first have it checked out by a Specialist in Green Roof Technology, in most cases the roof under consideration also needs to be checked by a structural engineer well acquainted with and recommend by the Specialist in Green Roof Technology

Waterproofing
Depending on the roof to be greened it may need to be waterproofed, on many structures this has already been done, on others it may be advisable to provide additional waterproofing, this can usually be combined with the need to provide an effective root barrier.

Root barrier
Care must be taken to provide an effective root barrier to keep the roots of the more vigorous plants from damaging the waterproofing. Here it is recommended to use weldable 1mm or thicker LDPE plastic sheeting.

Protection of the root barrier
The root barrier may need to be protected from mechanical damage. Depending on the surface of the roof that is to be greened it may be necessary to first lay a layer of geofabric on the roof as protection for the root barrier. It may also be required to cover the root barrier with a layer of geofabric, such as when no drainage is to be installed or when the plants are going to be planted into specially designed containers which are then placed up on the roof. Here again it is essential to get the advice of a specialist in Green Roof Technology.

Drainage
On most roofs some means of drainage will be required, again the need to keep the weight to a minimum is an important factor, fortunately with modern technology also come modern light weight products that are most suited to this requirement example of which are ENKADRAIN and DIMPLE DRAIN

Growing Medium
The choice of the growing medium is a most important consideration. What is required is a very lightweight well drained medium that also gives the highest degree of water retention to promote plant growth without becoming waterlogged. Experience gained in other counties in particular in Germany has shown that soil less growing mediums made up of lightweight materials such as expanded clay, perlite, vermiculite and volcanic rock  such as pumice and scoria are most suitable when mixed with varying quantities of well composted organic material or peat, to give a light weight soil free growing medium.  Here in South Africa we are a little restricted in the materials that are freely available at an affordable price. We have therefore opted for a carefully formulated and tested blend of expanded perlite, expanded vermiculite and well rotted, finely shredded bark compost which produces a very light weight growing medium with a high degree of water retention to promote optimum plant growth. This blend of materials gives us a very light weight material in proportion to its large water holding capacity so that when it is saturated the weight of the material is far less than that of a soil based material.

Selection of plant material
The ideal characteristics of the plants to be selected are that they are drought, temperature and wind resistant, that they are small and low growing, that they easily reproduce themselves from seed in the event that the plants die during times of extreme heat or drought. In addition it is of advantage if they have a mildly creeping habit so that they are easily able to fill gaps left by die back during times of moisture stress. However plants with a very vigorous creeping habit are mostly unsuitable. The plants selected must easily root themselves for easy and cost effective planting. For instance plants such as sedum are mostly used in Europe because cuttings of the desired species are made which are then  able to be simply distributed over the surface to be planted by hand. In Europe various species of Sedum have these ideal growing characteristics which results in them being almost exclusively used in roof greening projects.  Here in South Africa we are fortunate to have a very similar closely related genus to Sedum being Crassula which has many of  the characteristics of the ideal green roof plant. In addition we are also blessed with a wide range of other light weight low growing succulent plants which are to a greater or lesser degree suitable for extensive roof greening.

Here are some of the plants that are showing potential to be suitable for living roofs in Durban

For all of your Green Roof requirements contact Michael Hickman Specialist in Green Roof Technology at +27 82 061 2593 or email at info@ecoman.co.za