An Introduction to Designing Green Roofs in South Africa

Their design, construction and care

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 greening of roofs both existing and new.

1. Because the roofs are protected from the extreme and harsh effects of the elements, in particular ultra violet radiation and very high as well as low temperatures the waterproofing lasts very much longer. Temperature extremes at roof level are dramatically reduced, thus decreasing the need for expensive insulation against both heat and cold, heating and cooling costs are therefore vastly reduced.
2. Water runoff after rain is greatly 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, in addition less pollutants reach our watercourses and oceans.
3. The greening of large expanses of roofing also helps reduce the heat sink effect experienced in all large cities thus helping to reduce the local temperatures again this has the benefit in particular in warm climates of vastly reducing the air conditioning costs.
4. The greening of urban roofs can provide valuable habitat for a whole range of plants and wildlife from insects, to reptiles, to amphibians and birds even to small mammals that do not normally occur in city centres. For instance in parts of coastal 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 high up on city roofs.

Important considerations before undertaking the greening of a roof
Before considering to construct a green roof or to green an existing roof it is wise to first discuss the proposed green roof with a Specialist in Green Roof Technology.

Structural loading
It is essential that the roof to be greened be designed by or checked by a suitably qualified structural engineer to ensure it is capable of safely carrying the maximum expected design load of the fully saturated growing medium, pathways, plants as well as people who are expected to be on the roof at any given time.

Water and Root Proofing
It is essential that the roof to be greened has a durable and very reliable water proofing and root protection layer that can not be penetrated by the roots of the plants to be grown on it as it is a very expensive business to lift an established green roof to have a leaking roof repaired then to replace it. In particular the roof barrier must extend well above the surface of the growing medium and must be well attached or bonded to the structure to prevent water and roots from being able to enter behind it.

Bitumen Based Products
Here in South Africa the majority of roofs are water proofed using bitumen based products which do a very good job of waterproofing where no vegetation is allowed or required to grow on them. In Germany a number of roofs exist that have been successfully been waterproofed and root proofed with bitumen based products and which have not experienced problems, however they have mostly plants growing on them such as Sedum which do not have excessively invasive root systems. Here in South Africa I do not recommend growing plants directly on roofs which have been waterproofed using bitumen based products without the addition of a very strong root resisting barrier being constructed over the bitumen waterproofing such as 1 mm or thicker LDPE plastic sheeting fitted and welded together so as to prevent roots from damaging the water proofing.

Moisture Triggered Polyurethane Membrane
This is my preferred system which is highly effective and simple to install in addition it has been perfected in Germany and other parts of Europe over many years specifically where millions of square metres have been installed on green roofs.  A moisture triggered polyurethane membrane consists of a glass fibre reinforced liquid applied polyurethane membrane is made up of a seamless cold applied fully bonded, highly elastic, one-component, moisture-triggered polyurethane water and root barrier. The glass fibre matting (chop strand) reinforcement is bedded in the first coat of the liquid moisture cured polyurethane membrane giving a continuous seamless waterproofing and roof barrier. This system has the added great advantage that it has the ability to be moulded to the most complicated shapes that are created by the various details on roofs in a simple and very effective one operation waterproofing and roof proofing operation.

Protection of the root barrier
The root barrier may need to be protected from mechanical damage. Depending on the waterproofed surface of the roof that is to be greened in particular where LDPE plastic sheeting is to be used it may be necessary to first lay a layer of geofabric on the roof as protection for the root barrier. It may also be necessary to cover the root barrier with a layer of geofabric for protection against mechanical damage caused during planting and maintenance or when specially designed growing containers are to placed up on the roof for the plants to grow in. Here again it is essential to get the advice of a specialist in Green Roof Technology.

Fixing of a separate root barrier
Where a plastic sheet such as LDPE is laid over the waterproofing as the root barrier it must be well fixed to the structure well above the level of the growing medium using non-corrosive metal or plastic fixing strips in a way that prevents water and roots from entering behind it.

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. I have done trials with and have had excellent results using  Enkadrain®  Enkadrain®  or the local equivalent MacDrain® as well as using The Kaytech rooftop garden drainage technique consists of a tri-component system. The ZipCore™ HDPE sheet was laid directly onto the waterproofing with the flat side facing down and the cuspated side facing up. Joining of panels was done by simply overlapping by a single cuspation and clipping the cuspations together. The ZipCore™ was overlaid with a single layer of Geomesh™, followed by a layer of bidim® A2 geotextile. To prevent contamination of the cuspation voids during placing of the growing medium, the overlapping bidim® A2 was secured to the walls using packaging tape. The incorporation of Geomesh™ ensures that the bidim® filter layer does not bag into the spaces between the cuspations of the ZipCore™. The 1 mm gaps in the Geomesh™ permit unhindered through flow, while the nonwoven bidim® A2 geotextile provides a superior filter.

Containment
In many applications the area to be planted does not cover the entire roof so some for of containment needs to be provided. To date I have tested two methods the first being aluminium edging and the second being light weight concrete edging both methods have produced good results.

Surroundings and pathways
Areas of the roof that are not to be vegetated including pathways can be covered with conventional paving slabs, light weight concrete paving slabs or coarse gravel.

The Growing Medium
The choice of the growing medium is a most important consideration.
For roofs that have not been designed to take a heavy load a very lightweight well drained medium that also gives the highest degree of water retention to promote plant growth without becoming waterlogged which has suitable chemical parameters and has a good distribution of particle sizes. 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 to produce lightweight growing mediums. Where light weight is essential I have 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 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.
On roofs that have been engineered to be able to take a higher loading allowing the depth of the growing medium to be from 75mm to 150 mm research under the conditions experienced in Durban has shown that a blend of crushed brick, decaying granite, soil and well rotted compost works very well. Our research has also shown that most of our plants need considerably more organic material in the form of compost than those commonly grown in the colder, wetter latitudes such as in Germany.

Natural soils
Where deeper soil depths are required and the roofs have been designed to take the load natural soils may be used as long as they have a good distribution of particle sizes to ensure adequate drainage, organic compost may need to be added to aid in the establishment of the plants. Where the natural soils do not offer the required rate of drainage course sand or crushed brick may be added to improve the rate of drainage.
It is essential to sterilize all soil used to remove unwanted seeds as well as tubers and propagules of weeds such as Yellow Nut Sedge Cyperus esculentus this can be done very effectively with methyl bromide gas. Caution must be exercised to allow the gas to escape from the soil before plants are planted into it.
It is essential to add super phosphate, lime and slow release fertilizer to all growing mediums.

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 very wide range of other light weight low growing succulent plants which are to a greater or lesser degree suitable for growing on roofs.

Fertilizing Green Roofs
Both organic and non organic balanced to low nitrogen granular fertilizers are suitable to be used on Green Roofs. Slow release fertilizers although they have not been tested could be of benefit. The amount of fertilizer and the frequency of application will be dependant on a number of factors such as soil composition, soil depth, amount of precipitation or irrigation and plant type.

Careful observation of the plant growth and health taking into account the resultant growth rate required is going to be the only reliable indicator of the frequency and amount of fertilizer to be applied. Practice has proven that it is be best to give 3 to 4 or more light dressings of fertilizer per year rather than one or two large doses, this will greatly reduce the runoff of fertilizer and loss of nitrogen to the atmosphere in turn making more fertilizer available to the plant when it is needed greatly reducing the amount of fertilizer required to be applied per year. This will benefit the plant growth and reduce water pollution to a minimum. More applications need to be made in the summer than in winter.
When calculating the application rate the manufacturers application rate recommendations must be taken into account bearing in mind that the more frequently one applies fertilizer the less one would apply over the period of one year. To make this simple calculate the total amount of fertilizer to be applied for a period of one year as per the manufacturers recommendations, reduce this amount for instance by 40% then divide this amount by the number of applications it is intended to give the plants per year. A little trial and error is going to needed to be applied here. A light mulching with well rotted coarsely milled bark compost has proven to be beneficial under our conditions in Durban contrary to recommendations and common practice in Europe.

Only fertilize when the soil is sufficiently moist and is expected to remain so for some time.

Always irrigate immediately after fertilizing unless it is due to and does in fact rain within the next hour or two.

Never fertilize when the vegetation is in any way under moisture stress.

Getting the fertilizing correct is an important issue as is can make or break the success of the planting in the long term

Please note that this is not a complete do it yourself manual for green roof construction.
For more complete information I can be contacted at see below

Here are some of the plants that are showing potential to be suitable for Eco-Green Roofs in Durban