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®
the local equivalent MacDrain®
as well as using The Kaytech rooftop garden drainage technique
of a tri-component system. The
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™,
by a layer of bidim®
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®
geotextile provides a superior filter.
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
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
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.
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.
organic and non organic balanced to low nitrogen granular fertilizers
suitable to be used on Green Roofs. Slow release fertilizers although
not been tested could be of benefit. The amount of fertilizer and the
of application will be dependant on a number of factors such as soil
soil depth, amount of precipitation or irrigation and plant type.