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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
ground covers 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 1 mm 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. We have experimented with and have had
excellent results using both
ENKADRAIN
or
the local equivalent Maccaferri
MacDrain® 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.
Further 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 where weight is not a consideration.
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.
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
When
in
doubt always consult a Green Roof Expert.
Here are some of the plants
that are showing potential to be suitable for Eco-Green Roofs in Durban
