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The most important factor relating to any roof, whether it's a green roof
or not, is its long-term waterproofing function. The ultimate priority of
a green roof is to keep the building watertight.
Design
 The superimposed load
resulting from a landscaped roof must be taken into account when accommodating
the structural load. The calculation must also be based on its saturated
state. Below are examples of roof coverings and their approximate saturated
weights for comparison:
Gravel Surface 90-150 kg/m2
Paving Slabs 160-220 kg/m2
Vehicle Surface from 500 kg/m2
Extensive Green Roof 60-150 kg.m2
Intensive Green Roof 200-500 kg/m2
Recent strides in technology have advanced the properties of both extensive
and intensive green roofs, making then lighter, more durable and better
able to withstand the extreme climatic conditions of the rooftop environment.
The Eco-roof system is thoroughly engineered to address all the critical
aspects of design, including: the saturated weight of the system and load
bearing capacity of the underlying roof deck; moisture and root penetration
resistance of the waterproofing membrane; resistance to wind shear; management
of drainage; and the suitability of the proposed plant material. The Building
Logics' Eco-Roof membrane was developed in Germany where it has been successfully
used for over 30 years. This membrane incorporates a root rejecting material
so that the plants roots will not attack the membrane.
The Eco-roof system contains several layers of protective materials to convey
water away from the roof deck. Starting from the bottom up, a layer of insulation
(optional) is installed, a waterproof membrane is next, followed by a root
barrier, a drainage layer, a filter fabric designed to retain fine soils,
the engineered growing medium or soil substrate, and the plant material.
On steeper slopes, up to forty-five degrees, a raised grid structure is
installed to secure the growing substrate. A shallow layer of gravel or
pebbles is placed from 18" to three feet around the outside perimeter
of the roof and roof penetrations, providing additional drainage, fire control
and access to the roof for maintenance.
As with any roof, the most important aspect of a green roof system is its
ability to keep water out. The heart the green roof system is a multi-ply,
high strength, polymer-modified roofing membrane system.
Inorganic
Filter Layer- The filter layer prevents small soil particles from clogging
the drainage system below, while enabling water to nurture the surface plant
life. Typical materials are water-resistant polyester fiber mats or polypropylene-
polyethylene mats
Drainage Layer- The drainage layer allows excessive moisture to exit the
green roof system. On very shallow extensive green roofs the drainage layer
may be combined with the filter layer. Unimpeded drainage is assured in
green roof systems because the drainage layer is applied over the entire
roof area. The drainage layer forms an extremely stable and pressure resistant
sub-base. Drainage capacity must increase closer to the rainwater outlets,
so large quantities of drainage material, usually rounded stones, are installed
along the eaves and near outlets. These rainwater outlets need to be accessible
for seasonal cleaning and the stones form a separation barrier from the
vegetation.
Some drainage systems are more multifunctional. W.P. Hickman Systems' EcoRoof
System has gel-packs that provide water storage for the plants above and
a system of channels for drainage of excess water below. The system provides
additional protection to the underlying waterproofing element.
Waterproof Membrane- Eco-Roof waterproofing and reinforcing membrane is
installed on top of the structural deck. The membrane is an APAO modified
bitumen system, incorporating a unique root-resistant compound to prevent
roots from penetrating the roofing membrane and exhibiting exceptional tensile
strength, superior puncture resistance and excellent elongation characteristics.
Organic
The selection of drainage medium, depth and type of soil and selection of
plant specimens should be carefully chosen based upon local varieties of
plant species and local climatic conditions.
Soil Substrate- The growing medium or soil substrate can be selected from
several engineered mixes designed by various suppliers. By using a mixture
of native soil upgraded with organic or mineral additives (peat, humus,
wood chips, sand. lava or expanded clay) it is possible to achieve optimum
water retention, permeability, density and erosion control necessary to
support the green roof vegetation. The thinner the soil layer, the higher
the physical demands on the plants. Some problems in the past have been
the result of too shallow soil depths, resulting in root damage from heat
and frost fluctuations. The soil must be deep enough to protect the roots
from frost and heat damage.
Plant Material- The vegetation layer is the most vital and exciting part
of the green roof, and as such suitable and dependable plant material selection
needs to be assessed on a region basis. Characteristics of landscaping typically
used in extensive green roof systems include  shallow root systems, regenerative qualities and resistance
to direct radiation, drought, frost and wind. A much larger variety of plant
selections are available for intensive roofscapes due to the opportunities
offered by greater soil depths. Compatibility issues of green roof type,
anticipated use, temperature, humidity, rainfall and sun/shade exposure
are important elements for successful plantings of any kind. Native and
culturally adaptable plants used for the artificial environment of a green
roof need to be reviewed for heat and drought tolerance. This is extremely
important since most systems are designed to be low maintenance. The plant
make-up is customized to meet the specific function, environmental conditions,
climate zones and aesthetic requirements of the individual application.
The soil material is lightweight, long lasting and absorbent, able to retain
water to nourish the plant life. The plant materials usually require supplemental
irrigation and fertilization while getting established during the first
year. Due to the return of organic matter to the soil additional fertilization
is not usually necessary.
The desired seasonal visual impact of a green roof may also dictate the
plant material. The summer/ winter vegetative aspect changes just like it
would in any landscape. What happens to the green roof plants in the dormant
season? Depending on the ratio of herbaceous to evergreen plants selected,
the roofscape can have varying colors, heights and textures to create winter
interest.
Plant material can be applied to green roofs by several means: pre-vegetated
mats or blankets; direct on-site planting of sedum cuttings and/or seed
or root plants; hydro-planting; or any combination of these methods. The
types of plantings selected dictate their planting times. Root plants can
be planted throughout the entire growing period if they are sufficiently
watered. Plants used for extensive landscaping are cultivated in special
flat-bottomed planting trays. The sowing of sedum cuttings and/or seeds
is restricted to spring and autumn. Simple extensive green roofs should
not require any regular maintenance after the first two years.
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