Green Roofs.
COMPONENTS
Green roofs may be either
extensive or intensive. Your site characteristics
and objectives will determine which green roof is
appropriate for your property.
An extensive green roof
will have 8 or less inches of soil and vegetation
heights lower than 36 inches, typically. The
extensive green roof is low maintenance, more
economical, and usually offers the most green coverage
hence the greater environmental benefit.
The intensive
green roof will have soil depths between 8 inches and 4
feet and requires the normal maintenance functions
typical of a traditional garden. Intensive green
roofs are usually accessible to the public for
maintenance functions and often offer additional uses
such as recreational, social, contemplative, or
vegetative production activities.
The necessary components
may vary from extensive to intensive roofs and also by
roof site configuration.
Regardless, most green
roofs are generally comprised of four layers beneath
the plant material - the waterproofing
layer, the drainage layer, the root barrier, and the soil medium.
Waterproofing Layer
A waterproofing membrane
is laid directly on the roof decking. The layer is
commonly of bitumen, rubberized asphalt or compounded
thermoplastic composition. Depending upon the
composition, this layer is either loose laid or adhered
to the roof. The adhered system is usually bonded
to the concrete, steel or wood substrate through
hot-welding or permanent fusing. The advantage to
adhered systems over loose laid is the permanent seaming.
Thermoplastic
compositions tend to hold up better against acid
conditions caused by fertilizers and acid rain, for
example.
The membrane should be a
minimum 80 mil or greater in thickness for best
durability and performance. The waterproofing
membrane must also be impenetrable by roots and resist
the growth of algae and other organisms. Leak
detection systems can often be installed to make any
leakage easy to detect and locate.
Drainage Layer
The drainage layer must
simultaneously capture and retain rainwater or irrigated
water and also allow excess rainwater to drain away.
Drainage layers are typically of an egg carton style
configuration, offering closely spaced reservoirs to
capture water. Drainage layers are available with
reservoir depths between 1/4 and one inch. Between the
reservoirs drain holes are located to release excess
water so that it does not rise to the soil level.
The drainage layer is commonly made of formed
polystyrene.
Root Barrier Layer
The root barrier layer
prevents roots and soil from entering the drainage
layer. This is important for three reasons.
Roots and soil in the drainage layer reduce the
rainwater storage capacity of the reservoirs.
Second, roots submerged in trapped rainwater will rot,
introducing potentially fatal fungal diseases to the
plant material. Finally, roots in the drainage
layer are one step closer to the waterproofing medium.
The root barrier layer or
filter fabric is water retentive and, through capillary
action, transmits water from the drainage layer's
reservoirs to the growing medium so that the roots are
able to receive water.
Soil Medium
Specialized soil mixes
are usually used with green roofs. The precise mix
and amount will depend on whether the roof in intensive
or extensive as well as the types of vegetation grown.
Soil mixes for green
roofs are often a combination of expanded clay, shale or
slate; organic matter such as bark and worm castings;
and inorganic coarse matter such as limestone and
gravel. The extensive green roof might be 80%
expanded material whereas the intensive roof might be
55% expanded material.
The expanded material is
kiln-fired to over 2000 degrees Fahrenheit, causing it
to "explode", thereby creating pocks and voids in the
material. In this expanded state the material
offers an environment that has both good aeration and
water absorption capacity.
Green roofs benefit from
these specialized mixes because the mixes hold water in
reserve for the plant roots instead of soaking them and
provide a lightweight, disease retardant, relatively
wind-proof medium for good root health.
Extensive green roofs
generally have soil depths between 3 and 8".
Intensive green roofs are normally 8" - 4' in depth.
Engineers should be consulted to determine roof load
capacity and total saturated roof weight before
completing the estimating phase for any green roof
project.
Vegetative Layer
Plant material will vary
between extensive and intensive green roofs.
Extensive roofs will require highly drought tolerant
species growing 1 - 30" in height. The deeper the
soil layer, the higher the plant that can be sustained.
Intensive green roofs can sustain small and medium sized
trees if designed properly.
Sedums are popular
green roof plant choices due to their drought-tolerant
habit. Hundreds of varieties exist and extensive
green roofs can achieve varied aesthetic effects based
on creative combinations. Delospermas are another
family frequently present on the green roof.
Overall, plant trials are conducted throughout the
country and the world to test new finds and cultivars
and the palette of suitable plants is constantly
expanding.
Modular Systems
Several modular systems
are available. Modular systems are self-contained
units that require only the soil medium and the
vegetative layer for a functioning green roof.
These systems have advantages over area systems for
steeper-pitched roofs, smaller installations, or where
routine maintenance cannot be conducted at installation
height. Although they are not less expensive than
area systems, installation is much easier.
Cost
Costs vary widely.
Extensive green roofs may cost as little as $12 per
square foot, consistent with traditional roofing
systems, if the soil layer is very minimal.
Intensive green roofs may vary between $20 and $40 per
square foot[1], depending upon soil depth and
design complexity.
The installation costs
should be factored against the savings due to the
extended life expectancy a green roof provides versus
traditional roof life values, as well as the annual
energy savings and any qualifying tax credits.
[1]D'Antonio, Peter.
"Thermoplastic Waterproofing Membranes in Green Roof
System Construction 2004." Interface. Feb. 2004.
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