Agriculture Reference
In-Depth Information
and its relative vertical position in a living roof assembly is described here, while
the speciic design considerations are explored in detail in Chapter 4 .
1.4.1 Waterprooing layer
The purpose of the waterprooing layer is to protect the structure and underlying
building from water damage. It is usually a synthetic membrane. A well-installed
and reliable waterprooing layer is critical for any building, whether or not a living
roof is installed atop. Over the long term on a non-greened roof, leaks usually
result from mechanical breakdown of the membrane induced by UV exposure or
lexing from freeze-thaw cycles, or accidental physical compromises (holes, cuts)
by service, installation, and maintenance work on rooftop infrastructure. A prop-
erly designed and installed living roof should address each of these potential
compromises, by providing a physical insulating layer of protection above the
waterprooing membrane. Anecdotally, there is a 90-year-old living roof in
Germany that has never replaced the underlying membrane. A more conservative
estimate frequently heard is that a living roof system that fully covers the water-
prooing membrane should at least double its useful life.
1.4.2 Root barrier
Protecting the integrity of a building's waterprooing membrane is the highest
priority during the installation and long-term operation of a living roof system.
While avoiding plants with aggressive root structures (such as bamboo) is an
important line of defence, a physical or chemical root barrier provides another
layer of protection. In some cases wind-blown seeds may introduce plants with
aggressive roots, for example birch seedlings, and may be overlooked by mainte-
nance crews. The additional cost of a root barrier is considered small compared
to costs of repairing potential damage. Often, the root barrier is integrated into
the waterprooing layer to reduce separate layers and construction cost.
1.4.3 Moisture retention layer (optional)
A supplemental moisture retention layer or layers can enhance plant health and
stormwater retention. Direct physical contact between the held moisture and the
growing medium and/or plant roots is critical for functionality. Stored moisture
accessible by plant roots will reduce the duration and/or severity of plant stress
during extended periods between rainfall or irrigation events. Rainfall captured in a
moisture retention layer will contribute to evapotranspiration (ET) rather than direct
runoff, if the retained water is in direct contact with the growing medium and/or
plant roots. Moisture retention layers are therefore best placed at the base of the
root zone, between the growing medium and the geotextile separation layer. Con-
versely, moisture retention layers or materials placed in the upper zone of the living
roof cross-section may reduce root development through the growing media
proile, thus compromising the stability of plants in a usually windy environment.
Moisture retention layers may be made of synthetic materials or natural ibers.
Suitable natural ibers, fabrics or mats may be made from some types of peat,
 
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