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Greenroofs
must achieve a balance between adequate drainage and being able to store water and not dry out too quickly, and
this is accomplished by choosing appropriate drainage products and growing
media. But the selection of a specific plant community, greater plant diversity, or
simply the particular design parameters desired for
certain greenroofs may require additional water.
According
to Charlie Miller, "Ideally,
even thin systems work optimally with two layers, separated by a geotextile.
The lower level is very light-weight granular mineral material (usually a
fired clay). The
roots of the plants will penetrate through the geotextile and will concentrate
along the bottom of this layer.
Here they find the best conditions for survival (cool temperatures and
more consistent moisture).
If irrigation is included, we choose to introduce the water in the
granular layer – further encouraging propagation of roots at the bottom.
If the substrate is chosen to have good water retention qualities, this
system will support a variety of plants without irrigation.
When roots are encouraged to grow higher up in the profile, they are much more
vulnerable to the effects of varying temperature and moisture," (Charlie
Miller, personal communication, April 2000).
If the soil
substrate/drainage system cannot hold a certain amount of free water, then
additional forms of water storage may be necessary and can be supplied by several methods. Certainly, the most "ecologically correct" systems would be those that
are considered sustainable, requiring no or little human intervention.
Completely
self-sustainable passive
water retention systems include using ponding elements welded directly to the
protection membrane, and generally can be installed on roofs up to a 4% slope,
as shown below, courtesy of Roofscapes, Inc.,
www.roofmeadow.com:
Or, a more active system incorporating the
ponding ridges along with a drain with weir level regulator and an automatic
irrigation control is seen above from Optigrün, courtesy of Roofscapes, Inc.
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Photo Source: Optigrün Die Dachbegrüner
Planungsunterlage 98/99 Brochure |
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Optima,
Optigrün and ZinCo have developed active, sustainable, solar
powered automatic water collection and irrigation systems. These systems
also have the added sophistication of a maximum and minimum on/off switch to
make allowances for possible overnight rainfall filling the reservoir
naturally. For the ultimate environmental self-irrigation system, an
additional cistern can be incorporated to store excess rainwater which can be
recycled later (Optigrün, 1998/1999; and ZinCo, 1998).
The
drawings below show a solar-powered rainwater collection system from Optigrün-Italy
on the left and an irrigation detail by Roofscapes, Inc. on the right.
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Source: ZinCo International 3/98
Brochure |
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For
additional water retention capabilities, a damming piece can be installed into
the outlets, with inspection chambers or adjustable terrace grills for easy
maintenance.
Shown at left is ZinCo's Floradrain FD 60, with its large
drainage channels allowing up to 1.5" of water to be stored
underneath. The water reaches the plants by capillary action, and the
inspection chamber allows access for maintenance.
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Detail from Optima’s Planungs-Unterlage 9/97 manual. |
Optima has designed a self-regulating water retention, storage,
and automatic irrigation system utilizing small tanks (1 ½’H) which retain rainwater that has
percolated through the substrate. The system then hydraulically releases the
stored water automatically when the water dips below the pre-determined minimum
level.
As stated earlier, some drainage systems also incorporate water retention
capabilities; the stored water reaches the plants by capillary action. An
optional reservoir board layer or retention fleece mat, available from some companies, can be installed to retain and store small
amounts of water as well. Additionally, either a simple automatic drip irrigation system
with a
manifold delivering water at the base of the profile can be installed, or a more
complete (and heavy and costly) irrigation system can be incorporated into any
intensive greenroof design which can withstand the added weight.
Waterproofing | Insulation | Drainage | Filter Fabric | Growth Media | Plant Material | Water Storage & Irrigation
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