Agriculture Reference
In-Depth Information
3 Lockable access of the drainage points if the roof is a public space.
4 Coniguration, color and form of the drainage points that relate to the archi-
tectural language of the building/façade. The number and size of the drainage
points depends on the local building codes and, if already in existence, local
living roof policies. Accessibility of drainage elements, as well as other perfo-
rations and protrusions may be enhanced by a durable, pervious border (e.g.,
coarse aggregate, Figure 4.6 ). In some jurisdictions, it may be required. A per-
vious border provides a visual cue, making it easy for maintenance workers to
ind inspection or clean-out points. It may also trap debris such as plants
pieces blowing across the roof, preventing clogging of the drainage
opening(s). A pervious border may also be helpful if/where it is desirable to
keep protrusions dry, as is it unlikely to retain water (unlike the growing
media). Attention should be given to the material color chosen for the border.
Colors in sync with the roof's planting will avoid an impression of large geo-
metric forms which may compete with the overall system's planting design.
4.7 EXTENT OF ROOF GREENING
If the main purpose of a living roof for stormwater management is to eliminate
runoff from being generated from the rooftop, the irst steps are intercepting
and storing the majority of the rainfall that strikes the roof. Maximizing passive
rainfall capture is realized by continuous and thorough coverage of the roof
surface by living roof elements. Nonetheless, a minimum extent of unvegetated
elements must be acknowledged for safety and drainage (namely, pervious
edging and walkways), although even walkways can be placed over moisture-
retaining elements. Limiting the extent of impervious areas means they will not
notably contribute to runoff.
In a typical passive assembly, a living roof only captures rainfall occurring
directly over its surface. If the top of the growing media is lush with an adjacent
impervious roof area, a living roof still has little ability to manage runoff from
upslope impervious surfaces, and no ability to mitigate downslope areas, as the
growing media provides little wicking ability and very high permeability. In other
words, runoff will quickly pass vertically through the media to the drainage layer.
Where design elements such as picnic areas are incorporated, signiicant amounts
of runoff may be generated, just as with a conventional roof. An alternative
design approach might consider harvesting runoff from impervious areas, or
other nearby roof levels in a cistern for later, dry-weather distribution over the
surface of the living roof. Distribution would likely require active management,
using pipes and/or pumps as in an irrigation system, but could be run by solar
power (when plants would welcome additional water!).
Maximizing the extent of vegetated coverage provides beneits beyond storm-
water management. It helps plant survival. Impervious surfaces create a hotter
microclimate, which may exacerbate droughty conditions along the interface of
living and impervious elements. As discussed in the Introduction, any UV-exposed
 
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