Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 125

portion of a roof's waterprooing membrane renders it more susceptible to
failure.
Aesthetic outcomes may be achieved by containerized roof gardens: isolated
pot plants distributed around accessible roof areas. Roof gardens have also been
referred to as “landscape over structure” (Weiler and Schloz-Barth 2009), and
may sometimes be considered intensive living roofs. Containerized roof gardens
are not considered SCMS. They usually require regular irrigation and fertilizer to
maintain high expectations for aesthetics. The extent of coverage of the roof is
usually poor. The high maintenance and energy inputs required are further deter-
rents to qualify as environmental mitigation systems. As containers themselves
would add signiicant weight, a more effective alternative would be to distribute
the load in an extensive living roof coniguration, even in this case, if only a
portion of the total roof is vegetated.
4.8 IRRIGATION
The extensive living roof should be an environmentally-friendly and sustainable
system. This implies low energy input but maximum environmental beneits and
output, i.e., low maintenance and high stormwater mitigation capabilities ( Figure
4.11 ). When artiicial irrigation is required, off-grid water and electricity solutions
should be pursued ( Figure 4.12 ). This might mean solar or windmill-powered
mechanical systems that deliver non-potable (recycled) water for irrigation, so as
to minimize energy and potable (treated) water from the grid. Non-potable water
can be obtained from grey water or harvested rainwater.
Irrigation during establishment may affect the long-term performance of the
living roof plant community (Thuring et al. 2010). Where water availability is
restricted during establishment, plant root development and leaf size may perma-
nently be compromised, thus limiting water and nutrient uptake as well as pho-
tosynthesis (Boodley 1998). Demonstrating this point, droughts during the irst
two weeks of living roof plant establishment were found to be more damaging
to shoot development than when droughts occurred later in experiments, with
the plants that survived the early drought ultimately producing less shoot biomass
(Thuring et al. 2010).
The use of or need for irrigation is determined case by case. Possible scenarios
include the following:
1 Irrigation used only for the establishing phase for 2-3 years. In this case the
designer needs to evaluate the merits of a temporary system, which can be
removed. In the establishment phase, a provisional irrigational system above
ground, such as a sprinkler, should be used. Drip or sub-irrigation (without a
basal moisture retention fabric) may not be effective because of the non-
wicking characteristics of the growing media (Rowe et al. 2014).
2 Irrigation systems used long term, in most cases installed due to harsh climate
conditions for plants. Although a well-selected plant palette will incorporate