Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 108

media depth, D LR , other than what is calculated by Equation 4.1 is adopted, then
for planning purposes the maximum amount of stored water by the growing
media (rainfall retention) can be estimated by:

S w = D LR × PAW (4.3)

0 < S w ≤ 20-25 mm

Where S w = maximum water (rainfall) storage in the growing media (mm) per unit
area of living roof to be credited toward stormwater control. It is noted that, in
practice, at least this much rainfall must occur in order to ill up the entire storage
capacity. If an intensive living roof media with PAW = 0.25 and D LR = 200 mm were
proposed, Equation 4.3 suggests a maximum rainfall storage of 50 mm, but from
a regulatory perspective, credit for stormwater retention would only be given for
20-25 mm. As again, this is a rule-of-thumb estimate based on empirical evi-
dence at present; a local regulatory agency should decide on the upper limit.
Some agencies might consider increasing the possible retention credit above
20-25 mm depending on local performance data and/or as an incentive to
promote living roof implementation, acknowledging that there are many other
environmental beneits beyond stormwater mitigation.

4.2 WATER RETENTION TECHNIQUES

One of the most signiicant challenges to the continued health of the plant
community is access to water. Moisture stress is determined by growing
medium depth, moisture storage, underlying thermal mass, duration and
timing of shade, wind exposure (including discharges from air conditioners)
and local climate. On sloping roofs, the aspect and location on the roof inlu-
ence water availability and moisture stress, with stress higher toward the ridge-
line and on north-facing aspects (in the Northern Hemisphere) and lower near
the eaves and south-facing roofs. Shade can be created by placing aluminium
grates (e.g., a walkway access) or photo voltaic cells over an extensive living
roof, both reduce surface temperature and ET losses (Fifth Creek Studio 2012).
Moisture stress can be decreased by increasing the water stored in the growing
medium by:

• Usingwaterretainingmaterialsindiscretelayers,e.g.,coirblanketsorgeotex-
tiles, installed in direct contact with plant roots and/or growing media.
• Usingwaterretaining“pockets”insyntheticdrainagelayersthatrootscan
directly access. The generally non-wicking characteristic of living roof growing
mediums means that roots must be able to touch the stored water in order to
use (transpire) it.
• Increasingtheproportionofines(changingtheparticlesizedistribution)or
increasing the gradation of the growing medium to promote formation of
medium and small pores that retain water. Reducing the particle and pore size
distributions negatively inluence permeability, aeration and likely weight.