Planning considerations - Living Roofs in Integrated Urban Water Systems - page 64

d often unprotected exposure against strong winds, heat and cold on top of
buildings;
e a dynamic environment: plants and substrate change over time as plants
thrive, die and decompose, nutrients (or acid rain or salt spray) are added or
leached, and levels of organic matter luctuate.
3.4.2 Planning considerations for plant selections
Living roofs are artiicial landscapes, and as such impose speciic constraints on
the vegetation planted. Maintenance, climate tolerance, ET, accessibility, erosion,
aesthetics, biodiversity and xeriscaping inluence plant viability.
3.4.2.1 Plant maintenance demand
Maintenance plays a crucial role in the design process of a living roof. Every living
roof needs regular maintenance over its life span, as do conventional roofs. The
maintenance of an extensive living roof is typically more frequent until a resilient
cover is established. Thereafter, maintenance frequency ranges from weekly to
yearly or bi-yearly, depending on the design objective of the living roof and the
length of time a roof is designed to last (typically twice as long as bare membrane
or ballast roofs). Weekly maintenance is appropriate for a frequently used,
publicly accessible area of mown grass and annual lowerbeds that require
mowing, irrigation and seasonal replanting. An annual autumn maintenance may
be typical of an established living roof overlooked from a distance (or not at all).
For such sites, maintenance visits are required to check that drainage points are
clear so stormwater low is unimpeded; to remove plants with aggressive root
systems that can potentially damage the roof membrane, e.g., birch seedlings;
and to ensure overall living roof cover has not been compromised, exposing the
membrane.
Common failures related to maintenance (or lack thereof) include, but are not
limited to:
• lack of density of vegetation coverage - this might mean supplemental plant-
ing and/or irrigation in the summer months;
• pestilence;
• inadequate drainage due to clogging of drains and gutters, which can lead to
looding or diminished stormwater management performance through higher
than anticipated runoff volumes;
• irrigation system malfunction, leaks or clogging of nozzles, which can lead to
plant failure;
• insuficient weeding of taproot 2 and other undesired plants (especially if the
designer's planting concept needs to be static), leading to plant area failure
and unnecessary mechanical pressure on the root barrier.
The best prevention of failure in the maintenance and monitoring phase is to hand
the project over exclusively to trained living roof gardeners and plant ecologists.