Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 121

With respect to the growing medium's stability, design and installation can
draw from common geotechnical engineering experience for working with low
cohesion soils. Guidelines for erosion control and slope stability are relevant for
working at height on pitched living roofs.
Living roofs up to 15° (~27 percent) pitch are unlikely to require special design
or construction techniques. Nonetheless, as a precaution, construction of a 15°
pitch extensive living roof at the Auckland Botanic Gardens incorporated slope
breaks made from growing medium wrapped in coconut coir matting, and light
foot-tamping to secure growing medium in place (Fassman et al. 2013). The
system has not suffered from materials' slumping after three years.
The FLL (2008) suggests installation of structural anti-shear/slip protection
measures for roof pitches greater than 20° (~36 percent) to promote growing
medium stability. Specialty commercial products are available, including lexible
synthetic matrices, but a built-in-place batten structure may equally be suitable.
Whatever system is used should not compromise drainage, nor should it be reac-
tive. Poorly draining systems may drown plants and increase structural burden
beyond anticipated loadings. Common wood preservatives such as copper chro-
mium and arsenic present a potential to leach harmful contaminants into runoff.
Roofs with pitch 30-45° (~58-100 percent) require even more structural
control, and an altogether different approach to construction (FLL 2008). They
likely require signiicant irrigation during an extended (2-3 year) establishment
period, compared to lower demand over a 1-2 year establishment on lower
pitched roofs. Such steeply pitched roofs are unlikely to provide adequate storm-
water mitigation.
The surface of the growing medium may need further stabilization until plants
are established. Planting through a biodegradable erosion control mat placed
over the surface can prevent wind uplift of the growing medium, and wind or
bird uplift of immature plants (before roots are established). This simple tech-
nique may reduce supplemental replanting. Mats made of natural ibre such as
coconut coir will eventually degrade, whereas plastic or steel mesh may be visible
on the surface for the long term. Visibility may be problematic if plant coverage
is low or has less vigor (such as during a dormant season).
4.6.2 Parapets
Parapet height must not compromise safety for visitors or maintenance crews.
Minimum requirements are set by local building codes or government agencies
such as the Occupational Health and Safety Administration (USA), Canadian
Centre for Occupational Health and Safety, WorkSafe (New Zealand) and the
Health and Safety Executive (UK). If there is uncontrolled accessibility to the
public or people (other than a trained maintenance crew), structural roof rein-
forcement, ire regulations, erosion and roof uplift in windy conditions need
consideration.
A higher parapet tends to reduce or completely remove the at-grade viewing
experience of the living roof ( Figures 4.7 and 4.8 ). Alternatively, low roof parapets