Planning considerations - Living Roofs in Integrated Urban Water Systems - page 58

is often unwise to use existing architecture for the purposes of urban agriculture,
which necessitates the deep growing media (roughly 200 to 600 mm) of an
intensive living roof assembly (Peck and Kuhn 2001).
In some cases, such as Chicago's City Hall, buildings are planned for future
additional stories or use of roof space changes (for example, car parking/
helicopter pads are removed). These buildings are structurally well-suited toward
a living roof retroit. Creative thinking for Chicago's City Hall resulted in a living
roof with topography varying from a few centimetres to ~45 cm, and supports
trees, without additional structural support.
Initial planning of either new or retroit living roof begins with a licenced
structural engineer determining the structure's load bearing capacity. Structural
design must ensure the roof withstands the supplementary dead (static) loads from
the living roof assembly and the live (dynamic) loads from the luctuating mass of
the water, vegetation and, potentially, people. Live and dead loads are the two
classiications of gravity loads taken into consideration when calculating the struc-
tural capacity of a roof. Live loads constitute any luctuating or peripheral elements
to or on top of a structure. Dead loads constitute the weight of all static elements
that are essential to the structure itself. In New Zealand, live and dead loads are
referred to as imposed action and permanent action, respectively. Structural design
also includes wind uplift. Consideration of wind can result in higher loads at the
parapet area, if the roof assembly needs to be held in place by concrete pavers or
heavy gravel. In the retroit case, the engineer will determine how much loading
can be added to the existing bearing capacity of the roof. Extra structural
reinforcement may be required to support a prospective living roof.
Structural drawings are required by the landscape architect to assess the
potentially variable capacity of the roof's structure - including details of the roof
slab thickness or material, or cantilevered design. These elements inluence
feasibility of point and distributed loadings (e.g., sculptures and trees versus
growing media and low-growing vegetation). If a new roof is built, an
architectural roof plan and structural drawings should be readily available from
the client or architect. For retroits, existing architecture and structural drawings
may be more dificult to obtain, particularly in digital format. If plans for an exist-
ing building cannot be located, the architect or structural engineer should create
a current roof plan. This ensures that everyone is working with up-to-date
information.
3.3.2 Drainage considerations
The presence of any living roof does not compromise the need for ensuring
positive drainage. In other words, stormwater runoff must be able to discharge
off the roof without creating standing (ponded) water. Living roofs are not
restricted to near-“lat” (non-sloped or non-pitched) roofs. Living roofs may be
feasible on roofs up to a slope of approximately 10-15° without signiicant
modiications to the assembly compared to a “lat” roof installation ( Section
4.6.1 ).