Planning considerations - Living Roofs in Integrated Urban Water Systems - page 70

the roof's desired growing media and seeds, which are speciied by the horticul-
turalist for the roof's particular climatic conditions. It may compromise the desired
aesthetic and create an additional maintenance burden.
3.4.4 Ready-made extensive living roof assemblies
As the proprietary markets for living roofs have grown, so too have offerings of
ready- made or ready- to-install extensive living roof assemblies. One alternative is
a typically modular system of interlocking rigid trays that integrate the growing
medium and drainage mechanisms that are set atop the roof's waterprooing
membrane. Other forms include natural iber baskets or pouches that may be
placed over an external drainage layer. Biodegradable modules likely require
external mechanisms to hold them in place, as the container's edges are likely to
break down over time.
In all ready-made assemblies, the vegetation is either “pre-planted” or “pre-
grown,” where the latter provides instant full vegetative coverage upon installa-
tion. For pouches, plants grow through the pouch. Over the long term, modular
assemblies that provide a hydraulic connection between elements (e.g., the
growing medium and/or plant roots can access moisture in adjacent elements)
may maintain healthier plant communities. Pre-grown assemblies are likely more
resilient to damage, from wind and rain erosion, for example, as well as from
birds hunting for insects beneath plants that have not yet developed strong
roots.
The higher initial cost of a ready-made system may be offset by rapid installa-
tion. Advantages of providing an instant aesthetic, and the ability to swap poorly
performing elements provide some reassurance for risk-averse clients (Snodgrass
and McIntyre 2010). Modules also suit small retroit applications as they can be
carried up stairs and elevators, and placed over existing membranes of high
quality.
There is relatively little data to date in the academic literature documenting
stormwater management performance of ready-made assemblies (Carson et al.
2013; Gregoire and Clausen 2011). As long as a modular extensive living roof
forms continuous and substantial coverage of the roof's surface area, it is cur-
rently assumed that the stormwater retention and detention performance would
be on par with a continuous built-in-place extensive living roof.
At all costs, the living roof's designers should endorse environmentally benign,
natural material over manufactured material to reduce embodied energy of the
assembly. However, if budget, time restraints, or other contextual factors impose
a design ultimatum between a limited plant selection and no plants at all, the
authors suggest employing manufactured solutions, providing they comply with
the roof's design objectives.
3.4.5 Horticulturalist's checklist
The horticulturalist should look for the following characteristics when selecting
plants for the speciic project geographic location and building: