Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 98

surface of garden soils is also likely to be higher, increasing risk of fungal diseases,
to which many living roof plants are susceptible (namely succulents).
Some living roofs designed to promote biodiversity may deliberately use local
soils to make use of local seed burdens, particularly for roofs that are designed to
blend vegetation of the roof into the landscape (e.g., tussocks and pasture). If
natural soils are used, the initial weed maintenance may be high (especially if
soils are not weed-free). The lower permeability and iniltration rates of natural
soils compared to engineered living roof growing media means erosion control
fabrics or organic mulches are more likely needed to minimize risk of erosion. The
potential for erosion is also reduced if construction uses vegetated sods (a tradi-
tional European technique), where the vegetation has both a dense root system
and plant cover. The use of local, natural soils is likely to also increase the weight
compared to an engineered extensive living roof growing medium.
Altogether, the desirable living roof growing media characteristics include:
• Moderatetohighwater-holdingcapacity:
• providesrunoffretention(storage);
• supplieswaterforplantsbetweenrainevents.
• Highpermeability:
• rainfallthatexceedstheretentioncapacitymustpercolaterelativelyquickly
to the drainage layer to prevent overloading of the roof structure;
• preventswater-logging,assumingthedrainagelayerandoutletsarealso
adequate;
• preventsfreezinginwinter.
• Lowsystemweightatieldcapacityand/orsaturation(dependingontheload-
bearing characteristics of the roof).
• Adequatebearingstrengthtopreventcompaction:
• compactiondecreasesmediadepth;
• compactionmayoccurfromcrushingofbrittlecomponents.
• Resistancetodegradation:
• tophysicalcompactionviareasonablebearingstrength;
• todegradationviaoxidationorbreakdownofthegrowingmedium-such
as growing media with high organic matter or some foams - as degrada-
tion reduces inal growing medium depth, and can also lead to water-
logging;
• tobreakdownthroughsuccessivefreeze-thawcycles.
• Abilitytosupportplantlife:
• appropriatephysicalandchemicalcomposition;
• adequatedepthandtemperatureregime.
These desirable qualities are often at odds with one another. Weight increases
with growing media depth, thus increasing structural requirements, yet plant via-
bility improves in deeper media (Durhman et al. 2007; Getter and Rowe 2008).
However, media deeper than 150-300 mm are not necessarily advantageous, as
there is an optimum, climate-dependent depth beyond which highly drought-