Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 117

climate (i.e., precipitation frequency, amount and extreme wet periods), storm-
water runoff volume and velocity, and the impact of drainage on warm and
inverted roof structures. The drainage layer provides air circulation space, which
helps keep the insulation dry, a necessity for an inverted roof.
A drainage layer is invisible, but it carries the full weight of the living roof
assembly. The following questions need to be answered when choosing the
drainage layer:
1 Is the drainage layer capable of carrying dead loads designed on top (growing
medium, geotextile separation layer, fully mature plants, hard landscape)?
2 Is the drainage layer capable of carrying live loads (stormwater, snow, ice,
people)?
3 Does the drainage layer provide adequate low capacity to ensure water does
not accumulate within the growing medium (i.e., does the drainage layer
provide free drainage from the growing medium at all times)?
The ASTM provides testing methodologies to assess the low capacity of a drain-
age layer, depending on the materials used:
• ASTME2396-11StandardTestMethodforSaturatedWaterPermeabilityof
Granular Drainage Media [Falling-Head Method] for Vegetative (Green) Roof
Systems (ASTM 2011a).
• Theproceduredeterminesthewaterpermeabilityofcoarsegranularmaterials
(e.g., pumice, gravel or rock) used as drainage layers. It does not apply to char-
acterizing the growing media or synthetic sheet or board type drainage layers.
• ASTME2398-11StandardTestMethodforWaterCaptureandMediaReten-
tion of Geocomposite Drain Layers for Vegetative (Green) Roof Systems (ASTM
2011c).
• Theprocedureappliestoasyntheticsheet,matorpanelthatisspeciically
designed to convey water horizontally toward a roof deck, drains, gutters
or scuppers.
System failure is not easy to detect. Potential risks may arise from compromising
the layers' functions as a result of material defects or construction errors. These
risks include the following:
1 The weight of the living roof assembly may exceed the drainage layer's loading
capacity as recommended by the manufacturer, reducing the living roof's
drainage ability.
2 Material fatigue or material defects can compromise the drainage layer's per-
formance properties, i.e., drainage velocity and loading capacity.
3 The drainage layer has been forgotten in the planning process, incorrectly
sized for the expected stormwater runoff capacity, or the roof slope is com-
promised, in which case the living roof's overall performance may be at risk.
The result is a possible risk of ponding (standing water), which can create
unanticipated loading as well as a risk of surface low (on top of the growing
medium), which could cause erosion.