Every greenroof must have a
drainage layer to carry away excess water; on very shallow extensive greenroofs
the drainage layer may be combined with the filter layer. Unimpeded drainage is
assured in greenroof systems because the drainage layer is applied over the entire roof area. The drainage layer forms an extremely stable and pressure resistant sub-base (ZinCo
International brochure). Some believe a drainage layer is not necessary on
any sloped roofs due to nature's gravitational drainage capacity, but
single-source greenroof companies will most likely specify one.
Since 1995, the German
Forschungsgesellschaft Landschaftsbau Landschaftsentwicklung e.V. (The
Landscaping and Landscape Development and Research Society) has highly
researched various areas concerning greenroof design, including Draining;
Drainage Facilities; Water Retention and Permeability; Coefficient of Discharge;
Water Retention and Annual Coefficient of Discharge; and the Drainage Course
which includes material groups and types, requirements, etc. in their Guidelines
for the Planning, Execution and Upkeep of Green-Roof Sites (English Release,
American Society for the Testing of Materials
(ASTM) Green Roof Task Force passed the following ASTM standards regarding
E2398-05 Standard Test Method for Water Capture and Media
Retention of Geocomposite Drain Layers for Green Roof Systems
E2399-05 Standard Test Method for Maximum Media Density for Dead
Load Analysis of Green Roof Systems
Stay on top of
things by reading the ASTM Updates column by Ralph Velasquez here.
In March, 2005 Allan Wingfield, AIA wrote "The Filter, Drain, and Water Holding Components of Green Roof Design"
for Greenroofs.com as our Guest Feature Writer for that month. Allan
writes, "The purpose of the drainage layer is to remove excess water from the roof. The drainage system must consider surface water, sub-surface water and transporting water from the roof. A drainage course is to provide sufficient void space and slope to allow excess sub-surface water to be transported to roof drains or gutters where it can be removed from the roof. Surface water may be removed by proper sloping of pervious and impervious surfaces to roof drains or gutters. Roof drains, gutters, scuppers and associated piping then conduct water away from the roof.
"The primary functions of drainage may include: maintaining the overlying growth-supporting media in a drained condition; preventing chronic anaerobic conditions; providing the principal mechanism for discharging storm-event runoff including eliminating surface flow and minimizing seepage flow in the growth-supporting media. Secondary functions may include providing a suitable horizon for introduction of irrigation and increasing root-volume available for the plants.
"Drainage divides into two basic classes, aggregate drains and geocomposite drains, which may be combined or used separately in conjunction with the drain outlets. Aggregate type drain layers less than 4 inches in depth should be freely drained. With deeper layers drainage restriction can be used to provide water holding capacity. A number of granular materials may be considered including gravel & fines, lava & pumice, expanded clay & slate, and different recycled materials such as crushed roofing tiles or brick..."
Read the entire in-depth article here.
Keep in mind
that drainage capacity must
increase closer to the rainwater outlets, so large quantities of drainage
material, usually rounded stones, are installed along the eaves and near
outlets. These rainwater outlets need to be accessible for seasonal
cleaning (Hendriks and Hooker, 1994).
|These areas could also allow a separation
barrier of large rounded pebbles 500 mm or 20" wide around them from the
vegetation (ZinCo International Green Roof brochure, 1998).
Source: ZinCo International 3/98
Source: ZinCo International 3/98
Some drainage systems are
more multifunctional. For example, ZinCo's Floradrain line has troughs
that provide up to 40 mm or 1.5" of water storage for the plants above, and a system of
channels on the underside for drainage of excess water below. Special
holes allow aeration to the roots, and an additional moisture retention mat
below ensures a long-term water supply. The system provides additional
protection to the underlying waterproofing element. The Floradrain FD
60 is strong enough to act as shuttering to concrete. This enables
roadways, walls, and ballustrades to be constructed over the drainage layer,
thereby allowing unimpeded drainage beneath (ZinCo, 1998).
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