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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 professional association FLL - 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, 2002).

The American Society for the Testing of Materials (ASTM) Green Roof Task Force passed the following ASTM standards regarding drainage properties:

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 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). ZinCo Pebble Separation Barrier

Source: ZinCo International 3/98 Brochure

ZinCo's Floradrain  FD 60

Source: ZinCo International 3/98 Brochure

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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