Designer's Forum

Since 1997, Geosynthetics magazine (formerly GFR) has featured the popular Designer’s Forum column. These articles have been refereed, and often authored or co-authored, by Gregory R. Richardson, Ph.D., P.E., and currently a principal/senior consultant at the Raleigh, N.C.-based firm of Richardson Smith Gardner and Associates (“RSG Engineers”). For years, the Designer’s Forum has provided a well-coordinated synthesis of field-based experience and design engineering vision.

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  • Drainage and filter components for green roofs

    Sponsored by IFAI Expo

    The Purpose of Drainage for Green Roofs

    • Manage rainfall without sustaining damage from erosion/ponding of water
    • Provides void space and slope to transport subsurface water
    • Connects to roof drains, gutters, and scuppers and conducts water away from roof
    • Removes surface water from impervious surfaces
    • Maintains optimum growing conditions in the planting media

    The Purpose of Filter for Green Roofs

    • To prevent fine planting media particles from washing into the drainage layer
    • Prevents the reduction or clogging of drainage capacity
    • Usually a nonwoven geotextile, occasionally a woven fabric, is used

    Filter Components
    For retention:

    • The geotextile must have small enough openings to adequately retain the planting media
    • If not, soil particles will eventually clog the drain core

    For permeability:

    • The geotextile must have large enough openings to allow unimpeded flow of water
    • If not, hydrostatic pressure will build up and the planting media will not drain

    What are Geocomposite Drains?

    • A manufactured material that combines geotextiles and a polymeric core
    • The core composition varies in design and material
    • Geotextiles may be applied to one or both sides using heat bonding or adhesives
    • Geotextiles can filter, protect or hold water
    • Geotextiles are available in sheets or rolls that are overlapped or joined during construction

    Source: “How to Design Green Roofs & Landscapes” presented at the 2009 IFAI Expo, Sept. 22, 2009, in San Diego, hosted by the Geosynthetic Materials Association (GMA)