GMA Techline

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The Geosynthetic Materials Association (GMA) offers the GMA Techline, a resource for technical questions about geosynthetics. E-mail: gmatechline@ifai.com for fast, free, direct answers to your technical questions.

GMA serves as the central resource for information regarding geosynthetics and provides a forum for consistent and accurate information to increase the acceptance, and to promote the correct use, of geosynthetics.

Browse the entries below list to find past answers highlighted in Geosynthetics magazine.


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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)