The main function of the filter fabric/membrane
is to hold the engineered soil in place and still prevent small media particles, such as
plant debris and fines, from entering and clogging the drainage layer
below. Air and water are thus permitted to flow through while the drainage
layer and the actual drains are protected.
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 filter course is to prevent fine soil medium particles from washing into the drainage layer causing clogging or reduction of the drainage capacity of the drainage layer. The filter may be a separate layer or part of a geocomposite drain mat or board. The filter layer may be independent of the drainage layer or an integral part of a geocomposite drain. The filter layer typically should be installed parallel with the drainage layer.
"Geosynthetic fabrics, commonly known as geotextiles, are typically used and are available as woven and non-woven materials. These filters need to be tough to withstand abuse while other layers are installed, while being open enough to provide good flow for water into the drainage layer. In a granular or what is sometimes known as a single layer green roof the filter fabric is used to separate larger granular material for drainage from finer material in the growing media. The ability of the geotextile to allow water permeability is referred to as hydraulic conductivity by permittivity. The filter must allow good water flow in the normal direction (vertical for green roofs) while inhibiting the movement of soil medium particles into the drain layer. The apparent opening size, or the AOS, is a common physical property that is tested on geosynthetic fabrics used for filtering. This determines the size of the majority of the openings in a particular filter fabric and is determined by allowing very small glass beads of a known diameter (measured in millimeters) to pass through the fabric. The size is increased until a point when the fabric is able to hold back 95% of the beads. This is called the O95 or the AOS. It is desirable that some of the plant roots are able to pass through the filter. This is most critical for extensive green roofs where soil media depths are less..."
Read the entire in-depth article here.
Careful placement is required with overlaps of at least 100 mm
to 8 inches wide
along vertical edges up to the plant material layer, and should be finished with
a strip of self-adhesive bitumen membrane. Typical materials are lightweight water-resistant
polyester fiber mats or polypropylene- polyethylene mats (Hendriks and Hooker,
1994). These filter fabrics are the relatively inexpensive typical
non-woven, non-biodegradable landscape fabric types found at most home
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