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.
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.