Considerations in the selection and use of filter sheets include:
- the expected flow rate of water as it drains through the system
- substrate type – if components of the substrate have sharp edges, the filter sheet should be strong
- vegetation type – the filter sheet must allow penetration of roots, and certain roots will be more or less aggressive (for example, herbs versus trees roots)
The filter sheet can be either a woven or non-woven material. A non-woven material is preferable as it is more resistant to root penetration and can sometimes be used as a root barrier system. The filter sheet is not weatherproof and should not be exposed to sunlight for any length of time, and it should therefore be laid immediately prior to the installation of growing substrate. At boundaries or edges of planting beds, the filter sheet should be installed with upturns to at least the same height as the top of the growing substrate.