The filter layer is often positioned between the growth substrate and the drainage layer, and its main function is to keep plant debris and soil fines from entering and clogging the drainage layer below (Vijayaraghavan, 2016). Geotextiles are typically used as the filter layer in the green roof systems, and they can further provide a good level of structural serviceability for the green roof (high tensile strength). The small pores in the geotextile would also facilitate the water movement and enhance the water retention capacity of green roofs. Licht & Lundholm (2006) found the green roof with thick fabric has better water retention capacity (300% more precipitation) than the green roof without any fabric. 

Reference

Vijayaraghavan, K. (2016). Green roofs: A critical review on the role of components, benefits, limitations and trends. Renewable and Sustainable Energy Reviews, 57, 740–752. https://doi.org/10.1016/j.rser.2015.12.119

Licht, J., & Lundholm, J. (2006). Native coastal plants for northeastern extensive and semi-intensive green roof trays: Substrates, fabrics and plant selection. Paper presented at the The Fourth Annual Greening Rooftops for Sustainable Communities Conference.