The purpose of the root barrier layer is to shield the roof’s structure from plant roots that could enter from the upper layer of the green roof. The materials used for the root barrier layer ranges from plastic sheets to metal sheets (Vijayaraghavan, 2016). 

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