Root resistant film is an LDPE film. It is a durable material with good resistance to moisture and root growth. The root-resistant LDPE membrane is used to make a roof without root-resistant roof covering root-resistant. The use of this membrane does not make the roof waterproof.
You should place the root-resistant membrane on the roof first. You can then continue building the desired green roof.
Make sure that the foil fits together well by overlapping by 50 cm.