Root protection film is an LDPE film. It is a durable material which is highly resistant to moisture and root growth. The root-resistant LDPE foil is used to make a roof without root-resistant roof covering root-resistant. The roof is not made watertight by the use of this foil.
You should first place the root protection film on the roof. Then you can continue to build the desired green roof.
Make sure the foil fits well together with an overlap of 50 cm.