Re: Green roof waterproof base layer
Arcathan would be the waterproof membrane you are looking for. Application under a green roof is specific and requires 3 coats (with an average coverage rate of 2.2 kg / m²) so you would need 2 tubs of Arcathane. You should then place an air-laid woven fabric (or geo-textile) made for this purpose (to prevent any root growth going down on into the waterproof membrane).
Don't hesitate to contact me if you have any further questions,