The best sealing is simply one layer of EPDM which you can order in the larger construction store (at least you can here in the Netherlands). You can glue the EPDM to your roof. The material is water and root proof, which are the only requirements basically.
On top of that you'll need something that can both hold water and drain superfluous water (drainage layer, 4th layer from the top in this picture). On our green roof this was included in the package with the growing medium and plants. I'm not sure if you can buy it separately, but it looks something like this.
Some people like to add a protective layer between the EPDM and drainage layer but in my experience that is not really necessary as the EPDM is quite sturdy.