Hi Karen,
I am pretty sure you can get some sort of honeycomb patterned grid that you can put on. Sort of soft, flexible and stretchable that is maybe an inch or so deep that would in effect hold the soil in little cells. Or if it's a small area you could create your own grid out of timber. Sure I have seen this on a gardening programme.
Alterantively some green roof suppliers do a sort of all in one 'turf'. When I managed a visitor centre with a green rood they used sedum. It came on a growing membrane with everything it needed. there were several varieties seeded onto it so the one best suited to the location would gradually colonise the whole roof. The membrane was just rolled out like a roll of carpet. Only attention it needed was a slow release fertiliser; the company gave instructions on frequency, quanity etc. Within just a couple of years there were all sorts of plants seeded on the roof.
All I would say is make sure the shed structure is strong enough. The benefit of a growing membrane over soil and turf is its lighter. Soil and grass can retain a lot of water and therefore weight.
Hope this helps and if I can find any info on my old roof I'll pass it on.
Actually, just re-read your post, do you need soil if the aquadine retains the moisture, can the grass grow on that?
helen.