I believe you will need a concrete type roof or any type of roof that can hold lots of weight.  When you do a Green Roof/Living Roof with soil you need to factor in water/ grey water recycling; and how you can manage the drainage.  You need to take into consideration the type of plants/grass, that will be planted.  How thick the soil; how are you going to water the plants during dry periods, how do you ensure fast drainage in terms of heavy rain fall. 

I saw a documentary example of a living roof; this couple used vines/creeping plants on their roof; basically no soil, plants are watered using mist spray.  But they were living in a desert area.

I suggest you research on the amount of rain fall you get for the whole year, as well.

Start with the type of plants you want on your roof, work backwards on the suitable soil type (then you will know the amount of soil needed) which affect the strength of the roof to hold the weight, the water required.  You will need to calculate the amount of weight of Soil+ water soaked (i.e. after heavy rainfall).

Without a doubt waterproofing on the concrete roof is required; you will need to figure - worst case scenarios on roof leaks, repairs and maintenance.

Cheers.