If you are involved in a new build try not to follow conventional green roof logic. This tends toward the lightest solution possible. This is a built environment profession approach. This can lead to green roofs that are not resilient, especially during periods of hot weather. This can lead to disappointment in years to come. Especially in periods of extended drought. Any approach is possible but a good depth of substrate under a blanket, turf or a seed/plug approach is a must.
Therefore consider what you want to achieve and the decide on the structure you need. This is particularly important if you have delegated to an architect or designer. In the majority of cases the default approach for most design professionals is to go as light as possible. YOU dictate to the architect the load you want. YOU are the client.
If you are retrofitting you may be limited due to weight considerations. This may mean you may have to go for something lighter. In some situations such as sheds and some extensions, strengthening of the structure is very feasible. You, however, may consider this too much work and cost. Our advise is to seriously consider strengthening. You are going to live or someone else is going to live with the green roof for years to come. Why not do it well from the start to get maximum benefit and satisfaction from it in the long run.
Finally a green roof is not a building product. It is not inanimate. It is a living thing that changes with the seasons and the vagaries of our current climate changes! This is not a negative, though it can be perceived to be. It is a positive. Enjoy the visual changes through the seasons and the climate in a given year.
The guide will help you make the decisions you need to make.
Good luck and enjoy.