It sounds obvious but a green roof:
The Small Scale Green Roofs Guide provides details on what loads are possible on what joists and spans.
The guide goes it detail about what waterproofing can be used and you can install this safely to ensure that your roof doesn’t leak.
The step-by-step guide for new builds covers:
- The Roof Deck
- Waterproofing and Edge Detail
- The Layers
- Irrigation or not Irrigation
And remember this is a detailed construction guide not an overview. If you want to know which screw to use, the guide tells you (conversion may need to be undertaken to meet your situation!).
And if you have a shed, a garage or an extension the guide provides details of how you can take the above steps and where necessary adjust for the specific retrofit situation.