Green Roof

I looked at some details of various green roofs to work out what would be the best way to construct one.

Flat Green Roofs

Knowledge of green roof construction methods and the benefits of incorporating them in building design is steadily becoming more widespread in the UK.
Unlike pitched green roofs, flat green roofs can either take the form of
extensive (consisting of a thin layer of turf or sedum matting), or
intensive (characterised by a greater soil depth that can even
accommodate shrubs and trees). This more frequently takes the form of
roof-top gardens, providing valuable secluded green space in urban
settings.

Flat green roof build up

The build-up of a flat green roof will vary depending upon the intended use. We therefore suggest that you contact our technical department early on in the design process. However the diagram below shows a typical build-up for an extensive or semi-intensive green roof:

A standard roofing membrane and insulation layer is laid onto the underlying roof structure, then a studded, perforated drainage membrane such as Oldroyd Xv20 GreenXtra is used in addition to provide a drainage layer with reservoir capability. A filtrating drainage layer (Oldroyd Tp filter fleece) is laid down before adding the required soil loading and plantings, as shown in the diagram above.
In situations where the roofing membrane does not offer adequate root resistance a Safeguard Root Barrier layer may need to be included between the roofing membrane and the Oldroyd Xv20 Green Xtra drainage layer.
Other examples of typical green roofbuild-ups can be found in our library of CAD drawings.
For roof-top gardens, concrete decking is often used to provide a suitable load-bearing construction, whereas substantial reinforcement is not usually required for a lightweight flat turf roof.

http://www.safeguardeurope.com/applications/green_roofs_flat.php