PRINCIPLES:
The essential components of a green roof are:
TIMBER FRAME to hold the green roof elements onto the roof. In some cases, extra timber supports may be needed to ensure that the building can take the load.
ROOT MEMBRANE – this could be a pond liner or 300micron damp-proof membrane.
FILTER SHEET – this stops fine material from leaving the roof. It is generally a sheet like the type placed under patios to keep weeds out. It holds the fine soils but allows water to pass through. It can be bought at DIY stores.
MOISTURE BLANKET – essentially a woolly fleece that holds water. These can be bought commercial and are referred to as geotextile membranes. Cheap alternatives are cardboard or even old blankets/towels will do.
SUBSTRATE – this is the medium in which the plants will grow and should consist of aggregate such as crushed concrete/brick, limestone chippings, gravels or clay pellets. This should be at the base to free drainage. Above it, you need another layer, consisting of sand or soil [preferably subsoil] or both.
PLANTS – the species selected will depend on the substrate and the substrate depth. They could be seeded or plug planted.