Virtually all brown roof projects will utilise the soil and spoil from the immediate area to provide a growing medium. Some purists would argue that the only way to create a "true" brown roof is to leave this substrate to self-colonise over a period of several years. However, in practice a degree of human intervention is usually employed to assist in meeting specific biodiversity objectives. Such intervention will typically include:
This is usually achieved by dividing the roof into areas containing different local substrates (e.g. gravel in one area, topsoil in another area, crushed concrete in another area). This effectively provides a number of different environments on the rooftop, each favouring different species. In theory this creates habitats for a wider range of species on the rooftop, thus maximising biodiversity.
Intentionally introducing plant species from the local area - e.g. by collecting and scattering seeds from plants in the local area. This might be because the plants in question have been identified as being "target species" that the roof has been designed to support (e.g. to comply with a local biodiversity action plan). Alternatively the plants might help to provide a habitat for other target species, for example Common Toadflax might be introduced onto a brown roof intended to provide a habitat for certain species of bird. Another reason for seeding a roof rather than allowing it to self-colonise is to allow plant species with non-airbourne seeds to have a better chance of becoming established on the roof.
Certain rooftop features are often introduced as part of a brown roof design, either to maximise potential for biodiversity of to provide a habitat for a specific target species. Examples of common features are shown below: