To some extent all green roofs can be said to encourage biodiversity as they provide a habitat that supports life-forms that would be unable to survive on a more traditional roof structure.
Biodiverse roofs (also known as brown roofs or rubble roofs) share many of the qualities and benefits of green roofs, but differ in that they are designed to meet specific biodiversity objectives. Examples of such objectives could include:
Biodiverse roof build up
Typically, the build-up of a biodiverse roof will be similar to that of an extensive flat green roof (see diagram below). However rather than simply planting with sedum or turf blanket, a varied covering of plants, growing mediums, aggregates, etc... will usually be chosen with a view to meeting the relevant biodiversity objective.
To maximise biodiversity, a wide range of growing mediums will normally be distributed around the roof surface (usually sourced from the local area). In most cases, these are either left alone to allow the germination of wind-blown seeds, or alternatively, seeds collected from the local area are scattered over the roof area. Over time an ecosystem of plants and animals will develop on the roof.
Where the structure allows, it is generally considered beneficial to vary the depth of the soil/rubble etc... as this allows the roof to provide potential habitat for as wide a range of species as possible. The easiest way of achieving such depth-variation is often to place mounds of soil/aggregate directly over support pillars. It is important that a structural engineer is consulted when deciding on the landscaping of a biodiverse roof to ensure that the underlying structure can support both the overall weight of the roof and any high localised loadings caused by mounds of soil/aggregate.
Designing for a specific species objective
Where a habitat is being designed to support a specific species, conservationists will normally be consulted to assist in devising a roof-top habitat that will attract and support the target species. Examples of strategies for supporting specific species, in particular the Black Redstart, can be found at http://www.blackredstarts.org.uk/ .
Despite the fact that biodiverse roofs are designed to be "left alone" to allow an ecosystem to evolve, a small degree of maintenance is usually required. Typically this will involve the periodic removal of any plant species with aggressive roots that could damage the underlying drainage and waterproofing system.
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