Industrial brownfield sites can be valuable ecosystems, supporting rare species of plants, animals and invertebrates.  Increasingly in demand for redevelopment, these habitats are under threat.  "Brown roofs", also known as "biodiverse roofs", can partly mitigate this loss of habitat by covering the flat roofs of new developments with a layer of locally sourced material.  Construction techniques for brown roofs are typically similar to those used to create flat green roofs, the main difference being the choice of growing medium (usually locally sourced rubble, gravel, soil, etc...) to meet a specific biodiversity objective. In Switzerland, it is common to use alluvial gravels from the foundations; in London, a mix of brick rubble and some concrete has been used. 
The original idea was to allow the roofs to self-colonise with plants, but they are sometimes seeded to increase their biodiversity potential in the short term. Such practices are derided by purists. The roofs are colonised by spiders and insects (many of which are becoming extremely rare in the UK as such sites are developed) and provide a feeding site for insectivorous birds.  Laban, a centre for contemporary dance in London, has a brown roof specifically designed to encourage the nationally rare black redstart. A green roof,  above ground level, and claimed to be the highest in the UK and Europe "and probably in the world" to act as nature reserve, is on the Barclays Bank HQ in Canary Wharf. Designed combining the principles of green and brown roofs, it is already home to a range of rare invertebrates.