A brown roof is where the substrate surface is left to self-vegetate from windblown and bird lime seed dispersal
Biodiverse roofs and brown roofs are two different things:
- Biodiverse roofs are where seed or plants are introduced into the substrate at the time of construction.
- A brown roof is where the substrate surface is left to self-vegetate from windblown and bird lime seed dispersal.
Brown roofs are the best way of replicating exactly the wild urban spaces that are found at ground level. Similar plants will colonise the area.
They have become a popular type of roof garden finish in recent years as they are seen by many planners as a more natural, rugged urban feature and can offer a greater diversity of species as well as prolonged foraging for insects.
Brown roofs are very low maintenance and no irrigation is required. They still offer sound acoustic and temperature insulation properties to the building and will help to attenuate water runoff from the rooftop to a significant degree. The substrate level is normally up to 150mm, which offers a medium weight build-up, usually no heavier than 120kg per m2.
Often objects such as rotting tree stumps, stones and rocks can be introduced into the area to encourage insects and other wildlife.
Waste material from the construction process, such as cleaned rubble, can be introduced into the substrate, adding to a sense of recycling elements of the project.
However, caution must be exercised when using waste from the site to avoid contaminated material or sharp objects, which could damage the waterproofing being placed onto the roof.
After several years a brown roof will actually turn green as the wind-blown plants establish themselves and reach maturity.
A cross section of the layered system is below: