Extensive green roofs are designed to be lighter in weight, relatively cheap, not open to recreational use and requiring the minimum of maintenance. Their prime purpose is either ecological or for the environmental masking of buildings. Planting should be of drought-tolerant, wind- and frost-resistant species, such as sedums, herbs and grasses. Instant cover can be
created by the installation of pre-cultivated vegetation blankets where the immediate visual effect is required. Alternatively, a mixture of seeds, plant cuttings, mulch and fertiliser is sprayed onto the growing medium, and this will mature into the finished green roof over a period of between one and two years. The complete system with planting, soil, filter sheet, drainage, moisture-retention layer and root barrier will add between 60 and 200 kg/m2 loading to the roof structure, which must be capable of this additional imposed load. Typical soil depth is up to 100 mm. Limited maintenance is required to remove unwanted weeds, fill bare patches and apply organic fertilisers in the spring, and to remove dead plants and weeds in the autumn. An alternative approach to the fully planted extensive green roof is the biodiverse green roof which has some initial planting and incorporates natural features such as logs and boulders but which is then left to nature to develop with the local flora and fauna.