Extensive green roofs have a thin growing medium and require minimal maintenance, and in general due not require irrigation [some require irrigation initially]. They are generally less costly to install than intensive green roofs.

There are 3 types currently used in the UK:

1. Sedum mats

Sedum Mats - a sedum mat is a base layer of Polyester, Hessian, or porous polythene depending on the supplier, on which is laid the 2cm growing medium, on to which is sprinkled sedum cuttings. These grow into the substrate to maturity. When harvested the Sedum blanket is rolled up from the carrier upwards and delivered to site. When installed the Sedum blanket (including the 2cm of growing medium) is rolled out onto either 5 - 7cm of growing medium (standard method) or direct onto a moisture retention blanket (ultra light weight method).

Sedums are used because they are wind, frost and drought resistant not because they absorb water. Its ability to absorb water makes it drought resistant.

2. Substrate based roof

7cm of crushed recycled brick is placed on the green roof system and plug planted with sedums or with sedum mats applied. There is a misconception that green roofs are made of turf. Although some green roofs are made of turf this is not generally the case.

3. Green / Brown roofs for biodiversity

Similar to substrate based but can, in some cases, use recycled aggregate from site and generally left to colonise naturally or seeded with an annual wildflower mix or local seed source.

The original concept of the brown biodiverse roof was to use recycled brick and concrete form a local recycling plant. Although this was a good idea there are a number of problems:

1. the water holding capacity of the aggregate
2. quality of the aggregate
3. contractors eco dumping waste material on the roofs