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 three types currently used:

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 lightweight 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.

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.

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.

Additional benefits

Extended Roof Life
A green roof system protects the waterproofing membrane from climatic extremes, UV light & mechanical damage and in so doing almost doubles its life expectancy. Therefore, a good quality root resisting waterproofing system with a normal life expectancy when exposed to the elements of 30 years can be expected to last up to 60 years, thus saving clients the cost of re-waterproofing during the average buildings expected lifetime.

Reduction in costs of drainage
There are potential savings for clients in that a green roof installation could reduce the number of drainage outlets. A number green roof manufacturers have pointed out that quantity surveyors are unlikely to factor in these cost benefits, as they tend to separate the costs of roofs and drainage.

However, depending on the size of the roof and the height of the building, the cost benefit of drainage outlet savings could significantly offset the cost of a green roof.

Savings through the re-use of secondary aggregates.
The reuse of local or secondary aggregates can provide a cost saving during the construction of the roofs within a development. Although there will be a cost incurred if materials such as brick and concrete are used from site in terms of crushing to the required grade, screening and transportation within site this will be minimal. A 1000m2 green roof could potentially save the developer £10,000 off the cost of the materials needed on the roof.

Environmental

Air Quality
Extensive planting within cities is now widely recognised as a means of improving air quality. Therefore, green roofs contribute to the reduction of a number of polluting air particles and compounds not only through the plants themselves, but also by deposition in the growing medium itself.

Plants reduce carbon dioxide in the atmosphere and produce oxygen. Green roofs reduce the heat island effect, which is the main cause of ozone production. Plant roofs remove heavy metals, airborne particles, and volatile organic compounds. Being absorbed into the green roof system these polluting particles do not enter the water system through surface run off leading to improvement in water quality

Biodiversity and Wildlife
Green roofs are intrinsically of greater benefit to biodiversity than more traditional roofing methods. Many green roof manufacturers promote green roofs as benefiting wildlife, but with little evidence to demonstrate this. Of course ‘off the shelf’ green roof systems do provide benefits for wildlife compared to non-green roofs, but research in Switzerland and in London shows that green roofs need to be designed to meet specific local biodiversity conservation objectives.

Energy Conservation – improved thermal performance
Green roof systems are recognised as providing greater thermal performance and roof insulation for the buildings they are laid on. This can vary depending on the time of the year, and the amount of water held within the system.

Cooling [summer]
Poorly protected and insulated roofs can lead to substantial overheating of spaces beneath them. This can lead to the need for increased air-conditioning. A green roof not only acts as an insulation barrier, but the combination of plant processes [photosynthesis and evapotranspiration] and soil processes [evapo-transmission] reduces the amount of solar energy absorbed by the roof membrane, thus leading to cooler temperatures beneath the surface.

Thermal Insulation [winter]
Green roofs can help to reduce heat loss from buildings during the winter when root activity of plants, air layers and the totality of the specific system create heat and thereby provide an insulation membrane. However, the efficiency of green roofs as thermal barriers is dependent on the amount of water held within the system. Water retention can increase the amount of heat lost through the system and therefore any efficiency gains are dependent on daily conditions. It is therefore difficult to provide accurate figures on the net effect of green roofs on energy efficiency during the winter months.

Noise and sound Insulation
The combination of soil, plants, and trapped layers of air within green roof systems can act as a sound insulation barrier. Sound waves are absorbed, reflected, or deflected. The growing medium tends to block lower sound frequencies whilst the plants block higher frequencies. The amount of sound insulation is dependent on the system used and the substrate depth

Storm Water Amelioration
Green roofs store rainwater in the plants and growing mediums and evaporate water into the atmosphere. The amount of water that is stored on a green roof and evaporated back is dependent on the growing medium, its depth, and the type of plants used. In summer, green roofs can retain 70-80% of rainfall and in winter, they retain between 25-40%.