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                                                    Why Green Roofs? 

Traditionally there have been three broad classifications for green roofs - extensive, semi-intensive and intensive. However, whilst biodiverse roofs share many of the characteristics of an extensive roof, their increased specification merits a distinctly separate category.

Source: Green Roof Code of  Best Practice for UK 2011 (Prepared based on FLL guidance standard).

Extensive green roof

Extensive roofs serve as an ecological covering that provides society with environmental benefits and the building owner with life cycle cost benefits. A lightweight, low-maintenance roof system, typically with succulents or other hardy plant species (often sedum) planted into a shallow substrate (typically less than 100 mm) that is low in nutrients. Irrigation is not normally required. 

Biodiverse roof

A roof that is similar in composition to an extensive roof, but designed specifically to create a habitat that will attract a particular flora and fauna; whether replicating the original footprint of the building or enhancing the previous habitat. Includes a brown roof, which is a non-vegetated version. The growing medium is purposely-selected to allow indigenous plant species to inhabit the roof over time.

Semi Intensive green roof

An intermediate green roof type that can include characteristics of both extensive and intensive roofs. Typically requiring a depth of substrate between 100 mm to 200 mm, a wider range of plants can be included, compared to extensive roofs, including shrubs and woody plants. Irrigation and maintenance requirements are dependent upon the plant species installed. 

Intensive green roof

A version of a green roof, often referred to as a roof garden, that provides benefits akin to a small urban park or domestic garden. Designed primarily for recreational use, intensive roofs are typically configured with 200 mm+ of substrate and often require regular maintenance and irrigation.

                                                   Benefits of green roofs

Green, living and intentionally vegetated roofs are becoming more common in the World. Green roofs are one of the most readily-accessible sustainable technologies available to the construction industry and can be included as part of new buildings and (subject to structural checks), retro-fitted to existing buildings to provide the following benefits to the occupants of buildings, as well as the local setting:

Sustainable Drainage

Retention of water, through storage in the growing medium and evapotranspiration from the roof’s plants and substrate, reduces runoff volumes, reducing the burden on the sewer network and lowering water treatment costs. 

Detention of water, due to the time for water to infiltrate and permeate the substrate, reduces peak rates of runoff, helping to reduce the risk of flooding. 

Water quality improvements through the filtration of pollutants during the process of water infiltration. 

Biodiversity
Green roofs can replace habitat that has been lost as a result of urban development or create habitats for enhanced biodiversity to actively encourage flora and fauna into the area, for example, by providing food, habitat, nesting opportunities or resting places for creatures, such as spiders, beetles, butterflies, birds and other invertebrates.

Countering Climate Change & the Urban Heat Island

Vegetation consumes carbon during photosynthesis, positively removing emissions and helping to arrest climate change. The evaporative cooling of green roof vegetation also reverses the heat-reflecting effect associated with non-greened, impermeable surfaces that contribute to higher urban temperatures (known as the urban heat island effect).

 

By returning moisture to the environment through evapotranspiration, solar gain can be reduced.

 

The trapping of particulates and capturing of gases, ensure that air pollution levels are also reduced by green roofs.

 

Building Performance Enhancements

The evaporative cooling effect of green roofs, combined with the increased thermal mass of the build-up, can reduce the need for summer cooling (i.e. air conditioning), with a resultant reduction in carbon emissions. This additional mass also serves acoustic purposes, providing additional sound attenuation benefits.

 

Amenity & Health & wellbeing

Green roofs can benefit building occupants by providing valuable additional outdoor recreational areas, for a variety of possible uses, including amenity and recreation. Indeed, research suggests that such green space can improve the productivity of the workforce, reduce hospital patients’ convalescence times etc.  

 

Financial
Whilst future government policy may further increase the financial benefit to owners of buildings with green roof installations, green roofs can be seen to payback the initial investment by:


 

1. Increasing the life of the roof covering due to the vegetation cover protecting the membrane, thereby lowering thermal stresses induced by UV rays; and

 

2. Reduced energy costs due to the lower energy consumption demands attributable to the insulating effect of the substrate, planting & drainage layer.