Green roof definition

A vegetated landscape built up from a series of layers that are installed on a roof surface as ‘loose-laid’ or modular (that is, installed layer by layer on the roof or as pre-prepared layers in trays).Green roofs are constructed for multiple reasons – as spaces for people to use, as architectural features, to add value to property or to achieve particular environmental benefits (for example, stormwater capture and retention, improved species diversity, insulation of a building against heat gain or loss).

Green roofs can be assembled from a series of trays placed together

Green roofs can be assembled from a series of trays placed together

Vegetation on green roofs is planted in a growing substrate (a specially designed soil substitution medium) that may range in depth from 50 mm to more than a metre, depending on the weight capacity of the building’s roof and the aims of the design.

Green roofs have traditionally been categorised as ‘extensive’ or ‘intensive’.

Extensive green roofs are lightweight with a shallow layer of growing substrate of less than 200 mm deep, requiring minimal maintenance. They generally have lower water requirements and use small, low-growing plant species, particularly succulents. ‘Ecoroofs’ or ‘brown roofs’ are terms used to describe these extensive green roofs. Roofs that are designed and planted specifically to increase local plant diversity and provide habitat (food and shelter) for wildlife are known as ‘biodiverse green roofs’.

Intensive green roofs are generally heavier, with a deeper layer of growing substrate, and support a wider variety of plant types. Because they can support a heavier weight, they are readily accessed by people. Intensive green roofs need more irrigation and maintenance than extensive roofs, and are highly engineered landscapes, often built directly on structures with considerable weight load capacity, such as car parks. ‘Roof gardens’ or ‘podium roofs’ are terms also used to describe these types of green roofs. ‘Roof garden’ is used particularly for sites where less space is dedicated to the vegetation and growing substrate and more to hard infrastructure such as decking.

Until quite recently, extensive green roofs were usually designed as lightweight installations that were not publicly accessible, while intensive green roofs were designed as amenity spaces for people. The boundaries between these types of roofs are now less distinct, and terms such as ‘semi-intensive’ or ‘semi-extensive’ are used to describe roofs that show elements of these major categories.

Most commonly green roofs are installed directly onto the roof deck as a number of components laid on top of each other

The International Green Roofs Association and other international green roof industry organisations use these distinct roof classifications. However, in this guide, green roofs are described in terms of their particular characteristics, such as depth of growing substrate. The authors of the guide consider that as each green roof is unique and may incorporate a mix of design elements, strict classification of types is no longer as helpful or relevant as it once was. Around the world, designers and their clients, and those who install and maintain green roofs, are exploring a range of creative solutions.