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Green roofs

Historical perspective

Green roofs are not a new concept. In northern Europe they were a common element of vernacular styles of buildings up until the early 1900s, when the increased availability of cheap, reliable and low maintenance roofing materials impacted on their use.

Renewed interest in green roofs started towards the end of the 19th century in Germany where they were used for their fire retarding properties, and developed further during the early part of the 20th century as architects such as Le Corbusier, Walter Gropius and Frank Lloyd Wright embraced the concept of green roofs and roof gardens.

High profile examples of green roofs from the first half of the 20th century include The Lawn Road Flats (Isokon Building), Hampstead, London, (1933-1934), and on the building still known as Derry and Toms in High Street Kensington, London (1936-1938).

Types of green roof

Green roofs can be constructed on both flat (slope <10^o) and pitched (slope >10^o) roofs, although for the latter the stability of the materials used to create the roof and the water retention characteristics require careful consideration. Equally it is possible to construct a green roof on warm-deck (standard and inverted) and cold-deck roof constructions, although when constructing a green roof on an inverted warm-deck roof, a number of additional features should be incorporated within the design (e.g. extruded polystyrene insulation that is impervious to water).

The term “green roof” covers roofs, balconies and terraces, and includes roofs that are used by people as public open space and those that are deliberately inaccessible. The following three green roof categories are incresingly widely used:

Extensive – with thin soil, little or no irrigation requirements, low water retention and nutrient poor conditions for plants. Fig (right) - construction of an extensive green roof designed for biodiversity, Switzerland (courtesy S Wilson)
Intensive – with deep soil, irrigation requirements, high water retention and fertile conditions for plants. Fig (right) - intensive roof, Jubilee Park, Canary Wharf (courtesy G Kadas)
Simple intensive – constructed using various substrate depths, thus combining characteristics of extensive and intensive roofs. Fig (right) - simple intensive green roof (courtesy N Dunnett)

The type of green roof design will impact on the types of vegetation that can be grown, the likelihood of public access, structural considerations, maintenance requirements and cost.

Green roof build-up/ components

Including typical commercial green roof cross-section on a warm roof (Figure 9.1 in the guidance). Green roofing systems require a minimum of four components:

1. the root barrier – prevents the roots of vigorous plants penetrating through to the waterproofing and damaging the membrane. The root barrier can either be a biocide or a copper/ heavy grade polythene-based material.
2. the drainage layer – controls the water retention properties of the roof in combination with the substrate. Drainage layers can be composed of either granular materials (e.g. sand and gravel, lava and pumice, crushed brick etc.) or modular/ sheet systems.
3. the substrate (or growing medium) – provides the mechanical strength, pore structure, nutrients, chemical composition and drainage properties for the desired plant species. A wide range of natural and manufactured substrates are available.
4. the vegetation layer – can be established using vegetation mats (e.g. sedum mats), through plug-planting pot grown plants into the substrate, by distributing by hand seeds or cuttings or simply by natural colonisation.

 

Further components can be added depending on the specification or the particular system manufacturer, including filter membranes, moisture mats, protection boards, water retention systems, irrigation systems, special features to locate trees (e.g. anchorage fixings) and additional thermal insulation.

 

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