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Modern green roofs are a relatively new phenomenon. However, green/sod roofs have been around for centuries. The modern trend started when green roofs were ‘reintroduced’ to modern buildings in Germany in the 1960s, and have since spread to other countries. Today, it is estimated that about 10% of all German roofs have been ‘greened’. Green roofs are also becoming increasingly popular in the United States. Recent advances in technology have made green roofs lighter, more durable and better able to withstand the extreme conditions of the rooftop. This article looks briefly at green roofs.
What is a green roof?
A roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage and irrigation systems. Green roofs are the result of a complete underlying roof build-up system, providing continuous, uninterrupted layers of protection and drainage. A green roof is usually composed of; a waterproof membrane, followed by a root barrier, a layer of insulation, a drainage layer, the growing medium or soil substrate, and the plant material. A shallow layer of gravel or pebbles are placed from 18" to three feet within the outside perimeter of the roof, providing additional drainage, fire control, and access to the roof for maintenance. Vegetation selection is crucial to the long term success of the project, but green roofs need not,
necessarily, be green. The main issues to consider in all roofs are waterproofing and adequate drainage. However, in a
green roof, structural loading is also an important consideration.
Roof types
There are two basic types of roof greening covers:
* Intensive – also known as roof gardens, they have a greater depth of growing medium to support a wider range of planting, and often include shrubs and trees.
* Extensive - these are systems with a shallow depth of growing medium, low growing plants such as sedums, with no access other than for occasional maintenance The other main difference is that an Extensive green roof system is intended to be self sustaining.
Green roof construction
There are three accepted main methods of building up a green roof:
* Inverted roof - has the insulation layer above the roof deck, on top of the waterproofing layer; usually constructed over a concrete roof deck.
* Warm roof - has the insulation layer on top of the roof deck and the waterproofing layer on top of the insulation. In this system the insulation is always dry, giving a static thermal rating.
* Cold roof - has the waterproofing membrane placed directly on top of the roof deck, with the insulation installed below the roof deck, or the roof may be uninsulated.
Currently the most complete system of standards for green roof design is the German Landscape Development and Landscape Construction Research Facility’s (FLL).
Sustainability issues
Green roofs are recognised as important in the delay of rainwater run-off entering the storm water system, and the general retention of rainwater. Other recognised environmental advantages of green roofs include:
- Improved conservation and biodiversity
- Provision of new wildlife habitat
- Improved thermal insulation of buildings
- Reduction of airborne particulates
- Reduced rainwater runoff
- Enhanced roof insulation properties
- Attractive visual appearance
- Provide green space in urban areas
- Reduction in urban heat island effect.
Financial issues
Several studies have been carried out in Germany since the 1970s. About ten green roof research centres exist in the USA and greening activities are carried out in about 40 countries.
In a recent study on the impacts of green infrastructure, in particular green roofs in the Greater Manchester area,
researchers found that adding green roofs can help keep temperatures down, particularly in urban areas:
- Increasing roof life span dramatically
- Generally increasing the building’s value
- Improvement of thermal control of buildings
A green roof is often a quoted as key component in the design of an autonomous building. However, a green roof on an existing building is likely to require extra waterproofing, built up parapets, and general strengthening of the roof structure to accommodate the expected extra weight of soil and plants, and water retained by them. Brown roofs Post industrial contaminated land sites, often generally known as ‘brownfield’, can become valuable ecosystems, supporting rare species of plants, animals and invertebrates. Increasingly these sites are in demand for cleanup and redevelopment; hence, these habitats are under threat. ‘Brown roofs’, also known as ‘biodiverse roofs’, can partly mitigate this loss of habitat by introducing a
greening system to the flat roofs of new developments. However, instead of using virgin growing medium, the system uses a layer of growing material garnered from the site itself. This type of roof is also sometimes termed a ‘rubble roof’.
Construction techniques for brown roofs are typically similar to those used to create flat green roofs, the main difference being the choice of growing medium (usually locally sourced rubble, gravel, spoil etc...) to meet a specific biodiversity objective. Although the original idea was to allow the roofs to selfcolonise with plants, they are sometimes seeded to increase their biodiversity potential in the short term. The roofs are colonised by spiders and insects (many of which are becoming extremely rare in the UK) and provide a feeding site for insectivorous birds. The Laban Centre for contemporary dance, in London, has a brown roof specifically designed to encourage the black redstart, a rare bird which thrives on brownfield landscapes.
Further information
Roofing Handbook Information Sheet 25 Roof gardens Published by the Flat Roofing Alliance
Building green, a guide to plants on roofs, walls and pavements. GLA, 2004. Aimed primarily at developers, architects, planners, landscapers, designers and ecologists, this publication outlines the many human, social and natural benefits that come from using plants on buildings. Contains specific, practical advice on introducing vegetation to walls, terraces, courtyards and roofs.
Green roofs research advice note
Published by the British Council for Offices (BCO), this provides good basic information, general guidance and some examples including costs as well as a range of references and further sources of information on green roofs.
Green roof consultation
The aim of the code is to provide a genuinely informative and beneficial document. There will be two phases of consultation on the Code. The first will be in the early summer of 2010; the second phase will be in the late autumn of 2010. Green roof toolkit Published by the Environment Agency, contains general
guidance related to green roofs.
The Green Roof Centre
http://www.thegreenroofcentre.co.uk/
www.theNBS.com
