Green roofs: an introduction

Green roofs are the result of a complete underlying roof build-up system, providing continuous, uninterrupted layers of protection and drainage. Recent advances in technology have made them lighter, more durable and better able to withstand the extreme conditions of the rooftop.

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 inches to three feet (50cm to 1m) 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; in a green roof structural loading is also important.

Currently the most complete system of standards for green roof design is the German Landscape Development and Landscape Construction Research Facility’s (FLL).

Roof types

There are two basic types of roof greening covers: intensive (roof gardens) and extensive. Intensive planted roofs have a greater depth of growing medium to support a wider range of planting, and often include shrubs and trees. Extensive roofs are systems with low growing plants, such as sedums, with no access other than for occasional maintenance; this type of roof is intended to be self sustaining.

There are three 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.

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 recognized environmental advantages of green roofs include:

• Improved conservation and biodiversity.
• Provision of new wildlife habitat.
• Improved thermal insulation of buildings.
• Reduction of airborne particulates.
• Reduction in urban heat island effect.

Information Sources

More information on green roofs is available on The Construction Information Service. Subscribers to The Construction Information Service can link directly to the documents from the links below.

The Construction Information Service - Planted roofs

Architects' Journal

AJ Focus Membrane roofing. AJ Focus 08.1999
Examines some of the latest developments in membrane roofing technology including green roofs, roof boards ready-primed with bitumen, and testing the attachment strength of existing roofs in the light of increasing sales of flat roofing materials and a consolidation of UK manufacturers.

AJ Focus Roofing. AJ Focus 11.2000
Review of latest best practice developments in the area of sustainable roofing. The importance of taking a holistic approach to the assessment of the environmentally friendly performance of 'green' roof systems. Gives listing of manufacturers

British Board of Agrément

Certificate 05/4269 Anderson Monarflex. Rootbar membranes
Relates to Rootbar membranes, modified bitumen membranes designed for use in roof garden applications. The membranes are to be used as waterproofing on flat and sloping roofs with limited access as a partially or fully bonded system and in roof gardens (intensive planting) on flat roofs and green roofs (extensive planting). Includes Irish Building Regulations statement.

British Standards Institution

BS 648:1964 Schedule of weights of building materials

CIRIA

Interim code of practice for sustainable drainage systems
Provides a strategic approach to the allocation of maintenance for Sustainable Drainage Systems in England and Wales.

The Construction Information Service - Roof gardens

Architects' Journal

AJ Focus Landscape. AJ Focus 02.2003
Focuses on the resurgence of the roof garden brought about by improvements in roof deck technology and the need for energy efficient buildings. Also includes a list of manufacturers' data on general landscape items and costs for common items of work associated with landscaping and exterior works.

Architects' Journal Roof Gardens Credit Suisse administration centre, Uetlihof, Zurich. AJ 10.2.80

Architects' Journal Roof Gardens Arundel Great Court, London. AJ 9.4.80

Flat Roofing Alliance

Roofing Handbook Information Sheet 25 Roof gardens
There are two normal types of roof garden: Intensive Roof Gardens - generally speaking those containing natural soil that will support normal plant life and Extensive Roof Gardens which generally contain special sub-soils and require special plants. The overview provided in this Information Sheet is for guidance only so it is vital to check with the manufacturer that any specific product is suitable for the intended use.

Other sources of information

Guideline for the planning, execution, and upkeep of green-roof sites.
This guide to green roofs, published by the FLL, is available in English from http://www.roofmeadows.com or http://www.f-l-l.de.

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 outlinest he 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. Detailed specifications for the use of plants on buildings do not generally appear in the text, but comprehensive sources are listed in the technical information at the end of the book.
http://www.london.gov.uk/mayor/strategies/biodiversity/docs/Building_Green_main_text.pdf.

Planting green roofs and living walls. Timber Press, 2004.
Brings together data covering the actual logistics of how to implement plantings on roofs and building facades. Examinations of recent research underline the wide-ranging benefits of greening buildings, from recreational aspects to increased energy efficiency. Ofers important technical advice, along with horticultural recommendations pertinent to flat or sloped roofs and the greening of vertical surfaces.
http://www.ribabookshops.com/site/viewtitle.asp?pid=4891.

Green roofs research advice note
Published by the British Council for Offices (BCO), 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.
http://www.bco.org.uk/research/researchreports/detail.cfm?rid=45&cid=0.

The American Society for Testing and Materials (ASTM) has published a range of standards relating to plant selection, assessment of green roofs, and structural load calculations. These standards can provide an indication of some issues that should be considered and include:

E1991-05 Standard Guide for environmental life cycle assessment (LCA) of building materials/products

E2114-06 Standard terminology for sustainability relative to the performance of buildings

E2396-05 Standard test method for saturated water permeability of granular drainage media [falling-head method] for green roof systems

E2398-05 Standard test method for water capture and media retention of geocomposite drain layers for green roof systems

E2399-05 Standard test method for maximum media density for dead load analysis of green roof systems

E2397-05 Standard practice for determination of dead loads and live loads associated with green roof systems

See: http://www.astm.org/cgi-bin/SoftCart.exe/COMMIT/SUBCOMMIT/E0671.htm?L+mystore+idon1050+1155047337.

Related NBS information:

Articles:

• Wildlife on your roof
• Thatch roofing

Selected links:

• Roofs

Written January 2007

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