cibse ks11 - green roofs

Green roofs

CIBSE KNOWLEDGE SERIES

Direct and accessible guidance from key subjectoverviews to implementing practical solutions

CONTACT US AT:

The Chartered Institution of Building Services Engineers222 Balham High RoadLondon SW12 9BS

Membership enquiries: 020 8772 3650Events: 020 8772 3660General enquiries: 020 8675 5211

General information e-mail: [email protected]: http//:www.cibse.org

CIBSE is a Registered Charity, No. 278104

Further publications in the CIBSE Knowledge Series:

KS01: Reclaimed waterKS02: Managing your building servicesKS03: Sustainable low energy cooling: an overviewKS04: Understanding controlsKS05: Making buildings workKS06: ComfortKS07: Variable flow pipework systemsKS08: How to design a heating systemKS09: Commissioning variable flow pipework systemsKS10: Biomass heating

Green roofs

KS11

cover 25/9/07 12:21

The rights of publication or translation are reserved.

No part of this publication may be reproduced, stored in a retrieval system or transmittedin any form or by any means without the prior permission of the Institution.

© September 2007 The Chartered Institution of Building Services Engineers London

Registered charity number 278104

ISBN: 978-1-903287-87-3

This document is based on the best knowledge available at the time of publication.However no responsibility of any kind for any injury, death, loss, damage or delay howevercaused resulting from the use of these recommendations can be accepted by theChartered Institution of Building Services Engineers, the authors or others involved in itspublication. In adopting these recommendations for use each adopter by doing so agrees toaccept full responsibility for any personal injury, death, loss, damage or delay arising out ofor in connection with their use by or on behalf of such adopter irrespective of the cause orreason therefore and agrees to defend, indemnify and hold harmless the CharteredInstitution of Building Services Engineers, the authors and others involved in theirpublication from any and all liability arising out of or in connection with such use asaforesaid and irrespective of any negligence on the part of those indemnified.

Typeset by CIBSE Publications

Printed in Great Britain by Latimer Trend & Co. Ltd., Plymouth PL6 7PY

The Institution gratefully acknowledges contributions to this publicationfrom Anne Thorne Architects, Chit Chong (London Borough of Camden),Paul Early, Richard Garland (Sandwood Design and Build), Eric Hassell, andPaul Singleton (Icopal Ltd).

Crown copyright material is reproduced with the permission of thecontroller of HMSO and the Queen’s Printer for Scotland under licencenumber C02W0002935.

cover 25/9/07 12:21 Page 3

CIBSE Knowledge Series — Green roofs

Green roofsCIBSE Knowledge Series: KS11

Principal authors Cath Hassell (ech2o)Ben Coombes (ech2o)

Knowledge Series editorHelen Carwardine

EditorsHelen CarwardineKen Butcher

Publishing ManagerJacqueline Balian

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

2 Overview of green roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22.1 What are green roofs? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22.2 Key components of a green roof . . . . . . . . . . . . . . . . . . . . . . . . . .32.3 Types of green roof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32.4 Why are green roofs relevant? . . . . . . . . . . . . . . . . . . . . . . . . . . .5

3 Design considerations for green roofs . . . . . . . . . . . . . . . . . . . . .13

4 Constructing green roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

5 Maintaining green roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

6 Retrofitting green roofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

7 Costs and financial considerations . . . . . . . . . . . . . . . . . . . . . . . . .29

8 Legislation, regulations, and planning policy . . . . . . . . . . . . . . . .31

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32



1 Introduction

This publication is mainly aimed at design professionals who require a conciseunderstanding of the advantages of green roofs and the implications fordesign once specified. Key topics exploring the drivers for installing a greenroof are introduced with an overview of potential benefits to single buildingsand wider environmental goals.

The fundamental considerations and potential obstacles facing the designerare dealt with in a clear and concise manner with topics spanning the processfrom conception to implementation. Issues covered range fromunderstanding relevant legislation and whether existing buildings are suitablefor retrofit, through to specific dynamics such as choices of vegetation,substrate and drainage and their bearings on structural capacity, biodiversityand sustainable drainage systems (SUDS).

Important questions regarding cost, maintenance and aesthetic appeal arealso examined, giving designers valuable tools with which to assess anddiscuss with clients how best their goals are to be met.

The publication also covers issues for contractors during the build process,for building managers who are required to maintain green roofs and forclients who are considering incorporating green roofs into their buildings. The environmental advantages of green roofs are many and varied and it ishoped that this guide will result in more green roofs being incorporated intothe built environment in the UK.

1

2 Overview of green roofs

2.1 What are green roofs?

Roofs that are purposely fitted or cultivated with vegetation are known asgreen roofs or living roofs. There are different types of green roof and it isimportant to understand that they provide different habitats, affect storm-water runoff to varying degrees, and impact on energy usage and thermalperformance differently. They can also look very different and presentdifferent challenges during installation. They can be extensive (planted withsimple plants such as sedum, or allowed to seed naturally, where they arecalled biodiverse roofs), or intensive (planted with shrubs and even trees). Atpresent there are no specific British Standards relating to green roofs andtherefore best practice is to follow the German ForschüngsgesellschaftLandschaftsentwicklung Landschaftbau’s (Landscape Research, Developmentand Construction Society) Guidelines for the planning, execution and upkeep ofgreen roof sites(1).

Historically, turf roofs were common in the UK and Northern Europe butdecreased in popularity as the industrial revolution progressed. The modernemployment of green roofs started largely with the technical knowledge andpractice emerging from Germany in the 1970s and 1980s. Legislation wasintroduced to encourage the installation of green roofs and by 2001, 43% ofGerman cities provided incentives for green roof installation(2). Between 1989and 1999, German roofing companies installed nearly 35 million m2 of greenroofs and the rate is increasing(3). Industry figures suggest that 10% ofGerman roofs are greened, 80% of these green roofs being extensive sedumroofs. Other countries in Europe such as Switzerland and Austria as well asCanada and parts of the United States of America are increasingly installinggreen roofs.

In terms of proactive green roof policy, the UK lags behind the countriesmentioned above, but the past decade has seen a rise of interest andimplementation of green roofing in the UK. The drivers for this renaissanceare numerous and include an increasing awareness of sustainable issues,concerns over climate change, the desire to preserve green spaces in the faceof progressive urbanisation, and ecological concerns about local flora andfauna. It is estimated that there is a total of 200 million m2 of roof area in theUK which it would be possible to convert to green roofs with little or nomodification to the roof structure(4). These roofs are flat roofs on offices,schools, hospitals and other public buildings, as well as blocks of flats.

CIBSE Knowledge Series — Green roofs2

* Forschüngsgesellschaft Landschaftsentwicklung Landschaftbau e.V. (Landscape Research,Development and Construction Society) is an independent non-profit organisation founded in 1975.The FLL’s Guidelines for the planning, execution and upkeep of green roof sites are widely accepted as agood reflection of modern practice and technology


2.2 Key components of a green roof

Virtually all green roofs are made up of five key components as illustrated inFigure 1. These are:

— the planted layer (from sedum and grasses up to shrubs and trees)— the substrate which is the growing medium (and can be simple

geotextile 20 mm deep mats through to a soil mix which can be1500 mm deep on some intensive roof gardens)

— the filter layer which prevents the substrate blocking the drainage layer— the drainage layer which is a key component in effecting the amount of

storm-water which will be retained on the roof at any one time— the root barrier which protects the integrity of the roof from damage.

Insulation and a waterproof membrane are, of course, vital parts of any roof,not just green roofs. Table 1 details the elements and highlights theirproperties.

3

Figure 1: Cross section through atypical green roof

Vegetation

Substrate

Filter fleece/filter layer

Drainage layer

Root barrierInsulation

Waterproof layer

Roof

2.3 Types of green roof

There are three main types of green roof:

— extensive: which can be extensive sedum or extensive biodiverse— simple intensive*: which can also be biodiverse— intensive.

Most extensive sedum roofs, and many intensive green roofs, are supplied asa complete system, including everything from insulation and waterproofingmembrane through to specialist soil mixes and vegetation. These multi-layered ‘all-in-one’ systems are useful in many applications. When there is alimited amount of manoeuvrability in terms of structural capacity, having apre-made system with a verified saturated weight can be a big advantage. Inaddition, the client may require a particular vegetation type, which thesupplier of the roof can guarantee. Such systems are usually quicker to installand come with a manufacturer’s guarantee for the waterproofing.

* Simple intensive roofs are also referred to as ‘semi-intensive’ in some publications


Layer Purpose and considerations

Vegetation layer(see Table 3 formore detail)

Aesthetic appeal, biodiversity goals and albedo properties can alterthe choice of flora employed.

Pre-vegetated mats, rolls or panels (sedum mats):— immediate cover; most expensive option; must be fitted quickly

and secured until rooted.

Plugs:— pre-grown plug plants allowing choice of combining species; may

take some time to mature and reach good coverage.

Seeds and cuttings:— usually a cheap option; slow to produce good coverage; can be

hand-planted or distributed via spraying hydromulch; might needirrigation in early stages.

Raw colonisation:— likely to produce flora in harmony with surrounding ecosystem;

resulting coverage and aesthetics may be unpredictable.

Substrate layer Must be compatible with the choice of flora and in accordance withthe drainage properties the roof is to provide.

A large assortment of materials may be used, from recycledaggregates to prepared soils, expanded clay, lava and pumice.

Considerations must include porosity and permeability, water holdingcapacity, nutrient and mineral properties, as well as pH value.

Extensive roofs usually employ lightweight substrates with lownutrient levels.

Intensive roofs can employ specialist soil mixes and tend to havesubstrates with higher nutrient levels.

Peat should be avoided as it is unsustainable, breaks down quicklyand is too acidic for most plants.

Substrates must not use any materials that might seep pollutants intostormwater runoff.

Filter layer Essential to prevent the drainage layer from getting clogged up withplant debris or soil.

The material used should have a high tear strength and be resistantto perforation; rot-resistant PES or PP fleeces are ideal.

Drainage layer Crucial to the desired amount of water retention on the roof.

Lowers the chances of water ‘pooling’ on the roof.

Dictates the release of rainfall run-off.

Can benefit thermal properties of the roof.

Options include: — synthetic drainage (e.g. foams, mineral wools, plastic sheets and

geocomposites)— granular drainage (e.g. gravel, stone chips, recycled aggregates,

which can be recycled directly from site, and expanded clay)— simple drainage (using a free-draining substrate only).

Root barrierlayer

Inhibits roots from flora penetrating the roof membrane.

Must be chemically compatible with the drainage layer to prevent thebreakdown of the properties of both.

Must be positioned below insulation on inverted roofs to prevent itbecoming a vapour barrier.

Options include: — mechanical barriers (e.g. rubber mats, bitumen, slate-surfaced

layers and heavy-duty polyethylene)— anti-root compounds (some mechanical barriers are also

impregnated with biocides).

Table 1: Properties of essentialcomponents of greenroofsSource: based on a table in Building

Greener(5)


There is a move by some green roofs designers to encourage site-specificdesigns for green roofs, utilising the skills of drainage engineers for the SUDSdesign and ecologists for the biodiversity aspects, thus moving away fromgeneric engineered green roof systems. Depending on substrate depth, thesetypes of green roofs will usually be classified as extensive biodiverse or simpleintensive biodiverse (or even, on some roofs, a mixture of both). Eachcategory of green roof has different attributes, which are detailed in Table 2.

Extensive roofs can be further divided into those that are comprised of asedum mat, those that are hydro planted (rare in the UK) those that are plugplanted and those that are self seeded. The main differences between thesetypes are highlighted in Table 3 (see page 7).

2.4 Why are green roofs relevant?

Reducing energy use

Green roofs have been shown to impact positively on a building’s energyconsumption by improving its thermal performance, although the amount ofdifference this makes varies depending on daily and seasonal weather. Poorlyinsulated roofs, as found on many existing buildings, will lead to overheatingof spaces beneath them during the summer and excessive heating demandduring the winter. By retrofitting green roofs, both air conditioning andheating usage is decreased.

During the summer, the amount of solar energy utilised for evapo-transpiration from the plants and evaporation directly from the soil, reducesthe amount of energy available to be absorbed by the roof membrane andleads to a reduction in cooling load, which can be quite significant. During thewinter months, a green roof can add to the insulating qualities of the roof.However, the thermal performance is extremely dependent on the amountof water held within the green roof substrate and in a damp winter climatesuch as the UK generally experiences a green roof will add little to the overallthermal performance of the roof.

Climate change mitigation

Buildings are responsible for around 50% of the UK’s CO2 emissions. A highproportion of these emissions are from heating and cooling the internalenvironment. Reducing the energy consumption of the UK’s buildings willreduce their impact on climate change. Green roofs can significantly reducethe cooling load of a building and may have a positive effect on the heatingload.

5

Relevance of green roofs

• Energy use reduction

• Climate change mitigation

• Lessening the urban heat island effect

• Climate change adaptation

• Improving local air and water quality

• Promoting biodiversity

• Reducing storm water runoff as partof a SUDS strategy

CO2 emissions from commonly usedfuels for heating and cooling

Gas 0.206 kg CO2/kW·h

Oil 0.281 kg CO2/kW·h

Electricity 0.527 kg CO2/kW·h

LPG 1.496 kg CO2/ litre

Source: DEFRA (2007)


Type Overview of basic qualities

Extensive sedum Depth of substrate: usually 20–100 mm (can be up to 200 mm).

Suitable flora: mosses, grasses, succulents, some flowering plants; usually self sustaining;biodiversity average.

Accessibility: not usually designed for access by general public; needs to be accessible formaintenance.

Need for structural support: typically not an issue, though a structural engineer must still beconsulted.

Maintenance requirements: low, but will need yearly maintenance and possibly someirrigation during establishment stage; some suppliers of sedum recommend feeding with aslow release fertiliser in the spring and weeding three times a year.

Relative cost: low.

Thermal properties: good effect on summer overheating; negligible on winter insulation.

SUDS potential: attenuation of stormwater moderate to good; improves as depth ofsubstrate increases.

Extensive biodiverse (sometimes called‘rubble roof’ or ‘brown roof’)

Depth of substrate: variable but usually less than 100 mm.

Suitable flora: allowed to self-seed; potential for high biodiversity as seeded only with localplants that arrive naturally.

Accessibility: not usually designed to be accessible; needs to be accessible for maintenance.

Need for structural support: moderate demands on building structure; very site specific.

Maintenance requirements: very low, but will need yearly maintenance to remove treeseedlings.

Relative cost: low.

Thermal properties: slight effect on summer overheating, negligible on winter insulation.

SUDS potential: attenuation of stormwater good.

Simple intensive Depth of substrate: 150–500 mm.

Suitable flora: grasses, herbaceous plants (e.g. typical flowering garden plants) and shrubs;potential for high biodiversity if plants chosen carefully.

Accessibility: sometimes accessible to public but often designed only to be overlooked.

Need for structural support: moderate demands on building structure; very site specific.

Maintenance requirements: usually require some maintenance, dependent on goals.

Relative cost: moderate.

Thermal properties: excellent effect on summer overheating; negligible on winter insulation.

SUDS potential: attenuation of stormwater good to excellent.

Intensive (sometimes described as ‘roofgardens’)

Depth of substrate: 150–1500 mm

Suitable flora: all vegetation including lawns, beds, shrubs and trees; potential for highbiodiversity if plants chosen carefully.

Accessibility: usually fully accessible and therefore provide amenity.

Need for structural support: generally constructed over reinforced concrete decks.

Maintenance requirements: frequent; including irrigation, fertilising and weeding.

Relative cost: high.

Thermal properties: excellent effect on summer overheating; negligible on winter insulation.

SUDS potential: attenuation of stormwater good to excellent

Table 2: Types and basicattributes of green roofs


Lessening the urban heat island effect

The urban heat island effect is the temperature disparity between urbanisedareas and surrounding rural areas. Urban landscapes have a much higherproportion of tough, impermeable, darker surfaces (typified by conventionalroof surfaces) which favour the retention of heat. Urban surfaces can convertup to 95% of the net incoming solar radiation into heat(6) and in large citiesthis can result in a 4 °C variation between the city and surrounding areas,usually experienced at night. The urban heat island effect (see Figure 2) willincrease as summer temperatures increase and will therefore become evenmore of a problem in the UK in the future. During the hot summer of 2003night-time temperatures in London reached 8–9 °C higher than outlying ruralareas on a number of occasions.

Green roofs have very different evaporative, thermal and albedo (reflectivity)qualities from conventional roof types. Specifying a green roof over, say, abitumen roof effectively modifies the contribution of several factors key tothe impact of the urban heat island effect; evaporating surfaces are increased,storage of heat within the building fabric is reduced, and the local albedo ispositively altered.

Climate change adaptation

The fact that we are probably already committed to some continuation of thewarming climate of recent decades means that the urban heat island effectwill worsen. Even in the most optimistic of scenarios, whereby the emission

7

Type Description

Sedum mat Grown at ground level on geotextile fleece. Harvested andtransported to site in rolls. Provides an ‘immediate’ green roof.Specify a sedum mat grown in the UK to ensure varieties suited tothe UK climate. Some companies combine local wild flowers with thesedum to increase biodiversity.

Plug planted Allows choice of plants and layout design for client. Average densityof 15–20 plants per m2. Individual plugs of sedums or other floweringplants are planted into a growing substrate. There may be arequirement for weeding during the establishment stage.

Hydro planted Sprayed onto roof as a coagulate mixture containing seeds, plantcuttings mulch and fertiliser. Gives a diverse selection of plantvarieties. May take up to two years for full coverage. Fast andeconomic method for very large roof areas. There may be a highrequirement for weeding during the establishment stage.

Hand seeded Often the method used for biodiverse roofs. Fast installation andallows for local seed mixtures. Most weeds that arrive will be allowedto remain.

Naturalcolonisation

Another method for biodiverse roofs, allowing only naturalcolonisation of plant species to take place. Can be very slow. Weedsthat arrive will be allowed to remain.

Table 3: Types of vegetation coverfor extensive and simpleintensive roofs

Rural RuralPark Commercial

Suburbanresidential

Suburbanresidential

Urbancity

City

Illustrated heat island profileLa

te a

fter

noon

tem

pera

ture 91

88

84

33

31

29

°C °F

Figure 2: Sketch of an urbanheat-island profile Source: adapted from U.S. Environmental

Protection Agency website(7)

of greenhouse gases ceased immediately, the associated climatic changeswould continue because of the long lifespan of the gases in the atmosphereand the thermal inertia of the oceans(8). For this reason, adaptation to climatechange must now be considered as urgent a priority as mitigation. Climatechange is thought likely to exacerbate the urban heat island effect, impactnegatively on air quality, and increase the number of intense storms in thecoming decades. Subsequent sections give an overview of the ability of greenroofs to improve air quality and deal with storm water.

Improving local air and water quality

The urban heat island effect exacerbates ground-level ozone production,which is formed by a reaction between volatile organic compounds andnitrous oxides catalysed by heat and sunlight. It is classified as a pollutant andis the foremost component of smog. During the heat wave of 2006 theEuropean Environment Agency reported that safe levels of ozone weresurpassed often and in many places throughout Europe, including the UK(10).Through mitigating the urban heat island effect, as well as producing oxygen,green roofs can thus improve local air quality.

Having a living roof can also help to remove airborne particles, heavy metalsand volatile organic compounds from the local atmosphere. As thesecontaminants are retained by the green roof itself, their infiltration of thewater system through surface runoff is lessened, in turn improving localwater quality.

Promoting biodiversity

As more land is built upon, ensuring that biodiversity is retained is a keyrequirement for local councils. Whilst green roofs do not directly replaceground-based habitats and are not part of a ‘green corridor’, they can bethought of as green islands and, if well planned, can cater for a variety of floraand fauna unattainable on traditional roofs.

Different types of green roofs will produce different types of biodiversity.Biodiverse roofs are especially important as they can recreate the wastelandthat is lost when new building takes place. Within urban areas, it is very oftenbrownfield sites that are the most biodiverse. The best design may thereforebe one that supports a range of habitats for wildlife, including areas of shingle,gravel and stones and different substrate depths.

Designs can vary in different parts of the UK to meet local Biodiversity ActionPlans and to create habitats for local species. Green roofs can also be used torecreate habitat for some endangered species. This has been donesuccessfully in the London area, for example, to protect the habitat of the


Climate change impacts ontemperature

• In London average summertemperatures are predicted toincrease by 2.5 to 3.0 °C by the2050s under a high CO2 emissionsscenario.

• The number of ‘hot’ days (>30 °C)will increase by 5–10 days per year.By the middle of the century, ‘heatwave’ temperatures, such as thoseexperienced during the 2003summer, will be an average summer.

• August, the month with the mosturban heat island events, is likely tobe much warmer as the centuryprogresses. The table below showsaverage summer projectedtemperature increase, relative totoday, in 2080:

Emissions scenario Temp / °C

Min Max

Low +2 +3

High +5 +6.5

Source: adapted from Climate ChangeScenarios for the UK, UKCIP02(9)


redstart. The skylark, a species listed on the UK Biodiversity Action Plan, hasbeen known to breed on green roofs in Germany. There are examples ofrare lichens in Berlin, which are found on well-established green roofs andnowhere else within the city.

Designers need to be fully aware of any specific biodiversity aims of a projectin order to tailor a roof that will encourage, and not hinder, these aspirations.Many green roof manufacturers promote green roofs as benefiting wildlifebut supply limited or unspecific evidence to aid the decision process.

Of course ‘off-the-shelf’ green roof systems do provide benefits for wildlifecompared to traditional roofs, but research in Switzerland and in London hasrevealed that green roofs designed specifically to meet local biodiversityconservation objectives are far more successful to this end. A client should bemade aware that a specific biodiversity goal will likely affect the aestheticfinish of the roof as well as the degree of storm-water amelioration, andsome trade-offs may have to be made.

Specific biodiversity goals will usually demand a customised green roof ratherthan a generic, pre-manufactured system. However, commonly used sedummat systems can be purposely cultivated with local mosses and lichens overtime to alleviate this trade-off. Some manufacturers may also use varieties ofsedum native to the UK and a minority of companies now include wildflowerseeds in their sedum mats and blankets to increase diversity.

It may be worth contacting an expert on local biodiversity as flora and faunacan vary greatly between sites. They might even advise planning a roof topurposely encourage a distribution network between local sites, fostering thetransfer of biodiversity to areas where habitats have previously been lost. If a roof allows, a series of different mini-habitats can be employed to increasethe range of plants and insects. For example, consider having relatively bareareas of gravel, shingle and stones or having a few dry-rotting logs.

The client must also be prepared for the fact that green roofs will notreplicate perfectly the biodiversity of ground level. Some species of plantfound at ground level will not be suitable for anything other than intensiveroofs and the weight of substrate required by other species may rule themout altogether. It cannot always be expected that a roof designed to directlyreplace a habitat lost during construction will be immediately successful inconserving key species; speed of replicating the ground environment will notalways be instant, particularly if seeds have to grow on site. Table 4 detailsthe main properties of the various vegetation types commonly found onextensive and simple intensive roofs. Readers are directed to the EnglishNature Report No. 498(4) for in-depth advice on planting for a wide range ofbiodiversity.

9

Figure 3: Rare lichen, Berlin(top), two varieties of sedum(middle), lichen variety (bottom)


Type Properties

Sedum Most common plant for extensive green roofs; hardy and verydrought tolerant.

Evergreen, though if planted on very thin substrate may becomepatchy unless watered.

Relatively short flowering period in summer.

UK native species such as white stonecrop are available; this speciesprovides one of the more impressive flowering spectacles when inbloom, attracting a variety of insects including bees and butterflies.

Options:

— seeding: cheap though relatively slow to establish maturecoverage

— cuttings/plugs: quicker, though more expensive

— pre-grown mats/blankets: quickest, most comprehensivecoverage though also most expensive.

Mosses andlichens

Relatively slow growing.

Non-flowering.

A habitat for a massive range of microscopic animals andinvertebrates; these in turn are a food source for birds.

Moss carpets can be encouraged by using a layer of sandy soil,20 mm or less, which should be kept damp to attract moss spores.

Moss sods taken from other buildings and re-planted on a new greenroof will encourage colonisation even if they themselves fail toestablish.

Hydro planted Sprayed onto roof as a coagulate mixture containing seeds, plantcuttings mulch and fertiliser. Gives a diverse selection of plantvarieties. May take up to two years for full coverage. Fast andeconomic method for very large roof areas. There may be a highrequirement for weeding during the establishment stage.

Other plants Green roofs have been found to support a wide variety ofherbaceous plants.

Wildflowers (alsoreferred to as‘rooftopmeadows’)

In the autumn and winter seed-heads provide food for some birdswhilst desiccated stems and stalks provide shelter for manyinvertebrates. (For this reason it is better to remove dead flowers inlate winter rather than summer.)

Wildflower seeds will likely colonise a roof naturally butincorporating increasingly scarce wildflower annuals such asmarigold, cornflower, wild pansy and scarlet pimpernel will give anestablishing roof a head start.

A diversity of species stretches out the total flowering season,benefiting nectar-seeking insects.

Some species, such as wild marjoram and viper’s bugloss require asubstrate over 100 mm.

Seeding with grasses may result in wildflowers struggling to compete.

Local wildflower mixes are often available.

Calcareousgrassland

Limestone chippings and a blend of crushed brick and concrete willencourage the development of a calcareous meadow.

Usually species rich; capable of supporting 30 to 40 species per m2,so a good choice for small roofs with biodiversity aspirations.

Naturally such habitats are quite thin (50–100 mm) and so are idealfor extensive roofs.

Table 4: Choice of vegetation


Reducing storm-water runoff as part of a sustainable urban drainage systems(SUDS) strategy

Any built-up area needs to be drained to remove excess water. Traditionallythis has been achieved by underground pipe systems that convey water awayas quickly as possible to prevent localised flooding. However, as developmentincreases, this can impact negatively on existing storm-water drains causingthem to surcharge and create the potential for flooding further downstream.This method of draining the built environment also flushes surface pollutantsdirectly into the receiving watercourse. As the incidence of storm events ispredicted to increase in intensity and number due to the effects of climatechange, and the total volume of winter rainfall will rise (UKCIP02), methodsto mitigate storm-water runoff are increasingly being required in newdevelopments. SUDS are engineered solutions that aim to mimic naturaldrainage processes. They help to prevent localised pollution incidents andlocalised flooding as well as providing amenity and biodiversity.

Green roofs are considered as one method of source control of storm-waterunder a SUDS strategy. Once a green roof has established, both peak flowrates and total runoff volume of rainwater from the roof are significantlyreduced compared to a conventional roof. Green roofs store rainwater in theplants and substrate and release water back into the atmosphere throughevapotranspiration and evaporation. As green roofs are much easier toretrofit in the existing urban environment than most other SUDScomponents, their potential impact on reducing storm-water problems in theUK’s cities is especially high.

The amount of water that is stored on a green roof and evaporates into theatmosphere is dependent on the growing medium, its depth, type of drainagelayer and the type of plants used. The FLL Guidelines(1) should be followed toensure that actual runoff will be in accordance with calculated runoff. Sedumspecies are very effective at evapotranspiration but roofs of just sedums willnot meet comprehensive biodiversity aims. In summer green roofs can retain70–80% of rainfall and in winter they retain 10–35%(11). The difference is dueto a combination of more winter rainfall and less evapotranspiration by theplants because growth is not as vigorous during the winter months. Generally,the deeper the substrate the greater the average annual water retention.However, even a substrate of 20 mm will absorb 40% of average rainfall andthe remaining 60% will drain off at a much slower rate compared to aconventional hard roof finish(1). Control of surface runoff byevapotranspiration is more effective once a roof is established; time toestablish depends on the type of plants and method of planting (see Table 5)and needs to be taken into account at the design stage.

11

Climate change impacts onrainfall patterns

• In general across the UK winters willbecome wetter and summers willbecome drier. Annual precipitationwill remain the same in the north andwill fall by up to 10% across the restof the UK.

• Summer precipitation will fall acrossthe whole of the UK, possibly by upto 10% in the 2020s and up to 60%by the 2080s.summer, will be anaverage summer.

• Winter precipitation will rise by up to10% in the 2020s and up to 30% inthe 2080s.

• The incidence of storm events willincrease.

Source: Adapted from Climate ChangeScenarios for the UK, UKCIP02(9)


Between 30 and 40% of rainfall events are of such limited duration, or are atsuch low rates, that there is no measurable runoff from greenfield sites,whereas virtually all rainfall events produce some runoff from developedsites. A green roof can easily be designed to prevent runoff from all rainfallevents of up to 5 mm and, as part of a SUDS strategy, should reduce thevolume of surface or underground attenuation required at the siteboundary(12). In Germany, it is recognised that a green roof will have a definitepositive effect on storm-water runoff, and figures are provided for varioussubstrate depths at various rainfall rates as shown in Table 6. In the UK theEnvironment Agency recognises the same positive effects but the criteria formeasuring the amount of attenuation volume that can be offset are, atpresent, less clear and will depend on site specific situations.

Substrate depth/ mm State Surface runoff / %

Evapotranspiration/ %

50 Established 37 63

120 Established 28 72

100 New 58 42

Table 5: Evapotranspiration andsurface runoffSource: adapted from Schmidt(6)

Substrate depth/ mm Vegetation

Average annualwater retention

/ %

Average annualrainfall run off

/ %

20–40 Moss, sedums 40 60

40–60 Sedums and moss 45 55

60–100 Sedums, moss andherbs

50 50

100–150 Sedums, herbs andgrass

55 45

150–200 Grass and herbs 60 40

Table 6: Water retention inextensive green roofs(based on 650–800 mmannual rainfall)Source: FLL(1)

Potsdammer Platz, Berlin

Since 1995 this area has been extensivelyredeveloped with commercial and retailspace. 30% of the roof area (16000 m2)is covered with either extensive orintensive green roofs with substratedepth ranging from 100–600 mm. Only1% of annual rainfall runoff is allowedfrom the site. This is achieved by acombination of the green roofs andstoring rainwater runoff in a buffer lake.The collected rainwater is used forirrigation of the intensive green roofs andfor WC flushing. The green roofs retainbetween 60–70% of annual rainfall, thusreducing the size of the buffer lake.


3 Design considerations for green roofs

Once the design decision has been taken to incorporate a green roof ontothe building, several key issues need to be addressed to ensure a robustdesign that will translate seamlessly into a good build. Whilst the choice ofwhich type of green roof will often be dictated by the specifics of the site (forexample, whether it is new build or retrofit, any specific biodiversity aimsthat must be met, or storm retention parameters to meet), there is aplethora of further matters to consider and designers should address all thepoints that follow.

When should the decision be taken to incorporate a green roof?

This should be made as early in the design process as possible. Specifying agreen roof can, for example, have structural implications, or mean that SUDSfinal attenuation capacity is reduced. The pitch of the roof may be different tothat if a green roof had not been specified, or it may be harder to install solarpanels. Therefore, a late decision to incorporate a green roof will have animpact on many other design decisions of the building.

Will specifying a green roof affect planning consent?

Incorporating a green roof or roofs into a development may contributetowards planning consent. However, it is important that this is done at theoutset of the design process rather than added at a later stage to ‘green up’ adevelopment.

Is there a specific biodiversity aim?

Green roofs provide a habitat for plants, insects and birds on roofs wheregenerally there is no such habitat. Concerns over the displacement of localbiodiversity when new developments are built can also be lessened with theincorporation of a green roof or roofs. Reference should be made to section2.4 to understand how different types of green roof provide varying habitatsand meet different biodiversity aims.

Does the client require an immediate green roof?

Planting necessarily means that a green roof will take time to establish, butgives a more robust roof finish and allows planting of native species. Sedummats provide an immediate green roof effect but usually are less biodiverseand less effective at attenuating storm-water flows.

13

Main issues at the design stage

• Planning

• Structural considerations

• SUDS potential

• Biodiversity

• Fire prevention

• Roof orientation and pitch

• Offset of cooling plant

• Compatibility with solar thermal orPV panels

• Reuse of rainwater from a green roof

• Cost implications

Will the green roof affect the structural build up of the roof?

Structural loads on the roof depend on the type of green roof chosen as thefinished design, and a structural assessment is always required. Intensivegreen roofs with trees and where access for amenity is required will need tobe supported by a heavy steel or concrete structure. Many inverted roofs aredesigned to be covered in paving slabs, which act as ballast on the insulation.The substrate of a green roof can provide the necessary ballast in place of thepaving slabs, therefore resulting in no addition to the structural load. For low-rise lightweight structures, extensive green roofs are the only option andsingle ply membrane roofs will still usually need to be structurallystrengthened if a green roof is the design choice. It is important thatcalculations incorporate the load when the roof substrate is saturated, as thiswill be when the greatest loads are placed on the structure. Table 7 showsthe structural loads of different roof types, both conventional roofs and greenroofs. Tables 8 and 9 detail the saturated weight of different green rooflayers. Combined they show that many types of extensive green roofs, evenwhen saturated, will weigh less per m2 than many other types of roof.


Can a green roof be specified as part of a SUDS strategy?

Green roofs are classified as one of the methods of source control within aSUDS system. How effective they are at reducing rainwater runoff from theroof depends on the depth of substrate, type of plants and saturation andevapotranspiration rates. Reference should be made to section 2.4 for moredetailed information about how specifying a green roof aids storm-watercontrol.

Are green roofs a fire risk?

There is a potential fire risk during prolonged hot weather when thevegetation is dry. Although there are at present no mandatory fire standardsfor green roofs, it is recommended to follow the FLL standards(1), whichrecommend the use of pebble vegetation barriers as firebreaks. The FLL

Type of roof system Support load / kg·m–2

Gravel surface 90–150

Standard tiled roof approx. 150

Paving slabs 160–220

Extensive green roof 60–150

Intensive green roof 200–500

Table 7: Structural load forvarious roof typesSource: Frith and Gedge(13)

CIBSE Knowledge Series — Green roofs 15

Substrate layer:

— treated top soil 100 150–200

— open pore substrate for intensive green 100 100–130roof

— low-density open pore substrate for 100 80–130extensive green roof (e.g. burnt clay)

— high-density open pore substrate for 100 140–180extensive green roof (e.g. recycled aggregate, lava, pumice)

— layered mix (e.g. pumice, expanded clay) 100 70–100

— sedum mat (e.g. Bauder nylon loop base 28 40–50with substrate)

Drainage layer:

— expanded clay (dependent on size of pieces) — 50–80(per 100 mm)

— foamed glass (per 100 mm) — 25–35

— ceramic tiles (recycled) (per 100 mm) — 110–130

— gravel / sand and gravel (per 100 mm) — 150–180

— lava (per 100 mm) — 120–140

— pumice (per 100 mm) — 80–120

— foam tiles 50–65 2–2.5

— foam tiles profiled 75 25

— foam tiles profiled with filling 100 35

— foam tiles profiled with 2–8 mm of 140 50expanded clay

— plastic tiles 25 15

— plastic tiles with filling 40 20

— plastic tiles with 2–8 mm of expanded clay 60 25

— foam beads with polyethylene layer 30 6

— foam beads with polyurethane layer 35 25

Examples of common materials Standard Saturated for each layer thickness / mm weight / kg·m–2

Flora Height / mm Weight / kg·m–2

Extensive green roof with sedum, grasses,moss etc

50–100 10

Extensive green roof with soil, plants and smallshrubs (below 0.5 m tall)

100–150 15

Intensive green roof with larger plants and smallshrubs (below 1 m tall)

150–200 20

Intensive green roof with larger plants andsmall shrubs (below 3 m tall)

200–400 30

Intensive green roof with large plants and smalltrees (below 6 m tall)

400–1000 60

Intensive green roof with large plants and smalltrees (below 10 m tall)

over 1000 150

Table 8: Structural saturatedweights of commonmaterials used in greenroofsSources: Kolb and Schwarz(14)

Table 9: Typical weights ofcommon flora used ingreen roofsSources: Kolb and Schwarz(14)

recommendations further state that extensive roofs are unlikely to be ignitedby sparks provided that:

— substrate depth exceeds 30 mm and contains no more than 20%organic content by weight

— pebble or concrete paving fire breaks are provided in the vegetationevery 40 m with a minimum width of 1 m or a 300 mm high fire wall

— vegetation barriers are provided to all roof penetrations and in front ofwalls with openings

— vegetation barriers are kept clear of encroaching plants by routinemaintenance.

Is access for maintenance required?

As with all roofs, and especially flat roofs, maintenance is required andtherefore access must be considered. All green roofs require somemaintenance. Extensive green roofs need less maintenance than intensiveroofs, but there is still a requirement every year to take out large plants thathave self-seeded and could eventually damage the roof structure. Thereforeit is important to design for ease of access to all types of green roofs. Accessto the roof should be by internal access hatches (ideally) or alternativelysecure ladder points. There should be fall protection systems if operativeswill need to work near the edges of the roof. Health and safety concerns ascovered in the Construction (Design and Management) Regulations 1994(15)

(‘CDM Regulations’) must be addressed. As fall protection systems needyearly checking, a better option for small green roofs where edge rails cannotbe fitted, is to design them so that access can be by ‘cherry picker’. Section 5details necessary maintenance tasks.

Will a green roof need to be irrigated?

This depends on the type of roof but generally access to a water point shouldbe provided in the design for most green roof installations. Intensive greenroofs will need considerable watering throughout their life. Simple intensiveand extensive green roofs may require some watering while establishing, butafter that should not need watering unless there is a prolonged period(usually defined as six weeks) without rain. To reduce the need for irrigation,a green roof should be installed in the spring or autumn. Some green roofsuppliers recommend watering sedum blankets during the establishmentstage on south-facing roofs with steep pitches wherever they are installed;they also recommend watering during the establishment stage for any greenroof if installed in the drier parts of the UK (i.e. where the average yearlyrainfall is 650 mm or less). A biodiverse roof is designed to require nowatering.


Case study: access for maintenance

Green roofs were specified on a series ofdwellings, which had dual pitched roofs.The front roofs can be accessed by acherry picker. However, the rear roofsare only accessible though privategardens and as the dwellings are terracedthe only access is through the house. Thecherry picker cannot reach the rear roofsand so these are not being maintained. Asthis scheme was only completed in early2007 it is not yet clear what impact thiswill have but it is expected to be anegative one.


It is important to realise that water supplies are under increasing stress inmany parts of the UK and there is a considerable carbon load to water use(0.6 kg CO2 per m3). Serious consideration should therefore be given tocollecting rainwater on site if specifying any type of green roof that willrequire watering.

Will drought be a problem?

Depending on the type of flora, stage of establishment and importance ofaesthetics, a drought of six weeks’ duration will normally result in arequirement for watering on all types of green roof, apart from biodiverseroofs. As different types of sedum are more resistant to certain climaticextremes, employing a variety of sedum species can reduce the risk of anextensive green roof without access to a water point being badly affectedduring a drought. It usually takes five to eight years before a green roof isestablished with a variety of plants, which will be able to cope with mostmicroclimates.

Can rainwater from green roofs be collected?

Storm-water runoff from a green roof is less than that from alternative roofdesigns and the runoff will carry fine sediment held in suspension. However,rainwater can be collected from a green roof and used to water either theroof itself or around the building to water other vegetation. If fertilisers areused on the roof then the runoff should not be used to keep ponds toppedup as high nutrient levels lead to uncontrolled algae growth. It is notrecommended to use rainwater collected from a green roof back in abuilding. The water is discoloured, cannot be used for clothes washing and isespecially noticeable in white toilet pans.

Is the sizing of downpipes affected when a green roof is specified?

A green roof results in a reduction in total runoff volume and also a reductionin peak flow. The reduction in peak flow depends on the intensity of therainfall event and the level of saturation of the substrate before thecommencement of the rain event. When the substrate is saturated the lagtime can shorten to that from a hard roof, but peak flow reduction can stilloccur. Lui and Minor(16) demonstrated a reduction of 10–30% during a15 mm rainfall event with previously saturated 100 mm deep substrate. Giventhe varying substrate materials and depth available in manufactured greenroof systems in the UK market, advice should be obtained from the supplierfor any allowable reduction in downpipe size.

17

Figure 4: Rainfall amount, annualaverage 1971–2000Sources: Met Office (Crown

copyright)

Figure 5: WC flushed withrainwater from a greenroof showingdiscolouration effect

What restrictions are there on the pitch of the roof?

Extensive green roofs can be laid on pitched roofs up to 40 degrees.However, the higher the pitch the greater the detailing required, the morecomplicated the design and installation, the higher the cost and the lesschoice of access for maintenance. Therefore, when specifying a green roof itis best to design a roof that is flat or with a shallow pitch. Even a flat roofmust have a minimum fall of 1:60.

What restrictions are there on the orientation of the roof?

The roof orientation must be taken into account but a green roof will growon any aspect. On small dual pitched roofs, it is possible to site solar thermalpanels on the south pitch and have the green roof on the north pitch. Onlarger roofs this is not necessary as solar panels and green roofs arecompatible. Any over-shading of green roofs (either by neighbouring buildingsor by PV or solar thermal panels) will result in a different microclimate thatwill encourage colonisation by different plant species compared to a sunnysite. There are some sedum varieties that are shade tolerant, and they can bespecified at the outset if required.

Can a green roof be specified when there will be lots of mechanical plant on the roof?

Green roofs are well suited to commercial buildings where there will oftenbe air conditioning plant, water storage etc on the roof. As access will berequired for the mechanical plant, access for maintenance of the green roofwill be simplified. Attention needs to be paid to detailing around themechanical plant and to access across the green roof.

What roof outlets are required?

There should be a minimum of two outlets per roof as a precaution againstblockage. All outlets should be protected by an inspection chamber andsurrounded by a pebble vegetation barrier to prevent plant encroachment.

Will a green roof reduce heating and cooling demand in the building?

Research in Germany(6) and the US(3) has shown that green roofs will reducethe amount of cooling required in a building quite significantly. A green roofwill not absorb as much solar energy as a conventional flat roof and so theroof structure will heat up much less. At the same time evapotranspirationfrom the vegetation will dissipate heat that is absorbed and the substrateitself will act as an insulating barrier.



Some research has shown that a green roof can also slightly reduce heatingdemand, but the amount of heat loss mainly depends on roof insulationthickness. Potential savings in heating demand achieved by a green roofdepends on varying factors such as location in the UK, amount of growth,whether watered or not, amount of water held in the substrate, and will varythroughout the heating season on a daily basis.

Are green roofs compatible with solar thermal and photovoltaic (PV) panels?

Yes, but on a small pitched roof, once panels are in place there is not a lot ofspace for a green roof and a lot of detailing will be required, often leading toan excessive cost for the roof. In such cases a good design principle toconsider is a green roof on the north-facing pitch and solar thermal orphotovoltaic panels on the south-facing pitch.

On large pitched roofs, and all sizes of flat roof, both solar thermal and PVpanels can be combined effectively with green roofs. Indeed, it has beenshown that the cooling effect of a green roof leads to slight performanceimprovements from a PV system mounted on A-frames, as the roof is coolerand the PV cells work at a higher efficiency. The panels themselves must notbe overshaded by the vegetation on the green roof; therefore A-frames arepreferable to roof integrated or roof mounted panels. It is important torealise that the area under any panels will be shaded from sun and will not benaturally watered. The effect will be to create a different microclimate andattract different (especially shade-loving) plants.

What effect will a green roof have on the life of the roof?

Green roofs can increase the lifespan of a roof by protecting waterproofmembranes from environmental damage. The membrane is protected fromUV radiation, mechanical damage and the high thermal temperature changesexperienced on roofs, leading to an extended life.

What is the cost of a green roof?

A green roof will cost more than a conventional roof. Costs depend on thetype of green roof specified and it is important to remember that the £/m2

cost of a small green roof will be greater than that of a large one. However,specifying green roofs can reduce the amount of final attenuation capacityrequired in a SUDS system, and the cooling effect of green roofs will reduceenergy use during the lifetime of the building. Other cost issues are coveredin more detail in sections 6 and 7.

19

Will a green roof have a guarantee?

If the roof is bought as a package from a reputable firm and fitted by theirown recommended contractors, guarantees against faulty construction andfailure of the waterproofing layers will be provided. Some green roof systemscome with a guarantee of up to 20 years to cover defective product orworkmanship. Sourcing and specifying the different layers of a green rooffrom different suppliers will obviously have a bearing on the types ofguarantee you will be able to secure for a green roof as a whole (making aninformed design stage critical to eventual performance and maintenance).

Which manufacturer should be specified?

There are many different green roof suppliers now in the UK. Choose amanufacturer who is established and has their own guaranteed contractors.Most green roof manufacturers will work closely with a designer to giveadvice on specification, design, loading and thermal calculations.

Will specifying a green roof affect insurance premiums?

There should be no added insurance cost when a building is designed with agreen roof as long as all design standards as stated above are met.

What are the environmental impacts of specifying a green roof?

Although there are many important environmental advantages of specifying agreen roof, (all of which have been highlighted) it is important to recognisethat most green roofs are highly engineered solutions, which, as with allengineered solutions, will result in certain negative environmental impacts. Careful consideration should be given to all components of the green roof,choosing companies who use materials with the best environmental criterion.It is recommended that BRE’s Green Guide to Specification(16) rating system, orsimilar, should be used to compare the environmental impact of differentgreen roof materials.



4 Constructing green roofs

It is important to consider some of the key issues encountered during theinstallation of a green roof. The matters raised below are by no means acomprehensive list of the challenges to be confronted during the installationphase but will help contractors and designers understand the issues faced atthis stage of the design and build process. Plan for problems specific to thesite and ensure adherence to the manufacturer’s installation instructions. Theuse of approved contractors to install a green roof is recommended.

Are there specific issues for storing green roof materials?

Even though a roof structure is designed to take the additional structural loadof a green roof once installation is complete, it may be unable to support, forinstance, a pile of gravel or dozens of sedum rolls stacked together in onesmall area. Materials should be stored at low level and, carried, craned orpumped up when required. In the case of a biodiverse roof using site waste,the spoil collected from site will have to be sorted and stored separatelyfrom final site waste.

It is of paramount importance that the storage and delivery of materials isconsidered carefully and with other site works in mind.

What implications does installing a green roof have on the rest of the buildprocess?

There are different types of time management required. It is recommendedthat the green roof is not laid during the summer months and therefore thisneeds to be factored into the construction plan. Spring or autumn are thebest times to install a green roof.

Whereas a prefabricated drainage mat layer and sedum rolls are relativelyquick and simple to lay (requiring only unrolling and trimming to size), adrainage layer of sand and gravel and a soil substrate will inevitably takelonger to transport to the roof and fit. Hydro-planted roofs are quick to layas the substrate and growing medium are pumped up to the roof.

An immature green roof may also have to be secured whilst being establishedand usually it will be highly detrimental to tread on it. In such cases, otherworks requiring roof access will have to be completed before the green roofis laid. If the roof is being made up of individual layers, then the substratelayer can provide some protection to the roof membrane during constructionworks.

21

Main issues at the construction stage

• Materials management

• Time management

• Access for construction

• Health and safety

How is it best to access the roof?

Given the weight, amount and types of different material that need to reachthe roof, a crane will be needed on site during the green roof installationstage. This will have implications both for cost and what other work canproceed safely while the crane is in operation.

What are the health and safety implications during construction?

The design of a green roof must strive to minimise the risks that thoseinvolved in its construction are exposed to and relevant health and safetylegislation must be adhered to on site. Questions include:

— Are edges protected where necessary to minimise accidents from falls? — Does the transporting of materials to the roof demand machinery? — Have precautions been taken to prevent anything from dropping off

the roof? — Have trip hazards been removed or blocked off? — Is safe access provided?



5 Maintaining green roofs

Biodiverse roofs are designed to be low-maintenance and requiremaintenance just once a year. Extensive sedum roofs will require a moreproactive maintenance regime, with some manufacturers recommendingweeding three times a year and feeding once a year. Simple intensive roofs(unless a biodiverse type) and intensive green roofs designed with aestheticappeal or the functionality of a roof garden, will necessarily dictate a moreproactive maintenance regime, as with most gardens.

Who is responsible for maintenance?

Reputable contractors will include, or at least suggest and offer, an agreedperiod of regular maintenance. This is especially important whilst the roof isestablishing. Market-leading extensive green roof system suppliers will havetheir own approved contractors who will monitor progress immediatelyfollowing fitting and after the first full growing season (usually between12–18 months) before finally signing over maintenance duties to the client.

What are the relevant health and safety implications during maintenance?

There is increasing concern about heath and safety on green roofs duringmaintenance work. Fall protection (of whatever type) must be provided onall green roofs as maintenance will be required within 2 m of the edge of theroof. It is important that fall protection systems are themselves maintainedonce a year. Green roofs on low-rise buildings are now being designed sothat the complete roof can be accessed from a cherry picker to reduce therisk of accidents.

When should maintenance be carried out?

Most extensive green roofs and some simple intensive green roofs requiremaintenance on a yearly basis, which is normally undertaken each spring. Roofsnear to deciduous trees also need maintaining in late autumn to clear leaves.

What to inspect for?

Any roof clutter or blockage of drains is of primary concern, as is unwantedforeign growth, which could be detrimental to the plant species originallyestablished, biodiversity aims and even the roof structure itself. (Sycamoretrees and Buddleia, for example, can establish themselves and need to beremoved while still immature.)

23

Are weeds a problem?

Sedums are not very competitive plants and after a warm and wet summerareas of weeds can appear on an extensive sedum green roof. The weeds canbe cleared. Bare areas can be left to naturally regenerate with sedum, orcuttings of sedum can be placed on the bare areas, covered with substrateand watered. After 3–4 weeks, the cutting will be established. For thismethod to be successful the work must be carried out during spring orautumn as climatic extremes such as frosts or hot sun will severely inhibit thegrowth of the cuttings. Another option is to allow weeds (which will bemostly grasses) to grow in place of the sedum, and for sedum to be seenmerely as a stepping stone to a more biodiverse roof.

Is irrigation necessary on a regular basis?

Access to a water point is essential during the establishment stage of mostgreen roofs. This is of further importance to larger green roofs and those thathave been designed with flora, which demands more regular irrigation. Afteran extensive sedum green roof is established, it should not require wateringunless there is a drought. As a period of drought could coincide with ahosepipe ban, it is recommended that rainwater harvesting should beincorporated if specifying a green roof in water stressed areas. During adrought the requirement for watering will result in increased maintenance.

Should fertilisers be used on a green roof?

Many installers in Germany undertake maintenance and inspections every sixmonths for the first two years whilst the green roof is establishing itself andthen let nature take its course, resulting in a roof that has less sedum and isdominated by grasses with some wild flowers. Not fertilising is recognised asthe preferred option as, although specialist slow-release fertilisers can help agreen roof to establish by promoting plant growth, the variety of species willbe reduced and their use will result in increased nutrient levels in storm-water runoff which will negatively affect local water quality.

Is plant growth on vegetation breaks a problem?

Vegetation breaks have an important safety function to prevent the spread offire. They must be maintained at their original width and cleared of anyencroaching plants.

Is it possible to damage the roof structure during maintenance?

It is important that anybody working on a green roof understands the build-up of the layered components to avoid damage. Tools must be carefullychosen so as not to interfere or damage anything below the substrate.


Case study: Potsdammer Platz

In Potsdammer Platz, Berlin, 30% of theroofs are greened. Building occupiers arerequired to sign a declaration that theywill not treat the green roofs withfertiliser. This is to prevent increasednutrient levels in the buffer lake whichprovides both amenity and storage forthe rainwater harvesting system.


Can the drainage points fail and flood the roof?

All drainage points must be checked every year and cleared out if necessaryto ensure optimum performance.

25

Figure 6: Checking drainage points

6 Retrofitting green roofs

Due to the sheer number of suitable existing buildings relative to the numberof new-build projects, retrofitting green roofs on a wider scale can vastlyamplify the positive environmental impacts cited in this publication. One ofthe main drivers is as part of a refurbishment programme. If an existing roofis in poor repair, the opportunity for retrofitting a green roof should be givenserious consideration; it can help meet Building Regulations, extend the life ofthe roof and have environmental benefits both inside and outside of thebuilding. The fact that refurbishment is happening anyway may also makelocal councils and planning authorities more receptive to the idea of fitting agreen roof. Once the decision is taken to retrofit a green roof, almost all ofthe design, construction and maintenance issues for green roofs on new buildwill be relevant.

What are the structural implications?

If major structural work is required, the extra cost and labour involved candiscount the possibility of retrofitting a green roof. However, if an existingroof and its structure can support the extra weight of a green roof(remember it is vital to have a structural engineer assess this), there isnothing else, aside from finance, inhibiting retrofitting. Where structuralstrength is concerned, potential retrofits should be approached as individualcases and the merits and drawbacks of particular circumstances assessed andtaken into account. For example, with inverted roof structures the materialsused as ballast for the insulation such as paving slabs, stone pebbles or gravelare removed and the mass is replaced by the green roof. With planning thiscan mean no net change in structural load. (Refer to section 3 for in-depthstructural information.)

Are there specific cost implications for retrofitting a green roof?

Once the decision has been taken to upgrade a roof, either to improve itsthermal properties or because the lifespan of the waterproof membrane haspassed, the costs of upgrading the specification by adding a green roof arereduced. Equipment, machinery and labour are already on site. Some of theother costs, time and materials involved may also be offset during retrofitting;for example, as mentioned above, a green roof can remove the need forballast to hold down insulation, leading to an offset in costs.

Which buildings are suitable for retrofitting?

The retrofitting of green roofs has been successful in various situations. Thereare examples of retrofitting in circumstances as diverse as inverted concreteroofs (very common in the UK and therefore potentially a widespread


Main issues for retrofittinggreen roofs

• Structural implications

• Cost implications

• Suitable buildings

• Health and safety

Structural concerns may beoveremphasised

A survey of building professionals inLondon showed that while 92% ofdevelopers agreed that ‘the physicalstructure of many buildings prevents theestablishment of green roofs’, only 27%of structural engineers questioned heldthe same opinion.

Source: Frith and Gedge(13)

Case study: retrofitting a green roofto a timber roof

The Ethelred Housing Estate, London,had an existing roof of woodwooldecking slabs sheltered by a sheathingmembrane under age-brittle asphalt.

This construction is supported by timberjoists, which were shown to be strongenough to take the additional load of anextensive green roof.


application for retrofitting) and relatively lightweight timber joist roofs.Structurally, many office buildings are suitable for the retrofitting of greenroofs. Given their prominence in urban areas, large scale retrofitting of greenroofs to suitable commercial buildings would have a large impact onmitigating the urban heat island effect.

Hospitals are another promising application for the retrofitting of green roofsparticularly if they have varying height blocks, which allow patients a viewover them. There are successful examples in Switzerland where hospitalroofs have been greened.

The diversity of the UK’s school building stock precludes any generalisationover their suitability for retrofitting and the lack of research in this areameans that preliminary structural assessments are necessary on an individualbasis. Many schools in the 1960s and 1970s were typically lightweight, pre-fabricated, flat roofed buildings and the option of retrofitting extensive greenroofs in such cases should be considered under the ‘Building Schools for aFuture’ programme (http://www.bsf.gov.uk).

The potential for blocks of flats, particularly high-rise estates, is also vast,despite potentially trickier planning and consultation processes. Fitting greenroofs at high levels will improve the views of tenants in properties thatoverlook these roofs and can be carried out as part of improvements to thethermal properties of the blocks. The requirement for irrigation during theestablishment phase needs to be considered when retrofitting green roofs tohigh rise flats and a water supply (whether mains water or rainwater willneed to be supplied).

Are there buildings where retrofitting green roofs will be difficult?

The lightweight structure of standard, single storey industrial and commercialwarehouses usually render them unsuitable to bear the additional load ofeven an extensive green roof (once saturated) without major structuralupgrading. Though incompatible with the retrofitting of green roofs, thesizeable roof-space and numbers of such units can still have a positive impacton runoff if other forms of SUDS are employed.

Different considerations apply to housing where a tiled roof is designed tosupport 150 kg/m2. Extensive green roofs with thin substrate layers up to50 mm depth, weigh less than this even when saturated. However, the lessthe substrate depth, the more likely are the plants to require watering duringlong hot dry periods and the pitch on tiled roofs also requires more extensivedetailing. These factors combined with the extra cost for green roofs installedon pitches over 10% and the lack of economies of scale on small roofs meansthat retrofitting on individual housing is unlikely to occur in any great numbers.

27

Case study: London Boroughof Camden

During the summer of 2006, the LondonBorough of Camden retrofitted greenroofs to three high-rise blocks in thenorth of the borough. Despite the factthat the roofs were installed during aheatwave and there was limited accessfor irrigation, they had successfullyestablished by the following year.

Extensive sedum, ten months afterplanting

Extensive plug planted, ten months afterplanting

Are there health and safety implications specific to retrofitting green roofs?

There are many existing roofs with low parapet walls, which are ideal forretrofitting green roofs as part of a roof refurbishment. However safetyfeatures will not conform to current CDM Regulations(15). As maintenance willbe required within 2 m of the edge of the roof, a safety rail or fall protection(fall restraint rather than fall arrest) system must be retrofitted to the roof. Retrofitting green roofs may also have unintended consequences. An unusualhealth and safety issue was encountered by the London Borough of Camdenafter they had retrofitted a green roof and it became a magnet for some ofthe estate youth to congregate there; the conventional roof never had suchattractions. The problem was solved by increasing security to the internalroof access points.



7 Costs and financial considerations

The build costs when specifying a green roof are undoubtedly greater than aconventional asphalt finish or inverted roof finish. Indicative costs per m2 ofgreen roof are detailed in Table 10 for various roof types. It is important torecognise that these are guideline prices only and £/m2 costs will be higheron smaller roofs due to the extra detailing that is required and absence ofeconomies of scale. The table does not include installation costs and thesewill have to be ascertained to understand the total cost of the roof design asspecified. For example, a sedum mat has to be pre-grown and thereforeincurs greater production costs than other types of extensive green roof butits installation costs are generally less. Installation costs vary widely and arehigher in the UK than in Germany where the green roof market is verycompetitive and there is a high level of experience among the Germanconstruction industry in installing green roofs. Installation cost will fall in theUK as more contractors become experienced in installing green roofs.

29

Type of greenroof finish

Costper m2

Installation issues Maintenance costs

75 mm substratesedum plug roof

£25 Slower installation asplants installed separately

Quite high over first twoyears while systems bed in

75 mm substratesedum hydroseeded

£20–25 Fast installation; can taketwo years for plants tomature

Quite high over first twoyears while systems bed in

75 mm natural /biodiversity

£20–25 Fast installation; can takemany years for plants toestablish

Low

20–30 mmsedummat/blanket

£40–50 Fast installation;immediate green roofeffect

Low–medium

It is possible to offset some of the costs of a green roof at the design andspecification stage. For example, an inverted roof system requires ballast,either shingle or paving slabs, which can be replaced by a 75 mm substratebased green roof. Ballast paving costs in the region of £10–17 per m2. If asubstrate based biodiverse roof at a cost of £20–25 per m2 replaces paving at £17 per m2, then the additional cost of a green roof is reduced to£3–8 per m2. As green roofs are considered as a source control componentof SUDS, savings can be made further down the SUDS management trainwhen a green roof is specified. These savings can be significant if applied tomany roofs across large developments, especially as most downstream SUDSsolutions such as swales, detention basins and balancing ponds require landgrab, whereas green roofs do not.

Table 10: Average costs for varioustypes of green roofs

It is well established that operational costs of a building with a green roof willbe less in terms of the amount of cooling required. As the UK climate heatsup and the cost of electricity rises, these savings will increase. In Germany, itis estimated that the cost to install a green roof and maintain it over 40 yearsis about 43 (£29) per m2, compared to a possible saving of 70 (£48) per m2

from the reduced maintenance, energy saving, city water fee (saving fromstorm-water runoff) and increased life(18).



8 Legislation, regulations, and planning policy

Recognition of the relevance of the built environment to supportingbiodiversity, although still narrow, is certainly increasing. For example, theBiodiversity Strategy for England(19), declares the need to enhance theinclusion of biodiversity aspects into green buildings and explicitly referencesthe ‘green roofs for black redstarts’ work in Deptford, London, as a case inpoint.

In the absence of specific policy requiring green roofs to be installed, thereare a variety of different legislative instructions that promote theincorporation of green roofs into new developments. They are set out inTable 11 and the major points highlighted. Readers are referred to thedocuments themselves for greater detail. The key drivers are therequirement for SUDS in many new developments (as green roofs are onemethod of source control within a SUDS strategy) and the recognition thatgreen roofs promote and enhance biodiversity.

31

Legislation Key aspects relating to green roofs

PPS 25(20) (England) SPP 7(21) (Scotland)TAN 15(22) (Wales)PPS 15(23) (N I)

Legislation to assess flood risk. They all require the planningsystem to further SUDS by encouraging developers to adoptSUDS, wherever practicable, if necessary through the use ofappropriate planning conditions or by planning agreements. Greenroofs are recognised as source control for SUDS.

PPS 1(24) (Englandand Wales)

General references to protecting wildlife habitats and to enhanceas well as protect biodiversity.

PPS 9(25) (Englandand Wales)

Specific references to improving biodiversity. Most likely legislationunder which authorities will promote the installation of green roofs.

Area DevelopmentFrameworks

Used by planning authorities to set supplementary planningguidance. Often refer to biodiversity aims.

SupplementaryPlanning Guidance

A number of boroughs have potentially relevant SupplementaryPlanning Guidance on sustainable buildings

Water FrameworkDirective(26)

SUDS are promoted as the preferred drainage option. Greenroofs recognised as source control for SUDS.

Local BiodiversityAction Plans

Green roofs may be of benefit to any local species identified asrare if its habitat is approximated in the design.

BREEAM(27) A BREEAM (BRE Environmental Assessment Method) rating canbe improved if specifying a green roof on a building.

Code forSustainableHomes(28)

The Code sees SUDS as a core part of a sustainabledevelopment. Green roofs are recognised as a source control forSUDS.

BuildingRegulations

No direct reference to green roofs but thermal requirements forfinished roof structure must be met.

Wildlife andCountryside Act(29)

The Act states that in a new build situation where natural wildlifehabitat is being displaced a green roof can mitigate the loss.

Table 11: Legislation pertinent togreen roofs

References

1 Guidelines for the Planning, Execution and Upkeep of Green-Roof Sites (English version) (Bonn,Germany: Forschüngsgesellschaft Landschaftsentwicklung Landschaftbau e.V.) (2002) (ISBN 3-934484-59-x)

2 Grant G, Engleback L and Nicholson B Green Roofs: their existing status and potential for conservingbiodiversity in urban areas English Nature Report No. 498 (Peterborough: English Nature) (2003)(http://naturalengland.twoten.com/naturalenglandshop/docs/R498.pdf) (accessed 15/08/07)

3 About green roofs (website) (Philadelphia, PA: Penn State Center for Green Roof Research) (2006)(http://hortweb.cas.psu.edu/research/greenroofcenter/history.html) (accessed 15/08/07)

4 Grant G, Engleback L and Nicholson B Green roofs: their existing status and potential for conservingbiodiversity in urban areas English Nature Report No. 498 (Peterborough: English Nature) (2003)(http://naturalengland.twoten.com/naturalenglandshop/docs/R498.pdf) (accessed 15/08/07)

5 Building greener (London: CIRIA) (to be published)

6 Schmidt M ‘The evapotranspiration of greened roofs and facades’ Proc. Fourth Annual InternationalGreening Rooftops for Sustainable Communities Conference, Boston, May 11–12, 2006 (http://www.greenroofs.org/boston/) (accessed 15/08/07)

7 Heat island effect (website) (Washington DC: US Environmental Protection Agency)(http://www.epa.gov/heatislands/about/index.html) (accessed 15/08/07)

8 Wigley T ‘The climate change commitment’ Science 307 (5716) 1766–1769 (18 March 2005)

9 Climate change scenarios for the United Kingdom: The UKCIP02 Scientific Report (Norwich:TyndallCentre for Climate Change Research, University of East Anglia) (2002) (http://www.ukcip.org.uk/scenarios/ukcip02/documentation/ukcip02_scientific_report.asp) (accessed 15/08/07)

10 2006 was bad summer for ozone (webpage) (Croydon: Environmental Data Interactive Exchange ) (21March 2007) (http://www.edie.net/news/news_story.asp?id=12794&channel=0#) (accessed15/08/07)

11 Frith M and Gedge D Structural issues (webpage) (livingroofs.org) (2005) (http://www.livingroofs.org/livingpages/barstructural.html) (accessed 15/08/07)

12 SUDS design manual CIRIA Report C697 (London: Construction Research and InformationAssociation) (2006)

13 Frith M and Gedge D Storm water amelioration (webpage) (livingroofs.org) (2005) (http://www.livingroofs.org/livingpages/benwaterunoff.html)

14 Kolb W and Schwarz T Dachbegrunung, intensiv und extensiv (Stuttgart, Germany: Ulmer) (1999)

15 The Construction (Design and Management) Regulations 1994 Statutory instruments 1994 No. 3140(London: Her Majesty’s Stationery Office) (1995)

16 Liu K and Minor J Performance evaluation of an extensive green roof (Ottawa, Canada: Institute forResearch in Construction) (2005) (http://irc.nrc-cnrc.gc.ca/pubs/fulltext/nrcc48204/nrcc48204.pdf)(accessed 15/08/07)

17 Anderson J, Shiers D E and Sinclair M The Green Guide to Specification (Oxford: Blackwell Science)(2002)

18 Herman R ‘Green Roofs in Germany: Yesterday, Today and Tomorrow’ Proc. First North AmericanGreen Roof Conference — Greening Rooftops for Sustainable Communities, Chicago, May 29–30, 200341–45 (2003)

19 Working with the grain of nature; a biodiversity strategy for England (London: Department forEnvironment, Food and Rural Affairs) (2002) (http://www.defra.gov.uk/wildlife-countryside/biodiversity/biostrat/biostrategy1to4.pdf) (accessed 15/08/07)

20 Development and flood risk Planning Policy Statement PPS25 (London: The Stationery Office) (2006)(http://www.communities.gov.uk/pub/955/PlanningPolicyStatement25DevelopmentandFloodRisk_id1504955.pdf) (accessed 15/08/07)

21 Planning and flooding Scottish Planning Policy SPP7 (Edinburgh: Scottish Executive DevelopmentDepartment) (2004) (http://www.scotland.gov.uk/Resource/Doc/47210/0026394.pdf) (accessed15/08/07)

22 Development and flood risk Technical Advice Note TAN15 (Cardiff: National Assembly for Wales)(2004) (http://new.wales.gov.uk/docrepos/40382/4038231121/403821/403821/40382/403821/july04-tan15-e.pdf?lang=en) (accessed 15/08/07)



23 Planning and flood risk Planning Policy Statement PPS15 (Belfast: Department of the Environment forNorthern Ireland) (2006) (http://www.planningni.gov.uk/AreaPlans_Policy/PPS/pps15/PPS15.pdf)(accessed 15/08/07)

24 Delivering sustainable development Planning Policy Statement PPS1 (London: Department ofCommunities and Local Government) (2005) (http://www.communities.gov.uk/pub/806/PlanningPolicyStatement1DeliveringSustainableDevelopment_id1143806.pdf) (accessed 15/08/07)

25 Biodiversity and geological conservation Planning Policy Statement PPS9 (London: Department ofCommunities and Local Government) (2005) (http://www.communities.gov.uk/pub/833/PlanningPolicyStatement9BiodiversityandGeologicalConservation_id1143833.pdf) (accessed 15/08/07)

26 Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000establishing a framework for Community action in the field of water policy Official J. of the EuropeanCommunities L 327/1 (22.12.2000) (http://europa.eu.int/eur-lex/pri/en/oj/dat/2000/l_327/l_32720001222en00010072.pdf) (accessed 15/08/07)

27 BREEAM: BRE Environmental Assessment Method (website) (Garston: Building ResearchEstablishment) (http://www.breeam.org) (accessed 15/08/07)

28 Code for sustainable homes — a step change in sustainable home building practice (London:Communities and Local Government) (2006) (http://www.planningportal.gov.uk/uploads/code_for_sust_homes.pdf) (accessed 15/08/07)

29 Wildlife and Countryside Act, 1981: Elizabeth II, 1981 (London: Her majesty’s Stationery Office)(1981)

Further reading

Anderson/Monarflex/Icopal Green roof systems (Manchester: Anderson Monarflex Ltd.) (2006)(www.icopal.co.uk)

Bamfield B Whole life costs and living roofs — the Springboard Centre, Bridgewater (Norwich: SarnafilLtd.) (2005) (http://www.livingroofs.org/NewFiles/Living%20roof%20Bridgewater%20003.pdf)(accessed 15/08/07)

Bauder Technical manual (Suffolk: Bauder Ltd.) (2005) (www.bauder.co.uk)

Gedge D, Dunnet N, Grant G and Jones R Living roofs (Peterborough: Natural England)(2006)(http://www.english-nature.org.uk/pubs/publication/PDF/LivingRoofs.pdf) (accessed 15/08/07)

Grant G Green roofs and façades (Garston: BRE Press) (2006)

London’s urban heat island: A summary for decision makers (London: Greater London Authority) (2006)(http://www.london.gov.uk/mayor/environment/climate-change/docs/UHI_summary_report.pdf)(accessed 15/08/07)

Liu K and Baskaran B Thermal performance of green roofs through field elevation (Ottawa, Canada:Institute for Research in Construction) (2003) (http://irc.nrccnrc.gc.ca/pubs/fulltext/nrcc46412/nrcc46412.pdf) (accessed 15/08/07)

Munby B Feasibility study for the retrofitting of green roofs M. Eng Thesis Final Report CIV405 (Sheffield:The University of Sheffield, Department of Civil and Structural Engineering) (2005) (http://www.livingroofs.org/NewFiles/retrofittingofgreenroofs.pdf) (accessed 15/08/07)

Philippi P M ‘How to get cost reduction in green roof construction’ Proc. Fourth Annual GreeningRooftops for Sustainable Communities Conference, Awards and Trade Show, Boston, May 11–12, 2006(2006) (http://www.greenroofservice.com/downpdf/Boston%20Paper.pdf) (accessed 15/08/07)

33

cibse ks11 - green roofs

Documents

green roofs215

retrofitting green roofs

overview of green roofs

green roofs relevant

types of green roof

building servicesks03

pythe institution

use asaforesaid