What is a Green Roof or Living Roof - History and Background

Essentially a living roof forms a weatherproof covering with various layers supporting a range of plant cover. For some while now central Europe has led a resurgence in the installation of living roofs driven predominantly by the need to adapt to changes in climate and environment.

                                                 Types of green roof / living roof and application

There are many forms of living roof but for ease of explanation they tend to fall into 2 main categories extensive and intensive. A description of what that actually means can be found below with the addition of the bio-diverse roof which is fast becoming the covering of choice for many developments. These categories are fine for quick and simple reference but many feel that they are restrictive as its common practice to combine two or more types on a single installation.

                   Example of a Sky Garden Extensive Green Roofing System to a domestic Flat Roof

Extensive – an extensive living roof is not normally intended for human use and is unquestionably the most popular form. It’s relatively cheap to install, has lower weight loadings and requires minimal maintenance. Usually the plants are set in a light-weight growing medium or substrate of a depth of between 20mm and 150mm. The plants themselves tend to be highly resistant to drought, ground hugging species such as sedum.

Intensive – intensive living roofs are very much intended for human use and as such take on a more ‘landscaped’ feel with deeper substrates (at least 150mm up to a metre and beyond) catering for a wide variety of grasses, shrubs and trees. Maintenance is both essential and regular as lawns need mowing and shrubs pruning – an irrigation system should also be installed.

Bio Diverse/Semi Extensive – the principles behind these types of roof are driven by ecological concerns. Pioneered by the Swiss, the bio-diverse roof is steadily becoming a more common feature as policy makers seek to replace lost habitats for local flora and fauna. Sometimes known as ‘eco’ or ‘brown’ roofs they do not always cater for everyone’s tastes.

  Example of how you can even create a beautiful green roof on a pitched roof courtesy of Sky Garden

Often site debris such as crushed brick and concrete is used as a growing medium which is applied in a more landscaped way with variations of depth enabling mounds and craters to be formed. This allows a greater variety of mostly indigenous/local species to colonise.

The installation of logs, small rocks and sanded areas encourage invertebrates, birds and even lizards to use the roof. Initially the roof will look no better than a rough, stony, weed-infested area but in time will develop; constantly changing as different species come to the fore or take up residence.

During the growing and flowering seasons a mature roof can resemble something akin to an English meadow. Many people think they look messy and unkempt but in fact these roofs do more for the environment than sedum-based and intensive roofs.

The National Osteoporosis Centre Gets an Eco Friendly Green Roof

Protection Fleece and Resevoir Board	Laying the Substrate

                      Laying the Seedum Blanket                                            Finished Extensive Seedum Roof

                                                                  Why a Living Roof? What are the Benefits?

Consider this if you feel that your efforts won’t impact - figures by research institutes show that there is some 350sq/km of global deforestation per day and up to 120sq/km of global urban development every day!

To be able to re-green impermeable surfaces such as roofs gives us the ability to readjust the imbalance – plants store carbon, therefore, if we have more plants we can store more carbon. Living roofs won’t save the world but they serve as a fundamental tool with which to address the problem.

Much as the environmental arguments are compelling there are also very obvious benefits of creating a pleasant environment in which to live – something colourful perhaps, ever-changing and ultimately inspiring.

                                                                                  Features and Benefits

• Utility & Value – using the rooftop as an amenity where people can work, relax or socialise is a considerable benefit in these days where land and space is at a premium. The extra living space adds value to the property.

• Reduction of Urban ‘Heat Island’ – It has been proved that where there is urban sprawl there is an increase in temperature caused by impervious surfaces. As temperatures rise, air quality is compromised. Living roofs cool and steady the temperature and act as a humidity control thereby improving air quality.

• Storm Water Management – one of the key characteristics of living roofs is their ability to retain significant quantities of water and to release it through slower run off and evapotranspiration (a combination of evaporation and use by the vegetation).

This means less pressure is exerted on the sewer system thereby reducing flash flood risk. It may even be a viable option to use a living roof as an attenuation system rather than constructing subterranean tanks.

• Filtration of Dust & Pollutants – the vegetation absorbs carbon and stores it. This is known as carbon sequestration. Research on extensive roofs in America shows that more than 500 grams of carbon per square metre can be stored in this way. It is well documented that the vegetation also filters out other airborne dusts and pollutants. These processes improve the quality of the air we breathe.

• Replacement Habitats – bio-diverse roofs provide habitats lost through development.

• Aid to Planning Consent – local authority policies are increasingly favorable towards the installation of living roofs.

• Reduction in Power Demand – the insulation capacity of a living roof is problematic as a fully saturated roof will act as a cold bridge. Nevertheless the roof is not always at full saturation point so over the course of a year it has been proven to effect the power demands of a building - less demand for heating in winter and cooling in summer.

• Improved Aesthetics – [show picture of a flat roof next to picture of green roof] Text – which roof do you prefer?

• Recycle Materials – The system we use makes use of recycled plastics within build up, and wherever possible we use site debris as a substrate.

                                                                Choosing a Green Roof / Living Roof Considerations

• Financial - cost, of course, is a fundamental factor when making decisions but it shouldn’t be the driving force. Examine what the motivation is for installing a living roof – most green roofs will, to a greater or lesser extent, tick all the boxes above but to personalise your own its worth mulling over the following considerations:

• Weight – weight loadings are vital to calculate. On a new build this will be sorted by the architect but with a refurbishment it may be necessary to call in the services of a structural engineer as supports may require strengthening. Weights are calculated as ‘saturated’ ie when the roof is totally sodden with water. As a guide a simple extensive roof will command a saturated weight of around 100kgs/sq/m.

• Wildlife & aesthetics – do you want a picturesque garden with many flowers and exotic species? Or a more rustic scrub/meadow style that will provide a genuine habitat for local fauna? Or a combination of the two. Or a pitch and putt? Or a vegetable patch? All these and more are possible. You are only limited by your own imagination...

• Cheap, light-weight systems – be aware that there are systems on the market that are light weight and as such are quite cost effective to install. There are concerns with this type of system as they are nowhere near as effective. Fundamental layers in the build up are often removed or drastically reduced, weakening the system as a whole. These roofs will require more maintenance and are at more risk of dying off during prolonged periods of drought. Whether your motivation for a living roof is ecological, commercial or purely aesthetic it is a better option, long-term, to consider a more well balanced design.

• Maintenance – the maintenance requirements of a living roof centre around watering, feeding and weeding. Generally, extensive and bio-diverse roofs require considerably less than an intensive one. However, it really depends on what your vision of the roof is.

The first 12 months of a living roof’s life is the most crucial whilst the root systems establish so there would be a basic requirement to feed during growing periods and to water during prolonged dry spells. After that the main requirement will be for weeding out any invasive or unwanted plants which may spoil the balance of vegetation.

 

If aesthetics are not a consideration then roofs can be left pretty much to their own devices. With intensive roofs it is unquestionably necessary to have some sort of continued maintenance programme as lawns will need mowing and shrubs pruning and it may be necessary to enlist the services of a professional.

• Guidelines – how do you ensure that you have a good design? Most quality green roof suppliers and installers adhere to the German FLL (Forschungsgesellschft Landschaftsentwicklung Landschaftsbau e. V) guidelines as they are the only ones of any merit currently.

FLL is an independent research organisation set up by the German Government to develop a set of guidelines on all aspects of installing and maintaining green roofs. A copy of this volume is available in English from http://www.f-l-l.de/

                                                                              Installation Build Up

• Waterproofing - ensuring that you have a sound waterproof membrane as a base is essential. Some waterproofing systems are not root resistant so a root barrier will be required. To decrease the likelihood of there being a breach some sort of leak test is also recommended.

• Protection Fleece – much like a carpet underlay this layer protects the membrane form any abrasives that may work their way through the system.

• Drainage/Reservoir Board – this preformed plastic layer mimics the profile of egg cartons and is used to drain and store water. Coming in a range of depths, dependant on the type of roof garden to be installed.

• Filter Fleece – much thinner than the protection fleece, this one is laid between the drainage board and the growing medium (substrate) to prevent clogging.

• Substrate – a growing medium can take many forms from expanded shale and pumice aggregate to on-site debris such as crushed brick and concrete. It is vital to achieve the corrects depths for the plants above – thinner substrates require more maintenance, store less water, cater for fewer species and can have reduced survival rates. If using the pre-grown mats you should work to a minimum of 40mm depth as the mat itself usually has about 20mm of substrate. If plug planting or seeding its recommended that a depth of 50mm will suffice. Remember that greater depths of substrate mean greater benefits all round.

• Vegetation – when designing a green roof it is best to start with the top layer of vegetation. Consider what you want the roof to look like once it has reached maturity. The range of species available for use on green roofs is almost exhaustive but you must remember that you are generating an environment inwhich you expect a living thing to flourish, therefore, substrate type and depth must be considered along with fertilising regimes and irrigation to ensure plant survival. The vegetation itself can be installed in a number of ways:


Matting – what is commonly known as a sedum mat (much like turf in appearance) is laid over the substrate and watered in. This is quite a popular method in the UK as it gives the roof an instant ‘greening.’

Plug-planting – this method requires the installer to plant around 10 single plants per square metre. The coverage is not 100% initially and it will take around 12 months for a full establishment of vegetation. The advantage of this particular method is that you have a great deal more control over species selection and aesthetic design.

Hydro-seeding – this is a popular method on the continent as it’s quick and very cheap. After the substrate has been laid, shoots and seeds are broadcast over the relevant areas and a mulch of fibres, fertiliser and water is hosed over the roof from a tanker lorry below. Over a period of 12-18 months the shoots will develop and provide coverage.

                                                                               The Diadem System

At County Flat Roofing we use the Diadem green roof / living roof system as they provide a wide range of products and options for almost every eventuality. Quality and durability are hallmarks of the brand which use recycled materials wherever possible. Diadem is fast becoming an influential brand in central Europe, having originated in Hungary, and have a portfolio of prestigious projects across the globe. Growing mediums and vegetation are normally sourced from UK companies. Should you require any further information or would like to request a brochure please call us on 0800 098 8271 or contact us online.

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