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Green Roofs


RJ Evans are one of the UK's leading roofing companies. We are able to provide a full range of services to install green roofs. Our highly-experienced team covers all of London and the South of England, and are fully accredited roofing specialists. If you would like a quotation or any information on green roof systems please get in touch with us or call us now on 01277 375 511.


With the recent awareness of climate change, it has become apparent that the earth is heating up. We need to address this or face dire consequences. Building materials used (recyclability), insulation to cut the requirement of heating/cooling a building artificially as much as possible, conservation of wildlife and water preservation and reuse are key considerations in modern building design. An all-in-one solution incorporating all the above can be found in green roofs. Increasingly they have become a major part of any building scheme under construction. The reasons, why’s and how’s are explained below.

History of Green Roofs

The first use of green roofs can be traced back to the middle East to 500BC. The Vikings used them to insulate their buildings and they were used in the Middle Ages to aid the waterproofing of village buildings. In New World America houses were often constructed by using cut bricks of sod with the root sides facing upwards. Modern green roofs first started being incorporated into building design in Germany at the start of the 20th Century, culminating in over 13 million square Livia of green roofing in the same country today. Green roofs are the future and need to be embraced.

What Are The Benefits of Green Roofs?

Green roofs and the importance of them cannot be stressed enough. The world is undergoing a dramatic climate change, which is going to challenge building design considerations in the decades to come. Green roofs have many advantages. They offer wildlife sanctuary in built up areas. Increase, and maximise building amenities which allows roof areas previously redundant to be used as places of relaxation. An example of this would be roof gardens for urban residential properties.

Green roofs improve the air quality in the surrounding areas. Due to design of green roofs they have the ability to manage water run off. This results in a green roof reducing the stress on drainage when heavy rainfall occurs. Consequently, this reduces the risk flash flooding. Green roofs conserve and recycle water for essential building services such as bathroom facilities. They offer visual stimulation within city areas via increased green space.

Green roofs also help buildings maintain a steady temperature. This results in cooler temperatures in the summer and warmer ones in the winter. This leads to energy savings, from heating and air conditioning needs being vastly reduced. Overall, green roofs have many advantages both with regards to functionality and reducing a buildings carbon footprint.

Protects waterproof membrane and improves the environment.

A green roof reduces the carbon footprint by converting carbon dioxide into oxygen. In effect filtering out the bad air produced by pollution and putting back clean air by return.

Amenity gains such as roof gardens maximising roof space

Many roofs, especially in built up areas and tall buildings can be turned into roof gardens for residents and staff members. Offering a place to relax during the day. Normally this involves an intensive green roof build up and loadings are a critical consideration.

Insulation performance

The key benefit regarding insulation in green roofs is that they keep a building cooler in the summer and warmer in the winter. Reducing the need for air conditioning or heating to be used. This helps reduce the buildings carbon footprint and saves significant money as temperatures are controlled more naturally.

Retains water

The substrate and drainage pots on a typically designed green roof maintain water and therefore reduce water run-off. This alleviates strain on the surrounding drainage systems. It also ensures that the roof can be naturally maintained as far as plant watering is concerned. Sedum plants naturally retain water as a species and maintenance is kept to a minimum consequently.

Reduction in temperature extremes

Due to the build up of a green roof, they serve as a natural regulator of extreme temperatures. A substrate of 160mm for instance will drastically improve the insulation of a building. Keeping it cooler in the summer and warmer in the winter.

Protects the building from ultra-violet light

Most roof coverings, asphalt, single ply and felt are subject to UV degradation and ultimately the UV breaks these coverings down. This means a recovering is required. Green roofs totally cut out UV light and therefore greatly increase the life span of the waterproofing roof covering.

Reduces risk of damage to roofing membrane

The substrate build-up and design of a green roof acts as a natural barrier to roof damage. It offers at least 80mm of protection, making it difficult to penetrate down into the roof covering area

Increase the life expectancy of the roofing substrate

The main enemy of roof coverings is UV attack. A green roof completely cuts out UV impact on the roof covering which greatly increases the expected life expectancy and integrity of a roof.

Urban Heat Island Effect

When you look at many built up areas in the summer months from a distance, you often notice a ‘haze’ surrounding it. The areas often seem clouded in a shroud of mist. This is called the ‘heat island’ effect. Urban heat islands are caused when the air coming out of the buildings is hotter than the area surrounding it. This can be upwards of 5.6 C (10F) warmer. It explains why inner cities are often hotter in summer than immediate surrounding country areas. Green roofs actually help cool the atmosphere. Green roofs reduce dust and smog levels. They greatly assist in improving the microclimate of any inner city where they act as a humidifier. Carbon dioxide is reduced, and more oxygen is generated. The ‘green footprint’ produced provides a natural wildlife habitat, enabling birds and insect life to especially prosper. Basically, green roofs replicate or put back the natural footprint of the building. Currently in the UK alone there is over 200 million m2 roof area that would benefit from a green roof without having to change the buildings’ structure. The opportunity to improve the working city climate depends on the willingness to embrace the change required to achieve it. The benefits of putting these redundant barren roofs to good use include:

• Increased green space in urban areas for recreation

• More enjoyable areas to reside or work due to increased green areas ( for example, roof top gardens)

• Consent for planning made much easier as the negative impact on the area is drastically reduced.

• Better environments mean that property values increase due to area being a more pleasant place to live and work.

The benefits that greater insulation from having a green roof are many. The main ones being:

The deeper the substrate depth the better thermal insulation values. This cuts down on additional fuel bills in the colder months and lower electrical bills due to cooler buildings in the warmer months. Also, noise from traffic is greatly reduced, especially in areas near airports and busy motorways.

The research paper "Cooling the cities – A review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments" (x) has found green roofs drastically reduce and control the heat leaving the building. They offer a natural insulation blanket onto the area (the ceiling) where most heat emits a building. From a short distance away, you will often see on a hot day a blurry mist emitting form buildings and built-up areas. This is due to the increase in heat produced by buildings in relation to the surrounding temperature. The more efficient the insulation, the greater this heat release is reduced. It is why cities are always the hottest areas during warm periods of weather. Cities are hotter than surrounding areas consequently.

Green Roof Drainage

Water is becoming an increasingly valuable commodity. Having a green roof reduces the requirement for drainage of excess water as between 70% and 90% of rainwater is retained in the substrate. The moisture in the substrate evaporates slowly and this reduces the drainage capacity
required on the building. It also reduces the risk of flooding of the surrounding areas due to drainage and sewer system overload.  Most city road and building materials are designed to not support water, concrete, asphalt etc and all rainfall is directly running off into the overworked sewerage and drainage systems. These often can’t cope with hard rainfall and flash flooding is often the direct consequence.

Typically, maximum direct water run off following heavy rainfall on a gravel roof is up to 90%. A green roof reduces this to 25%. If you multiply this by multiple buildings in city areas the support this offers to the drainage infrastructure is obvious.

What Is The Best System For A Green Roof?

This totally depends on the functionality or intention of the roof and the loadings the building has been designed to withstand. You could not install a intensive green roof onto a timber deck for example as the weight loading would normally be too high. So there are a number of green roof options to consider.

Green Roof Types

Extensive Green Roofs

The actual definition of an extensive green roof is ‘‘The cultivation of plants which are wind, frost, & drought resistant, which require very little maintenance and are self-propagating. The bulk of this type of vegetation will consist of mosses, succulents (sedums), herbaceous plants and grass’. This consists in the main of sedum and herb type plants and the substrate depth is 60mm to 70mm. Extensive green roofs tend to deliver brightly coloured flowers and vegetation and range from 5 to 150mm in height. Extensive green roofs are the most popular and fastest establishing types of green roofs. It is quite usual that certain plant species will dominate the field area after a given amount of time. If rockery type sedum species and wild flowering plants are used, then a minimum substrate depth of 80mm is required. These plant types provide ground cover with plants heights up to 300-400 mm. Micro sedum plant segments can also be considered as an option. These can be pumped onto the roof and prove to be a very cost-effective option on larger areas. In this situation you will always find that certain plant species will prosper and dominate the field area.

The importance of substrate depth and quality is very important. Be it for an extensive green roof or an intensive green roof. The substrate depth dictates how well any roof design establishes itself. Without enough substrate, any green roof will simply not be able to establish itself and many empty field areas will soon be obvious. Longer term, the entire area will simply fail. Modern green roof substrates generally consists of a mixture of free draining organic material, often this is a volcanic crushed rock, coupled with recycled certifiable waste products such as clay bricks and tiles, that provides a low nutrient growing medium that is also water retentive. Sedum blanket is placed upon this in identical fashion to laying a lawn. There is a drainage system installed directly onto the roof substrate (felt, liquid roofing, PVC, asphalt) normally in conjunction with a root barrier ability within the roofing substrate itself or physically placed on the roofing substrate surface. All green roof weights are calculated from a saturated viewpoint. Typically, extensive green roofs weigh 80kg/m2.

Biodiverse Green Roofs

The definition of a biodiverse green roof is ‘the creation of a landscape designed to replace that which has been lost beneath the footprint of the building, so as to attract and retain the wildlife which would otherwise have been lost’. These designs tend to include rockery and paving areas. Biodiverse green roofs would typically require the same substrate depth as extensive green roofs. These are normally constructed using a plug plant method. This is where the selected plants are physically implanted into the areas meaning that a more accurate design is possible. Biodiverse are typically calculated to weigh similar to extensive roofs. 80kg/m2

Intensive Green Roofs

Intensive green roofs require much more design consideration. It is unlikely that a timber roof deck could support such a build up as the loading factors can be considerably more than an extensive/biodiverse type roof. Loads of around 250 kg/m2 can be expected. A concrete deck is often required among the design considerations. Intensive green roofs offer very exciting design options, including shrubs, bushes, and small trees. Seats and recreation areas can also be added so that the roof can then easily double as place where staff or residents can enjoy green areas in built up areas. A definition of an intensive green roof would be ‘A landscape which includes a wide variety of vegetation. This varies from lawn grass to large trees, usually including a significant element of hard rock type landscape. Intensive roofs require a high degree of maintenance to control the vegetation and keep it developing as it should.’

The factors required for the construction of any green roof are:

• Waterproofing

• Roof barrier

• Moisture matt

• Drainage element

• Filter membrane

• System substrate

• Plant level

The natural needs of green roofs are:

• Drainage

• Air

• Water

• Nutrients

What Sedum Plants Make Up Green Roofs?

It is very important to understand and specify the type of roof finish a client requires. Specifying the correct vegetation landscape dictates entirely the finished roof. If the wrong plants are specified, the roof will simply fade and die. For example, below is a chart that illustrates some various sedum plants and where and when they best flourish.

The Key Components Of A Green Roof System

Moisture Matt

• Retains any moisture and nutrients

• Will not degrade or rot.

• Fibre mats with polypropylene carrier

• Typically retain up to 5 or 7 l/m2

• Laid loose with 100mm laps

Drainage Layer

• Ensures drainage of the roof is constant

• Provides water storage to maintain substrate moisture

• Perforations ensure drainage and air into the substrate

• Provides further protection to the substrate

Filter Sheet

• Prevents substrate fines from clogging the drainage layer

• Manufactured from polypropylene

• Loose laid with 150mm laps

Green Roof Substrates

• Good plant anchorage

• Stable and resistant to wind and water erosion

• High aeration with water saturation

• Excellent absorption and buffering

• Low weight

• Free of weeds, diseases, and pests

• Fire resistant

• Excellent water retention

Key elements Of Green Roof Substrates

There are 5 types of substrates for a green roof.

• Extensive – ‘Sedum carpet’

• Extensive – ‘Rockery type plants’

• Semi intensive – ‘Heather & lavender’

• Intensive – ‘Roof Garden’

• Intensive (acidic) – ‘Cranberry bogs’


• A green roof must be designed as part of the building and should not be a post construction consideration.

• The green roof must be designed within the constraints of the building capability so that the integrity of the building is not at risk.

• Maintenance of a green roof is important. It will otherwise revert to and grow as nature would dictate.

Green Roof Slopes

• Extensive roofs – minimum 1:80 slope

• Intensive roofs – May be laid without falls

• Slopes up to 20°  - simple to achieve

• Above 20° - additional supports required to resist shearing forces

Choice of vegetation

• Choice of plants so that they thrive

• Substrate mixes to support the plants

• Local climate

• Water, air, nutrients

• Maintenance

Why Green Roofs Are A Great Choice

• Safe

• Flourishing

• Permanently functioning

• Economical

• Ecologically beneficial

Get In Touch With Any Questions About Green Roofs

If you would like a quotation or any information on green roofs please get in touch with us or call us now on 01277 375 511. One of our friendly team of green roofing experts will be happy to help you.