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Design Considerations

Green roofs provide numerous benefits, however, it is worth remembering that a client will often have their own priorities. By considering each benefit at the earliest stages of design it will be possible to create a green roof which is multi-responsive.

number 41What are the considerations when planning a green roof?

a) Access and Fire Risk


As with all roofs, and especially flat roofs, maintenance is required and therefore access must be considered. All green roofs require some degree of maintenance. Low maintenance should not be interpreted as no maintenance. Extensive green roofs require less maintenance than intensive roofs, but there is still a requirement every year to clear gutters and unwanted vegetation. Therefore it is important to design for ease of access to all types of green roofs.

Access to the roof should ideally be through internal access hatches or alternatively by secure ladder points. Full roof containment is the preferred safety option, if this is not possible, there should be fall protection systems for operatives to work on the roof. Health and safety concerns as covered in the Construction (Design and Management) Regulations 2007 ('CDM Regulations'). All fall protection systems must be checked annually.

It should be noted that the CDM Regulations place a legal responsibility on clients and designers to take into account the fact that a building must be designed to be constructed, maintained, altered and demolished safely. As part of a project's commitment to safe practice, it is strongly recommended that all those involved in the process take note of these responsibilities.

Edge protection and personnel containment must be in place during the installation of any roofing processes in line with current HSE guidelines. A means of safe access by a roof anchorage system, or better still a total containment system for roof inspection and maintenance is strongly recommended.

Fire Risk

The first green roofs of modern times were installed as fire prevention measures. Although experience in Germany has shown that the risk of fire is small, dry vegetation on green roofs does have the potential to catch alight following prolonged hot weather. Although there are no mandatory fire standards for green roofs at present, it is recommended that the following measures are put in place to reduce risk:

- Vegetation barriers (intentionally un-vegetated strips) of pebbles (20mm - 40mm) or paving (concrete) at 500mm wide to all roof penetrations and in front of all up-stands

- Pebble or concrete paving in the vegetation at every 40m run, with a minimum width of 1m or a 300mm high fire wall

- Vegetation barriers are kept clear of encroaching plants by routine maintenance

Tests in Germany (DIN 4102-7) have demonstrated that extensive roofs are unlikely to be ignited by sparks provided that the substrate base is a minimum of 30mm and contains no more than 20% organic content by volume. Therefore it is recommended that green roof substrate does not contain more than 20% organic content by volume.

b) Management of Design


If the green roof is bought as a package from a reputable firm and fitted by approved contractors, guarantees against faulty construction will be provided. Some green roof systems come with a guarantee of up to 20 years to cover defective product or workmanship. Sourcing and specifying the different layers of a green roof from different suppliers will obviously have a bearing on the types of guarantee you will be able to secure for a green roof as a whole system (making an informed design stage critical to eventual performance and maintenance). The vegetation element will never gain a full guarantee due to it being a living organism, however some firms will replace losses that occur during the first 12 months or agree to ensure that a certain % of cover is maintained. It important to ensure a maintenance regime is put in place to ensure the vegetation establishes.

Manufacturers and Supply

There are an increasing number of green roof component suppliers in the UK. Look for products which have guarantees and include the supplier in the design process as they will advise on options, loading, drainage and thermal performance.

Planning Consent/Permission

Incorporating a green roof (or roofs) into a development may gain support which can help to secure planning consent. It may be possible to retrofit green roofs on certain buildings without planning consent, although the planning authority should be consulted if there is any doubt.

c) Roof Structure

Limited Roof Space

When a green roof is installed on a commercial building, there will often be an air conditioning plant, water tanks and other equipment located on the roof. As access will be required to service these features, access for the maintenance of the green roof can be provided via the same route. Attention needs to be paid to the detailing of edges where a green roof skirts around mechanical structures and there may be a requirement to provide paved access across a green roof. Any green roof, no matter how small, brings benefits, therefore is worth including on a building even when space is limited.

Outlet and Downpipe Requirement

There should be a minimum of two downpipe outlets per roof as a precaution against blockage. All outlets should be protected by an inspection chamber and surrounded by a pebble vegetation barrier to prevent plant encroachment. A green roof results in a reduction in total runoff volume and peak flow. The reduction in peak flow depends on the intensity of the rainfall event and the level of saturation of the substrate before the rain event began. When the substrate is saturated the lag time can shorten to that of a traditional roof, but peak flow reduction can still occur.

Given the varying substrate materials and depths available in manufactured green roof systems in the UK market, advice should be obtained from the supplier for any allowable reduction in downpipe number or size.

Pitch of the Roof

Any pitch of roof can be greened, to the point where the surface is vertical. Where surfaces over 45° are to be greened they require technology closer to that used for living walls. Living walls are outside of the scope of this document. In order to green steeply pitched roofs, more complicated designs and installation methods are required and this can increase cost and limit access for maintenance. Roofs from 'flat' (normally 3°) up to a 10° pitch can be greened with standard green roof products. A flat roof normally has a minimum fall of 1:60 to encourage drainage. Beyond 10° additional support is required.

Roof Pitch Formula

Structural Capacity

Structural loads on the roof depend on the type of green roof chosen and a structural assessment is always required. Intensive green roofs with trees and where access for amenity is required will need to be supported by a heavy steel or concrete structure. Many inverted roofs are designed to be covered in paving slabs or pebbles, which act as ballast on the insulation. The substrate of a green roof can be calculated to provide the necessary ballast in place of the paving slabs, therefore resulting in no addition to the structural load. For low-rise lightweight structures, extensive green roofs are the only option and lightweight roofs will usually need to be structurally strengthened if a green roof is incorporated. It is important that calculations incorporate the load when the roof substrate is saturated, as this will be when the greatest loads are placed on the structure.

d) Design Factors


Designers need to be fully aware of any specific biodiversity objectives for a project in order to ensure that a roof includes the appropriate habitat or biodiversity features. In order to attract particular species, specialist advice should be sought from ecologists with knowledge of the bioregion, species requirements and green roofs. Clients and other stakeholders should be made aware that specific measures designed to benefit biodiversity may affect the appearance of the roof, or could change the performance of the roof in terms of rainwater attenuation or cooling. As with many building projects, trade-offs have to be made.

In considering the conservation of biodiversity it is advisable to determine the distribution of local habitats in the vicinity, and any aspirations to build networks of natural greenspace or green infrastructure. If a roof design allows, a series of different mini-habitats can be deployed to make the roof more appealing to a wider variety of plants and insects. However, an ecologist should be consulted on the minimum area that any one habitat requires. Features that are frequently included on biodiverse green roofs to attract invertebrates are: varying depths and composition of substrate, un-vegetated areas, stones, rope and decaying wood.

It may take a while for a green roof to create the appropriate replacement habitat - It cannot be assumed that a green roof will be immediately successful in conserving key species. It should also be noted that green roofs cannot replicate all habitats that occur at ground level. Certain species of plant require deep soils or shelter which cannot be feasibly created at roof level, and some soils which occur at ground level may be unsuitable for inclusion in a green roof build-up.


Extensive (including biodiverse) green roofs do not usually require irrigation, although they are often watered during the establishment phase (the first 4 to 6 weeks after installation). Semi intensive green roofs should not need to be watered unless there is a prolonged period (usually defined as six weeks) without rain.

Intensive green roofs are often irrigated. Deeper substrates are less vulnerable to drying out as they can store more rainwater, but the types of plants tend to be more water dependent, so irrigation can be necessary.

To reduce the need for watering during establishment, a green roof should be installed in the spring or autumn. Consideration should therefore be given to collecting rainwater if specifying any type of green roof that will require irrigation. Water supplies are under increasing stress in many parts of the UK and there is a considerable amount of embodied carbon in potable water (0.6 kg CO2 per m3). Climate change is predicted to result in more heat waves which will increase demand for irrigation.

Rainwater Harvesting

Storm-water runoff from a green roof is reduced compared to a traditional roof. However, rainwater can still be collected from a green roof and used for any non-potable applications. Water may be discoloured with leachates from the green roof substrates, however the colour can be used as a good indicator of the building's environmental credentials. Where rainwater is harvested from green roofs, fertilisers should not be used. High nutrient levels in water can lead to problems with algae blooms.

Solar Thermal and Photovoltaic (PV) Panels

On roofs, both solar thermal and PV panels can be combined effectively with green roofs. Indeed, it has been shown that the cooling effect of a green roof leads to performance improvements from a PV system mounted on A-frames, as the roof is cooler and the PV cells work at a higher efficiency. The panels themselves must not be over-shaded by the vegetation on the green roof; therefore A-frames are preferable to roof integrated or roof mounted panels. It is important to realise that the area under any panels will be shaded from sun and will not be naturally watered. The effect will be to create a different microclimate and attract different (especially shade-loving) plants.

number 42What are the key technical design requirements of a green roof?

a) Waterproofing

The most important element of any roof, green or otherwise, is that it is water resistant to ensure the integrity of the building is not compromised. It is important that the waterproof component of the roof is confirmed as being sound before proceeding with the installation of the green roof. A green roof should only be installed over a root barrier, which can be incorporated into the waterproofing itself or may consist of an additional membrane on top of the waterproofing. There are many waterproofing products on the market and the supplier of the material will have the technical data to specify which of their products are suitable for use under green roofs. Some manufactures will have Test Certificates from the FLL or other testing facilities to verify the products.

The installation of the waterproofing and its detailing to perimeters, outlets and protrusions through the roof must take into account the depth of the green roof build up. The waterproofing should always be 150mm above substrate level at internal up-stands and protrusions, and at least 100mm at external perimeters.

The waterproofing system must be leak tested and certified, immediately before the installation of the green roof. It is also a good idea to make a photographic record of the area to be greened. This can be used as a reference for any seams, joining features or as evidence of the condition of the area.

b) Up-stand Details

All up-stands, roof perimeters, outlets and protrusions through the roof should be protected by an un-vegetated barrier such as 20 to 40mm rounded shingle or concrete paving slabs. This should be the same depth as the substrate and no less than 500mm across. The waterproof layer should always rise at least 150mm above any other element (substrate or vegetation barrier) on the vertical face. Where waterproofing material is used on the vertical faces of up-stands and parapets, the material should be double skinned or metal coping should be brought down to vegetation barrier level. This will ensure that there are no weak points due to UV rays or climatic stress where there is no green roof coverage.

c) Vapour Control Layers

When specifying the roof construction below a green roofing system, the minimum required performance of the vapour control layer needs to be considered and calculated. The presence of water above the roof waterproofing membrane affects the rate at which internal moisture vapour is transmitted through the roofing system. If a vapour control layer with a low resistivity is used the vapour drive could be negative, resulting in internal condensation and poor thermal performance. The vapour control layer supplier should be able to advise which product will suit the situation best.

d) Outlet Inspection Chambers

All outlets and downpipes from the roof should have easily accessible chambers with removable covers. This prevents any material being washed down the outlets, but also enables direct access for maintenance and inspection.

e) Loading Weight

It is fundamental that the saturated weight of the proposed green roof system is obtained and the information is given to a chartered structural engineer. All loading values are based on saturated loads plus other relevant considerations.

Green roofs put a greater loading on buildings than cold roofs (where the insulation is on the inside of the roof) or warm roofs (where the insulation is on top of the roof, but does not use ballast to hold it in place). However, extensive green roofs do not place greater loads on buildings designed to have warm or inverted roofs (where the insulation is on the outside of the roof and is designed to be held down by gravel ballast or paving slabs). In these situations a green roof can be used as the ballast, though care should be taken to ensure the green roof is as heavy as the ballast would have been.

Substrate manufacturers and suppliers of green roof systems will be able to provide product data sheets on each of the components comprising any particular green roof build up.

number 43What are the aims of the green roof?

Every green roof is being designed and installed for a reason, those reasons usually corresponds to one of the recognised benefits of green roofs. The advantage of a green roof as a technology, is that whichever benefit is being prioritised, some aspect of the others are also achieved.

From the list below decide what the reason is for your green roof (more information on each point can be found in the Benefits section), and keep that reason in mind as you go on to design the rest of the roof.
Reasons for a green roof Design requirements
a. Water attenuation Substrate depth of at least 80mm*, ideally deeper < 200mm.
Reservoir / drainage board with water holding capacity.
Unrestricted water escape from roof for excess water.
b. Increase roof lifespan Substrate depth of at least 80mm*.
Double skin exposed waterproofing.
c. Improve local air quality Substrate depth of at least 80mm*, ideally deeper < 200mm.
High percentage of roof area covered by vegetation.
Varied types and heights of plants.
d. Cooling effect on building/reduce Urban Heat Island Effect Substrate depth of at least 80mm* ideally deeper < 200mm.
High percentage of roof area covered by vegetation.
Varied types and heights of plants.
e. Conserving biodiversity Substrate depth of at least 80mm* ideally deeper < 200mm.
Varied depths of substrate, undulate from one depth to another across the roof.
Varied types and heights of plants.
Include 'natural features' (see the Biodiversity and Planting pages)
Note: Habitat replacement is different from biodiversity conservation, an is also covered in the Biodiversity and Planting pages.
f. Improving water quality Substrate depth of at least 80mm* ideally deeper < 200mm.
Specific substrates and minerals can be used to filter out particular elements.
Note: Water from green roofs with 'common' substrates will often have discoloured runoff.
However, some elements may have been extracted from the water as it passes through the green roof.
g. Reduced sound transfer Substrate depth of at least 80mm* ideally deeper < 200mm.
h. Provide amenity space Substrate depth of at least 80mm* ideally deeper < 200mm where planted.
Protection mat
Drainage board
Good safe access and stable walkways.
Edge protection of at least 1.2m height from top of substrate or walkways.
i. Provide aesthetic impact Substrate depth of at least 80mm* ideally deeper.
High percentage of roof area covered by vegetation.
Varied types and heights of plants.
May require irrigation.

number 44What are the access requirements of the green roof?

Types of Green Roof Green roofs are typecast according to their depth and maintenance requirement. The following names for different green roofs have been adopted by the construction industry: