Green Roof Guidelines

Green Roof Guidelines

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.

a) Access and Fire Risk

Access

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.

Guarantees

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

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.

d) Design Factors

Biodiversity

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.

Irrigation

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

A green or living roof is a roof or deck onto which vegetation is intentionally grown or habitats for wildlife are established. There are different types of green roof and it is important to understand that each type functions and looks different. This is because each green roof is unique and provides a different type of habitat, water storage capacity and energy saving potential.

Green roofs can be extensive — implying shallow substrate depths and load nutrition, through to intensive — which can be similiar to roof gardens.At present there are no specific British Standards relating to green roofs. However most if not all elements of green roof materials are covered by either a BSI standard or a German DIN standard, as described in the German FLL Forschngsgesellschaft Landschaftsentwicklung Landschaftbau’s (Landscape Research, Development and Construction Society) Guidelines for the planning, execution and upkeep of green roof sites. The 2011 GRO Codr provides UK specific green roof recomendations.

Historically, turf roofs were common in northern Europe, but decreased in popularity as the industrial revolution progressed.

Roof gardens became popular on multi-storey buildings in Berlin in the late nineteenth century and continued to be constructed on large city-centre structures throughout the twentieth century.

However, modern extensive green roof practice was stimulated by improvements in roofing and waterproofing technology emerging from Germany in the 1970s and 1980s. Legislation was introduced to encourage the installation of green roofs, and, by 2001, 43% of German cities provided incentives for green roof installation (Lenart 2001 Lenart, Claudia. 2001. "Garden in the Sky ," Utne Reader, (104): 20 — 21).

Other countries in Europe such as Switzerland and Austria also have a long tradition in using green roofs. More recently Canada, parts of the United States of America, Japan and Singapore have experienced a growth in green roof uptake.

Grass Roof in Oswego Illinois, USA, from Greg Robbins

In terms of statute and policy, the UK lags behind continental Europe, however individual cities are championing green roofs. For example, the Living Roofs and Walls, Technical Report: Supporting London Plan Policy 2007. states that:

Policy Living Roofs and Walls

The Mayor will and boroughs should expect major developments to incorporate living roofs and walls where feasable and reflect this principle in LDF policies.

It is expected that this will include roof and wall planting that delivers as many of these objectives as possible:

  • accessible roof space
  • adapting to and mitigating climate change
  • sustainable urban drainage
  • enhancing biodiversity
  • improved appearence

Boroughs should also encourage the use of living roofs in smaller developments and extensions where the oppurtunity arises.

Sheffield City Council is also leading the way with its Planning Conditions, requiring new building in excess of 1000m or 10 dwellings to have 80% vegetated cover.

The UK has seen a rise in the interest and implementation of green roofs over the past decade, due to increasing awareness of sustainable development and the need to adapt the built environment to mitigate the effects of climate change. Other factors include the desire to preserve green spaces in the face of progressive urbanisation, and targets to conserve urban biodiversity. It is estimated that there is at least 200 million m of roof space in the UK which could be greened with little or no modification to the roof structure. This consists of mainly flat roofs on offices, schools, hospitals, other public buildings and housing blocks.

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.

a) Access and Fire Risk

Access

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.

Guarantees

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

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.

d) Design Factors

Biodiversity

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.

Irrigation

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


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