Living roofs explainer and FAQ

A living roof is a roof with vegetation growing on it. To be more precise, the load-bearing roof structure is covered by successive layers of waterproofing/root barrier, insulation, drainage/water retention layer, substrate/growing medium and vegetation. These ingredients vary depending on the type of living roof.

The living roof system governs how the living roof is constructed, determining the thickness of each component part and the thickness and weight of the roof as a whole. Most systems – available from a variety of manufacturers – include the following components:

• Load-bearing roof structure
• Waterproofing and root barrier (usually combined)
• Insulation
• The drainage and water retention layer (usually combined)
• The substrate or growing medium
• Vegetation.

Beware: different systems and suppliers prescribe different specifications such as depth of substrate/growing medium.

See: How do you choose a living roof system, supplier and installer?

To ensure that your living roof will be both safe and long-lasting, you must not only choose a system, supplier and installer with a proven track record in living roofs, but also one that adheres to the German FLL Guidelines for the Planning, Construction and Maintenance of Green Roofs, as well as UK guidelines, including The GRO Green Roof Code of Best Practice.

The living roof industry and regulatory environment in the UK is younger than in Germany, which is considered to be the world leader. German regulations and guidelines – known as FLL – are much more established and stricter than other countries. Other countries, including Britain, are adopting German guidelines into their own.

There are three categories of living roof: 1. green roof; 2. brown roof or biodiverse; and 3. blue roof.

1. A green roof is planted with vegetation during install. There are three types of green roof, requiring successively greater depth of substrate/growing medium:
   a) Extensive roofs mostly comprise self-sustaining, low-growing plants, frost and drought tolerant, such as mosses, succulents, herbs and grasses, with bulbs and tubers. The most commonly used plant on extensive roofs are sedums, which studies of the oldest roof installations have shown are the most enduring.
   b) Semi-intensive roofs typically include plant species which are less demanding than intensive roofs, such as wild flowers, herbaceous perennials, grasses and small shrubs.
   c) Intensive roofs look like a residential garden with a wide range of plants, including perennials, grasses, bulbs and shrubs, in some cases trees and lawns.
2. A brown roof mimics the ‘brown field’ site that existed before the building was constructed. It is left to self-seed via wind or bird, with plants and wildlife that were displaced by the build. A biodiverse roof uses a similar substrate to a brown roof, but involves a crafted mix of seed or planting being introduced by the installer, usually on the recommendation of an ecologist.
3. A blue roof incorporates a sustainable drainage system (SuDS), which retains water following a downpour, to prevent the damaging run off and manages how the water is slowly dispersed. Commonly, a green roof is installed above the blue roof – sometimes called a blue-green roof.

Green roofs; Brown roofs; Blue roofs

According to GRO and the German FLL Guidelines for the Planning, Construction and Maintenance of Green Roofs, extensive (sedum roofs) require a substrate with a minimum depth of 80 mm, semi-intensive roofs (e.g. herbs or wild-flowers) require a substrate between 100 and 200mm and intensive (mixed planting roofs) should have a minimum depth of 200mm.

A historical study of green roofs built in the 1970s and 1980s in Germany, suggests that roofs will suffer loss of substrate as they age, making it particularly important that guidelines on substrate depth are followed.

A living roof can deliver the following benefits, according to BREEAM (Building Research Establishment Environmental Assessment Method):

1) They benefit people and community, by improving health, wellbeing and comfort, as well as heritage and local character, by:

• Being more aesthetically pleasing than many conventional roofing solutions, so improve the city’s roofscape and reduce the visual impact of development.
• Helping to reduce the urban heat island effect by absorbing heat into the plants and substrate rather than reflecting it to the local environment.
• Roof gardens encourage people to go outside and engage in outdoor activities.

2) They reduce construction and operational impacts, by:

• Reducing surface water runoff and can act as a natural buffer helping to prevent flash flooding. See: Blue Roof.
• Improving air quality, through plant respiration.
• Reducing noise through a building’s roof, both inside from external noise such as road traffic, and outside from internal noise.

3) They help mitigate climate change, by:

• Reducing a building’s energy consumption and lowering the cost of heating in winter and cooling in summer.
• Reducing the urban heat island effect by absorbing heat into the plants and substrate rather than re-emitting it to the local environment. Plants help remove heat from the air through evapotranspiration.

4) They can provide habitats for wildlife and improve biodiversity.

Other claimed benefits of living roofs include:

1) Increasing the effectiveness of solar panels, by reducing the ambient temperature of the roof area, evidence from Germany, where biosolar (combined living roof and solar panels) are more common, suggests.

2) Improves the life expectancy of the roof, by protecting the roofing materials from direct ultra–violet radiation and extreme temperatures.

SEE: How do living roofs reduce energy costs of a building?

According to BREEAM (Building Research Establishment Environmental Assessment Method):

• Green roofs and green walls “can help to reduce demands for artificial heating and cooling by contributing to insulation performance and reducing demands through solar gain and wind generated heat loss. This both helps to reduce energy demand and its associated costs as well as conveying resilience to future climate change in a visible manner, thus assisting in demonstrating corporate social responsibility”.

Studies:

• Research has found that green roofs can reduce heat flow through building roofs in summer by approximately 80%.
• Research by Nottingham Trent University found that a green roof kept a building 4.5°C warmer in winter and 15°C cooler in summer than a conventional roof.

Various studies have found that living roofs, particularly those with deeper substrate and a mix of substrates, have had a beneficial impact:

• More diverse vegetation provides foraging resources for urban bees.
• The creation of mounds provides refuge for invertebrates from extreme temperatures.
• Careful planning of substrate mix improves invertebrate species richness e.g., spiders, beetles, sometimes rare species.
• The provision of habitat resources, like perches, food and water, can accommodate the breeding needs of ground-nesting birds.

German FLL Guidelines states that roofs with a high degree of structural diversity, e.g. intensive, extensive green with raised areas and woody vegetation have the ‘highest faunistic diversity’. It is also recommended that roofs should be connected to the surrounding landscape by greening facades.

Green roofs; Brown roofs

The construction and planning guidelines of many countries and cities explicitly require provision for green roofs, walls, roof terraces and sustainable drainage systems SuDS. Some places have gone further, introducing incentives and enshrining urban greening into law.

The direction of travel in the UK is clear from the guidance for developers and planners in London.

Sustainable Design and Construction SPG provisions (2014) for London states: APPENDIX 4, 6.4.3.

“New development should incorporate Sustainable Urban Drainage Systems and green roofs where practical with the aim of achieving a Greenfield run-off rate, increasing bio-diversity and improving water quality. Surface water run-off is to be managed as close to source as possible.”

Living Roofs and Walls Technical Report: Supporting London Plan Policy (2008) recommended the following policy statement:

“The Mayor will and boroughs should expect major developments to incorporate living roofs and walls where feasible 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 appearance.”

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

The report also made the following recommendations:

• A minimum of 70 per cent of the roof space should be vegetated to provide maximum benefit for SuDS, building energy performance and biodiversity.
• At least 25 per cent of the total roof space in any one development should be accessible to residents and/or workers.
• A roof with an average depth of 100mm substrate with 80 per cent of the substrate having an average holding capacity of approximate 2 litres/10mm/m2 equivalent providing a potential minimum capacity 20 litres/m2.

See: Blue Roof; Green roof

There is no such thing as a no-maintenance living roof. Like any ground-level garden, if left alone, nature will soon take over. Different roofs require different maintenance regimes. Pritchard & Pritchard recommends the following maintenance plans:

• Extensive or sedum green roofs are low maintenance, requiring four visits to inspect and tend per year.
• Brown and bio-diverse roofs are low maintenance, requiring four visits to inspect and tend per year.
• Semi-intensive roofs such as wild-flower roofs, require monthly visits to inspect and tend.
• Intensive or mixed planting roofs require monthly visits to inspect and tend.
• Roof-top gardens and podiums require monthly visits to inspect and tend.

A historical study of green roofs built in the 1970s and 1980s in Germany, found that without maintenance, roofs suffered loss of substrate, lower pH, less biomass, and more soil organic content than younger roofs.

Maintenance regimes for living roofs require the following tasks (according to German FLL Guidelines):

• Irrigation
• Fertilizing
• Removing unwanted foreign vegetation
• Cutting back ground cover
• Pruning woody plants
• Repeat seeding
• Repeat planting
• Pest control
• Keeping technical installations vegetation-free
• Keeping edging/safety margins and surface coverings free from dead leaves and overgrowing vegetation
• Removal of refuse
• Plant protection
• Refilling of substrate in cases of erosion
• Inspection and functional testing of irrigation systems
• Inspection of the drainage system and inspection shafts
• Mowing, scarifying; aerating; top-dressing of lawns
• Mowing wild-flower meadows.

See: Living roof maintenance; Roof-top garden maintenance.

A properly installed, irrigated and maintained living roof should last for over 40 years. There are living roofs in Germany built using a similar light-weight type of system used today (pioneered in Germany) that date back to the 1970s, for example Pliensau cemetery, near Stuttgart.

Arguably, the oldest living roof in the world is on top of a 400+ year-old medieval fortified tower house, Torre Guinigi, in Lucca, Italy.

A biosolar roof combines a living roof with solar panels. If properly designed, installed and maintained, these two roof types are not only very compatible, the living roof may also increasing the effectiveness of solar panels, by reducing the ambient temperature of the roof area. Evidence from Germany, where biosolar roofs (combined living roof and solar panels) are more common, suggests this is the case.

Every living roof installation is bespoke. The cost calculation takes into account the following factors:

• Roof size
• Roof pitch
• Elevation, access and requirement of crane
• Location
• The load-bearing capacity of the roof – is it sufficient to support the living roof?
• Living roof type: Brown and extensive or sedum green roofs are cheaper to install than intensive (mixed planting) and blue SuDS roofs
• Mixed use: e.g. public access see: Roof-top gardens or solar panels. see: Biosolar
• Maintenance package. Brown and extensive or sedum green roofs are lower maintenance than intensive roofs. See: Living roof maintenance