Benefits

Green roofs are used to:

• Grow fruits, vegetables, and flowers
• Reduce heating (by adding mass and thermal resistance value) and cooling (by evaporative cooling) loads on a building — especially if it is glassed in so as to act as a terrarium and passive solar heat reservoir
• Increase roof life span
• Reduce stormwater run off — see water-wise gardening
  

• Filter pollutants and CO2 out of the air — see living wall
• The soil and plants on green roofs help to insulate a building for sound; the soil helps to block lower frequencies and the plants block higher frequencies.
• Filter pollutants and heavy metals out of rainwater
• Increase wildlife habitat in built-up areas — see urban wilderness

The following benefits can be achieved with virtually all green roof infrastructure systems.

• Economic Benefits - cost savings opportunities for the building owner include:
• Protection of roof membrane resulting in a longer material lifespan (it is estimated that green roofs will last up to twice as long as conventional roofs), resulting in decreased maintenance and savings in replacement costs;

• Savings on energy heating and cooling costs, depending on the size of the building, climate and type of green roof. Using a Micro Axess Simulation model, Environment Canada found that a typical one storey building with a grass roof and 10 cm (3.9 inches) of growing medium would result in a 25% reduction in summer cooling needs. Field experiments by Karen Liu in Ottawa Canada, found that a 6 inch extensive green roof reduced heat gains by 95% and heat losses by 26% compared to a reference roof.
  

Sound Insulation

Soil, plants and the trapped layer of air can be used to insulate for sound. Sound waves that are produced by machinery, traffic or airplanes can be absorbed, reflected or deflected. The substrate tends to block lower sound frequencies and the plants block higher frequencies.

A green roof with a 12 cm (4.7 inches) substrate layer can reduce sound by 40 decibels; a 20 cm (7.9 inches) substrate layer can reduce sound by 46-50 decibels.

Improved Air Quality

A green roof will not only absorb heat, decreasing the tendency towards thermal air movement, but will also filter the air moving across it.

1 m2 (10.76 ft2) of grass roof can remove between 0.2 kg of airbourne particulates from the air every year

Carbon Dioxide/Oxygen Exchange:

Through the process of photosynthesis, plants convert carbon dioxide, water and sunlight/energy into oxygen and glucose. This cyclical process supplies animals and humans with oxygen and food.

1.5 m2 (16.15 ft2 ) of uncut grass, produces enough oxygen per year to supply 1 human with their yearly oxygen intake requirement.

Temperature Regulation

Moderation of the Urban Heat Island Effect:

Through the daily dew and evaporation cycle, plants on vertical and horizontal surfaces are able to cool cities during hot summer months. In the process of evapotranspiration, plants use heat energy from their surroundings (approximately 592 kcal per L of water) when evaporating water. One m2 (10.76 ft2) of foliage can evaporate over 0.5 litres of water on a hot day and on an annual basis the same area can evaporate up to 700 litres of water.

This process reduces the 'Urban Heat Island Effect' in the summer. The 'Urban Heat Island Effect' is the difference in temperature between a city and the surrounding countryside. It is mainly due to the expanse of hard and reflective surfaces, such as roofs, which absorb solar radiation and re-radiate it as heat. Reduction of the 'Urban Heat Island Effect' will also reduce the distribution of dust and particulate matter throughout the city and the production of smog. This can play a role in reducing greenhouse gas emissions and adapting urban areas to a future climate with warmer summers.

Green roofs can play a role in reducing the urban heat island (UHI), augmenting the existing vegetation, but the precise amount of coverage is still uncertain. Dr. Brad Bass in collaboration with a modelling group at the University of British Columbia under the direction of Dr. Roland Stull, ran a mesoscale atmospheric simulation for the City of Toronto with green roofs. The city's vegetation reduced the UHI by up to 1 degree C over approximately 1/4 of the City. Using a green roof coverage of 50%, this cooling was extended to approximately 1/3 of the City and increased the maximum cooling to 2 degrees C.

Although the green roof coverage was high, Dr. Bass estimated that only 6% of the roofs were fully irrigated as the model decreased irrigation to those parts of the city that were not fully urbanized, decreasing it to zero in totally natural areas of the city. This suggests that the actual green roof coverage to obtain these results could be much smaller than 50% although the exact requirement is difficult to determine due to a number of uncertainties that emerged in this modelling exercise.

Building Insulation:

Historically, green roofs have been used to insulate buildings. Shading the external surface of the building envelope has been shown to be more effective than internal insulation.

Green roofs insulate buildings by preventing heat from moving through the roof. Their insulation properties can be maximized by using a growing medium with a low soil density and a high moisture content and by choosing plants with a high leaf area index (i.e. the bigger the leaves, the better). This could play a role in reducing greenhouse gas emissions and adapting urban areas to a future climate with greater incidences of drought and extreme heat.

Industrial Cooling:

The Possman Cider Cooling and Storage Facility in Frankfurt, Germany yielded a 2-3 year payoff of their green roof system through savings in heating and cooling costs, as well as in equipment costs, since additional cooling towers had become unnecessary

Creation of Microclimates:

A green roof will have a noticeable impact on the heat gain and loss of a building, as well as the humidity, air quality and reflected heat in the surrounding neighbourhood. In conjunction with other green installations, green roofs can play a role in altering the climate of the city as a whole.

On a summer day, the temperature of a gravel roof can increase by as much as 25 °C (77 °F), to between 60-80 °C (140 - 176 F). Covered with grass, the temperature of that roof would not rise above 25 °C (77 °F), thus resulting in energy cost savings.

20 cm (7.9 inches) of substrate with a 20-40 cm (7.9 - 15.7 inches) layer of thick grass has the combined insulation value of° 15 cm (5.9 inches) of mineral wool.

Rooms under a green roof are at least 3 - 4 °C (5.4 - 7.2°F) cooler than the air outside, when outdoor temperatures range between 25-30 °C (77 - 86 °F).

Water

Stormwater Retention:

Water is stored by the substrate and then taken up by the plants from where it is returned to the atmosphere through transpiration and evaporation.

In summer, depending on the plants and depth of growing medium, green roofs retain 70-90% of the precipitation that falls on them; in winter they retain between 25-40%. For example, a grass roof with a 4-20 cm (1.6 - 7.9 inches) layer of growing medium can hold 10-15 cm (3.9 - 5.9 inches) of water.

Water Filtration:

Green roofs not only retain the rainwater, but also moderate the temperature of the water and act as natural filters for any of the water that happens to run off.

Temporal Delay of Stormwater Runoff and Reduced Runoff Volume:

Green roofs reduce the amount of stormwater runoff and also delay the time at which runoff occurs, resulting in decreased stress on sewer systems at peak flow periods.

Preservation of Habitat & Biodiversity

Habitat:

Rooftop habitats can play one of two roles: a 'stepping stone' habitat connecting natural isolated habitat pockets with each other, or an 'island' habitat remaining isolated from other habitats at grade.

Green roofs can be specifically designed to mimic endangered ecosystems/habitats, including the prairie grasslands of the midwest US, the rocky alvars of Manitoulin Island and the Great Lakes Region in Canada.

The Toronto City Hall Demonstration Project features a black oak prairie ecosystem and native plant butterfly plot

Flora & Fauna:

Green roofs designed for minimal maintenance are very protected and can become home to plants easily damaged by walking and to birds that nest on the ground. Since the soil on these green roofs is also less likely to be disturbed, it becomes a safer habitat for insects, and the deeper the soil the more diversity the roof can support.

In Germany, for instance, research has shown that green roofs can support anywhere from 10 to 40 different insect species and have even been found to harbour nesting bird species.

The Toronto City Hall Demonstration Project features two urban agriculture plots that are growing a variety of annuals and perennials.

A green roof is often a key component of an autonomous building.

A 2005 study by Brad Bass of the University of Toronto showed that green roofs can also reduce heat loss and energy consumption in winter conditions.

In a recent study on the impacts of green infrastructure and in particular green roofs in the Greater Manchester area, researchers found that adding green roofs will help keep temperatures down, particularly in urban areas: “adding green roofs to all buildings can have a dramatic effect on maximum surface temperatures, keeping temperatures below the 1961-1990 current form case for all time periods and emissions scenarios. Roof greening makes the biggest difference…where the building proportion is high and the evaporative fraction is low. Thus, the largest difference was made in the town centres.”

Types

Green roofs can be categorized as intensive, "semi-intensive" or extensive, depending on the depth of planting medium and the amount of maintenance they need. Traditional roof gardens, which require a reasonable depth of soil to grow large plants or conventional lawns, are considered "intensive" because they are labour-intensive, requiring irrigation, feeding and other maintenance. Intensive roofs are more park-like with easy access and may include anything from kitchen herbs to shrubs and small trees.

"Extensive" green roofs, by contrast, are designed to be virtually self-sustaining and should require only a minimum of maintenance, perhaps a once-yearly weeding or an application of slow-release fertiliser to boost growth.

Extensive roofs are usually only accessed for maintenance. They can be established on a very thin layer of "soil" (most use specially formulated composts): even a thin layer of rockwool laid directly onto a watertight roof can support a planting of Sedum species and mosses.

Another important distinction is between pitched green roofs and flat green roofs. Pitched sod roofs, a traditional feature of many Scandinavian buildings, tend to be of a simpler design than flat green roofs. This is because the pitch of the roof reduces the risk of water penetrating through the roof structure, allowing the use of fewer waterproofing and drainage layers.

Greenroofs range from small, simple, owner-constructed designs on sheds and homes, to extensive proprietary systems installed on large commercial and industrial buildings. Of the latter installations, there are two basic types: intensive and extensive.

Intensive Green Roofs

Intensive roofs typically have deeper soils and irrigation systems, allowing for a much wider range of plant types and sizes. They provide significantly better insulation. Intensive green roofs are usually designed to be accessible (available to be walked on or used as amenity space). This type of eco-roof requires more technical expertise to design and install, and more maintenance once built than extensive types.

Extensive Green Roofs

These roofs are characterized by their low weight, low capital cost and minimal maintenance. Soils (or growing medium) are very thin, and irrigation is rare, allowing only smaller, hardier plants to survive. Extensive green roofs are more suitable for larger areas and for sloped roofs. They are easier to retrofit onto existing roofs but are not usually designed for public accessibility.

While differences exist between the range of plants possible for extensive versus intensive green roofs, those best suited to any rooftop environment are:

• Drought-tolerant, requiring little or no irrigation after establishment
• Self-sustaining, without the need for fertilizers, pesticides, or herbicides
• Able to withstand heat, cold, and high winds
• Very low-maintenance, needing little or no mowing or trimming
• Perennial or self-sowing

Suitable plants also have growth patterns that allow them to thoroughly cover the soil, and include a variety of species to ensure establishment of a self-maintaining community which is appealing to animal and human users alike.

Brown Roofs

Industrial brownfield sites can be valuable ecosystems, supporting rare species of plants, animals and invertebrates. Increasingly in demand for redevelopment, these habitats are under threat. "Brown roofs" can partly mitigate this loss of habitat by covering the flat roofs of new developments with a layer of locally sourced material. In Switzerland it is common to use alluvial gravels from the foundations and in London a mix brick rubble and some concrete has been used.

Although the original idea was to allow the roofs to self colonise, they are now seeded to increase their biodiversity potential in the short term. The roofs are colonised by spiders and insects, many of which are becoming extremely rare in the UK as such sites are developed and provide a feeding site for insectivorous birds. Laban, a centre for contemporary dance in London, has a brown roof specifically designed to encourage the national rare Black Redstart.

(In 2003 Laban won the coveted RIBA Stirling Prize.) The highest green roof [160m] to act as nature reserve in the world is on the Barclays Bank HQ.[20] This has been designed using the principles of the brown roof idea and is already home to a range of rare invertebrates.

Disadvantages

Green roofs have more demanding structural standards. Some existing buildings cannot be retrofitted with a green roof because of the weight load of the soil and vegetation. Depending on what kind of roof it is, the maintenance costs could be higher. Green roofs also place higher demands on the waterproofing system of the structure both because water is retained on the roof and due to the possibility of roots penetrating the waterproof membrane. Installing adequate waterproofing systems and root barriers can increase the cost of the roof.

Costs

The cost depends on what kind of roof it is, the structure of the building, and what plants can grow on the material that is on top of the roof. In the Spring 2007 issue of the Green Roof Infrastructure Monitor (Green Roofs for Healthy Cities web site) , Jörg Breuning reflects the wind and fire loads of green roofs and how German insurance companies handle extensive Green Roofs.

Some cost can also be attributed to maintenance. Extensive green roofs have low maintenance requirements but they are generally not maintenance free. German research has quantified the need to remove unwanted seedlings to approximately 0,1 min/(m²*year). Maintenance of green roofs often includes fertilisation to increase flowering and succulent plant cover. If aesthetics is not an issue, fertilisation and maintenance is generally not needed. Extensive green roofs should only be fertilised with controlled release fertilisers in order to avoid pollution of the stormwater. Conventional fertilisers should never be used on extensive vegetated roofs. German studies have approximated the nutrient requirement of vegetated roofs to 5gN/m². It is also important to use a substrate that does not contain too much available nutrients. The FLL-guidelines specify maximum allowable nutrient content of substrates