Grass Is 'Greener' On The Roof

• 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.

• 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.

• Potential to reduce the size of HVAC equipment on new or retrofitted buildings (capital and operational savings).
• Potential to reduce the amount of standard insulation used.
• Potential to incorporate cooling and/or water treatment functions.
• Potential for local, regional, and national market exposure, depending on the uniqueness of the project.
• Potential to reduce or eliminate roof drains.
• Potential to meet regulatory requirements for stormwater management.
• Potential to reduce community resistance to new developments.

• Provision of amenity space for day care, meetings, and recreation;
• Aesthetic appeal, increasing the value of the property and the marketability of the building as a whole, particularly for accessible green roofs. For example, American and British studies show that "good tree cover" adds between 6 to 15 per cent to the value of a home. Green roofs offer the same visual and environmental benefits.
• Satisfying the aesthetic needs of people looking down upon the roof from adjacent buildings.
• Potential to improve employee productivity.

• The Fairmount Waterfront Hotel in Vancouver used to grow herbs, flowers, and vegetables on its accessible roof, saving its kitchen an estimated $30,000 a year in food costs.

Herb Garden on Vancouver's Fairmount Waterfront Hotel (courtesy David Walker)

• Potential for faster approval process for new projects, as is the case in Chicago
• Potential for reduced stormwater/wastewater charges from your municipality or utility.
• Potential to reduce the size of stormwater management ponds or cisterns, resulting in cost savings.
• Potential for grants related to energy efficiency and/or green roofs.
• Potential for density bonusing / larger floor area ratio.
• Potential to satisfy regulatory requirements for green roofs.
• Potential to score more than 7 credits under the US and Canadian Green Building Council LEED certification system.
• Potential for satisfying minimum parkland / green space set aside, requirements.
• Potential for greenhouse gas emissions trading credits, stemming from energy savings.

Public Benefits

• Suppliers and manufacturers of roofing membranes and root repellent layers;
• Suppliers and manufacturers of drainage layers, landscaping cloth, curbs, irrigation systems and other specialty products;
• Suppliers and manufacturers of substrate, light-weight soils and amendments;
• Garden nurseries specializing in plants specifically for green roofs;
• Design and engineering professionals;
• Contractors and landscapers; and,
• Companies supplying maintenance contracts.

• Cost savings from increased stormwater retention and decreased need to expand or rebuild related infrastructure.
• Decreased cost of meeting greenhouse gas reductions and adapting to climate change by reducing the "Urban Heat Island Effect" and the need for interior building insulation.
• Decreased need for health care services from reductions in ground level ozone resulting from a reduction in the urban heat island.
• Increased worker productivity and creativity.
• Decreased need for health care services and medication due to the benefits of passive experiences with nature and vegetation.
• Extending the lifespan of landfil sites by reducing re-roofing material waste.
• Opportunities to recycle aggregate and compost.

• 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 up to 2 kg (4.4 lbs) of airborne particulates from the air every year, depending on foliage type.

• 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.

• 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.

• 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.

• 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 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.

• 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.

• 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.

• Urban greening has long been promoted as an easy and effective strategy for beautifying the built environment and increasing investment opportunity.

• Psychological studies have shown that the restorative effect of a natural view holds the viewers' attention, diverts their awareness away from themselves and from worrisome thoughts thereby improving health.
• People living in high-density developments are known to be less susceptible to illness if they have a balcony or terrace garden. This is partly due to the additional oxygen, air filtration and humidity control supplied by plants but also from the therapeutic benefits that result from caring for plants. The variety of sounds, smells, colours and movement provided by plants, although not quantifiable, can add significantly to human health and well being.
• Patients in the same hospital, recovering the from the same operation, were studied as to the restorative effects of views onto a landscaped courtyard versus a brick wall. The patients with the green view had shorter post-operative stays, took fewer moderate and stronger painkillers and had fewer negative evaluation comments from the nurses.

• A garden on the roof is often considered safer than a garden at grade. For example, because access to the roof is often restricted to building tenants or employees, there is less likelihood of assault or vandalism.

• Green roofs can help to address the lack of green space in many urban areas. Studies show that leisure activities in natural settings such as gardens and parks, are important for helping people cope with stress and in meeting other non-stress-related needs.