Green roof
A green roof or living roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage, and irrigation systems. Container gardens on roofs, where plants are maintained in pots, are not generally considered to be true green roofs, although this is debated. Rooftop ponds are another form of green roofs, which are used to treat greywater. Vegetation, soil, drainage layer, roof barrier, and irrigation system constitute the green roof.
Green roofs serve several purposes for a building, such as absorbing rainwater, providing insulation, creating a habitat for wildlife, and decreasing stress of the people around the roof by providing a more aesthetically pleasing landscape, and helping to lower urban air temperatures and mitigate the heat island effect. Green roofs are suitable for retrofit or redevelopment projects as well as new buildings and can be installed on small garages or larger industrial, commercial and municipal buildings. They effectively use the natural functions of plants to filter water and treat air in urban and suburban landscapes. There are two types of green roof: intensive roofs, which are thicker, with a minimum depth of, and can support a wider variety of plants but are heavier and require more maintenance, and extensive roofs, which are shallow, ranging in depth from, lighter than intensive green roofs, and require minimal maintenance.
The term green roof may also be used to indicate roofs that use some form of green technology, such as a cool roof, a roof with solar thermal collectors or photovoltaic panels. Green roofs are also referred to as eco-roofs, oikosteges, vegetated roofs, living roofs, greenroofs and ''VCPH ''
Environmental benefits
Thermal reduction and energy conservation
Green roofs improve and reduce energy consumption. They can reduce heating by adding mass and thermal resistance value, and can reduce the heat island effect by increasing evapotranspiration. 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. A modeling study found that adding green roofs to 50 percent of the available surfaces in downtown Toronto would cool the entire city by.Through evaporative cooling, a green roof reduces cooling loads on a building by fifty to ninety percent, especially if it is glassed-in to act as a terrarium and passive solar heat reservoir.
A concentration of green roofs in an urban area can reduce the city's average temperatures during the summer, combating the urban heat island effect. Traditional building materials soak up the sun's radiation and re-emit it as heat, making cities at least hotter than surrounding areas. On Chicago's City Hall, by contrast, which features a green roof, roof temperatures on a hot day are typically cooler than they are on traditionally roofed buildings nearby. Green roofs are becoming common in Chicago, as well as in Atlanta, Portland, and other United States cities, where their use is encouraged by regulations to combat the urban heat-island effect. Green roofs are a type of low impact development.
A 2023 meta-analysis found that green roofs reduce rooftop surface temperatures by an average of 30 °C during summer months, providing significant mitigation of urban heat island effects.
A 2023 systematic review of 86 studies reported that extensive green roofs typically reduce roof surface temperatures by 15–30 °C during peak summer conditions, while also lowering indoor temperatures by 1–4 °C depending on substrate depth and regional climate.
In the case of Chicago, the city has passed codes offering incentives to builders who put green roofs on their buildings. The Chicago City Hall green roof is one of the earliest and most well-known examples of green roofs in the United States; it was planted as an experiment to determine the effects a green roof would have on the microclimate of the roof. Following this and other studies, it has now been estimated that if all the roofs in a major city were greened, urban temperatures could be reduced by as much as.
Water management
Green roofs can reduce stormwater runoff via water-wise gardening techniques. Green roofs play a significant role in retrofitting the Low Impact Development practices in urban areas. A study presented at the Green Roofs for Healthy Cities Conference in June 2004, cited by the EPA, found water runoff was reduced by over 75% during rainstorms. Water is stored by the roof's substrate and then taken up by the plants, from which it is returned to the atmosphere through transpiration and evaporation.Green roofs decrease the total amount of runoff and slow the rate of runoff from the roof. It has been found that they can retain up to 75% of rainwater, gradually releasing it back into the atmosphere via condensation and transpiration, while retaining pollutants in their soil. Many green roofs are installed to comply with local regulations and government fees, often regarding stormwater runoff management. In areas with combined sewer-stormwater systems, heavy storms can overload the wastewater system and cause it to flood, dumping raw sewage into the local waterways. Often, phosphorus and nitrogen are in this category of environmentally harmful substances, even though they are stimulating to the growth of plant life and agriculture. When these substances are added to a system, it can create mass biological activity since they are considered limiting factors of plant growth and by adding more of them to a system it allows for more plant growth.
Ecological benefits
Green roofs create natural habitat as part of an urban wilderness. Even in high-rise urban settings as tall as 19 stories, it has been found that green roofs can attract beneficial insects, birds, bees and butterflies. A recent list of the bee species recorded from green roofs highlights both the diversity of species, but also the bias towards small ground-nesting species. Rooftop greenery complements wild areas by providing stepping stones for songbirds, migratory birds, and other wildlife facing shortages of natural habitat. Bats have also been reported to be more active over green roofs due to the foraging opportunities these roofs provide. Research at the Javits Center green roof in New York, NY, has shown a correlation between higher numbers of certain insects on the roof, particularly moths, with an increased amount of bat foraging activity. Research from 2023 in St. Louis, Missouri, showed that urban rooftop food gardens support diverse bee populations, enhancing urban pollination and biodiversity.Green roofs also serve as a green wall, filtering pollutants and carbon dioxide out of the air, helping to lower rates of diseases such as asthma. They can also filter pollutants and heavy metals out of rainwater.
Carbon sequestration
An additional environmental benefit of a green roof is the ability to sequester carbon. Carbon is the main component of plant matter and is naturally absorbed by plant tissue. The carbon is stored in the plant tissue and the soil substrate through plant litter and root exudates. A study on green roofs in Michigan and Maryland found the above-ground biomass and below ground substrate stored on average between 168 g C m−2 and 107 g C m−2. Variations occurred among the different species of plant used. Substrate carbon content averaged 913 g C m−2 and after the subtraction of the original carbon content the total sequestration was 378 g C m−2. The sequestration can be improved by changing plant species, increasing substrate depth, substrate composition, and management practices. In a study done in Michigan above ground sequestration ranged from 64 g C m−2 to 239 g C m−2 for S. acre and S album. Also, by increasing the substrate depth would allow for more area of carbon storage and diversify the types of plants with greater potential of carbon storage. The direct carbon sequestration techniques and methods can be measured and accounted for. Green roofs also indirectly reduce CO2 given off by power plants through their ability to insulate buildings. Buildings in the US account for 38% of the total carbon dioxide emissions. A model supported by the U.S. Department of Energy found a 2 percent reduction in electricity consumption and 9-11% reduction in natural gas when implementing green roofs. Through this, a 2023 comprehensive review highlighted that green roofs also contribute to carbon dioxide reduction through both direct sequestration and indirect mechanisms, such as decreasing building energy consumption and mitigating urban heat islands.Other
- Help to insulate a building for sound; the soil helps to block lower frequencies and the plants block higher frequencies
- If installed correctly many living roofs can contribute to LEED points
- Increase agricultural space
- Green roofs not only retain rainwater, but also moderate the temperature of the water and act as natural filters for any of the water that happens to run off.
Costs and financial benefits
With the initial cost of installing a green roof in mind, there are many financial benefits that accompany green roofing.
- Green roofing can extend the lifespan of a roof by over 200% by covering the waterproofing membrane with growing medium and vegetation, this shields the membrane from ultra-violet radiation and physical damage. Further, Penn State University's Green Roof Research Center expects the lifespan of a roof to increase by as much as three times after greening the roof.
- It is estimated that the installation of a green roof could increase the real estate value of an average house by about 7%.
- Reduction in energy use is an important property of green roofing. By improving the thermal performance of a roof, green roofing allows buildings to better retain their heat during the cooler winter months while reflecting and absorbing solar radiation during the hotter summer months, allowing buildings to remain cooler. A study conducted by Environment Canada found a 26% reduction in summer cooling needs and a 26% reduction in winter heat losses when a green roof is used. With respect to hotter summer weather, green roofing is able to reduce the solar heating of a building by reflecting 27% of solar radiation, absorbing 60% by the vegetation through photosynthesis and evapotranspiration, and absorbing the remaining 13% into the growing medium. Such mitigation of solar radiation has been found to reduce building temperatures by up to and reduce energy needs for air-conditioning by 25% to 80%. This reduction in energy required to cool a building in the summer is accompanied by a reduction in energy required to heat a building in the winter, thus reducing the energy requirements of the building year-round which allows the building temperature to be controlled at a lower cost.
- Depending on the region in which a green roof is installed, incentives may be available in the form of stormwater tax reduction, grants, or rebates. The regions where these incentives will most likely be found are areas where failing storm water management infrastructure is in place, urban heat island effect has significantly increased the local air temperature, or areas where environmental contaminants in the storm water runoff is of great concern. An example of such an incentive is a one-year property tax credit is available in New York City, since 2009, for property owners who green at least 50% of their roof area.