Benefits of Green Roofing

Waterproofing Durability

The addition of a green roof acts a protective covering for the waterproofing system, protecting it from UV radiation, thermal shock and atmospheric pollutants. Due to the protection from these aggressive factors, as an industry rule of thumb, the green roof will help to roughly double the life expectancy of the waterproofing.

BREEAM Ratings

BREEAM (Building Research Establishment’s Environmental Assessment Method) is a commonly used method of establishing a buildings environmental performance. It sets the standard for best practice in sustainable building design, construction and operation and has become one of the most comprehensive and widely recognised measures of a building’s environmental performance.

BREEAM assessments can be carried out on new construction, major refurbishment to existing buildings, new construction to an existing building (e.g. an extension) or a combination of new construction and major refurbishment to an existing building and an Existing building fit-out. There are also a variety of BREEAM Schemes for different types of building usage, including courts, education, industrial, healthcare, offices, retail, prisons, multi-residential and data centres. Each of these differs in scope and technical criteria.

Biodiversity
In October 2010, in Japan, World governments agreed to the Strategic Plan for Biodiversity 2011-2020 and the Aichi Targets as the basis for halting and eventually reversing the loss of biodiversity of the planet. To build support and momentum for this urgent task, the United Nations General Assembly declared the period 2011-2020 to be “the United Nations Decade on Biodiversity, with a view to contributing to the implementation of the Strategic Plan for Biodiversity for the period 2011-2020” (Resolution 65/161).

Green roofs provide valuable support to the UN objectives, enabling the utilisation of the generally unused roof space to deliver valuable environmental benefits.

Biodiverse green roofs can be designed to create living space for flora and fauna, supporting local or national Biodiversity Action Plans (BAPs). They can enhance the buildings sympathy with the surrounding landscape, replace lost biodiversity and providing additional BREEAM credits under the Land & Ecology category.

Additional features, such as timber, shelter, stones and other roofscape furniture can be incorporated into the roof design to achieve particular ecological objectives, such as attracting invertebrates.

Drainage and Flood Risk

The Council for the Protection of Rural England (CPRE) has calculated that an area of green space equivalent to one hundred football pitches has been lost to development in England every day since 1992 (Meadow madness: why the loss of England’s grasslands continues uncontested, 1998).

This development replaces permeable ground with artificial ‘hard’ surface through which rainwater cannot permeate and, with up to 95% of ground space rendered impermeable in a densely populated area, flooding is the result.

Most roof surfaces, like ground level surfaces, shed rainwater immediately, resulting in drainage systems being further subject to abrupt increases in rainwater flow rate and volume.

Substrate based green roofs provide a significant contribution to the control of water run- off at source, reducing the quantity of rainwater run-off compared to exposed or stone ballasted roofs surfaces. On average a green roof system retains 35% of the yearly rainfall and extends the time taken for the remainder to leave the roof.

Retained water will be subsequently released to the atmosphere through;

  • Photosynthesis: the uptake of water from the substrate and biochemical incorporation by the plants.
  • Transpiration: the uptake of water from the substrate and release to the atmosphere by the plants.
  • Evaporation: of water from the substrate by the sun.
  • Evapotranspiration: wind driven evapotranspiration from substrate and plant layers.

Solar PV Performance

The inclusion of a green roof with solar PV panel installations provides a positive influence on the performance of the solar units by preventing the overheating of the panels that leads to reduced energy production.

Roof Temperature Increase +25°C +30°C +35°C +40°C +45°C +50°C +55°C +60°C +65°C +70°C +75°C
Electricity Generation -0.50% -2.50% -5.00% -7.50% -10.00% -12.50% -15.00% -17.50% -22.50% -22.50% -25.00%

As air rises during the day dark roof surfaces absorb and re-radiate solar radiation (see Albedo Affect). The addition of a green roof prevents the roof temperature elevating, for example, under British climatic conditions, work at Trent University indicates that planted roofs can have markedly lower temperatures compared to the unplanted variant. Typically with a mean daily air temp of 18.4°C an unplanted membrane roof will have a surface temperature of 32°C compared to 17.1°C for a roof covered with a 100mm extensive type plant build up.

Evaporation of retained moisture will also help keep the PV panels cool and at optimum operational temperature.

Carbon Capture
A two year small scale study of 12 sedum green roofs conducted by the Michigan State University found that the roofs absorbed up to 375 grams of carbon dioxide per square. Whilst the volume sounds small if a city of around 1 million inhabitants greened all of its roof space it would remove as much carbon from the atmosphere as taking 10,000 mid-sized SUVs and trucks off the road for a year.

The study did however take a rounded view of the benefits and stated that due to carbon footprint of the green roof materials it takes seven years for the roof to offset the carbon used for its construction and become truly carbon negative.

Albedo Effect

With urban density increasing the Albedo or urban heat island (the solar heating of an urban environment) effect is impacting on UK cities at an ever-increasing rate, manifesting itself through City’s having a higher air temperature than the surrounding suburban or rural areas.

A significant contribution to reducing the Albedo effect can be made by simply introducing a green roof to a building and utilising the thermal mass, and release of retained moisture into the atmosphere through;

  • Photosynthesis: the uptake of water from the substrate and biochemical incorporation by the plants.
  • Transpiration: the uptake of water from the substrate and release to the atmosphere by the plants.
  • Evaporation: of water from the substrate by the sun.
  • Evapotranspiration: wind driven evapotranspiration from substrate and plant layers.

Thermal Performance

In Summer months Green roofs absorb great quantities of solar radiation through the growth of plants and evaporation of water from the substrate and plant tissues. These factors negate the high daily thermal temperature swings at roof level reducing:

  • Heat Flow Into The Building During Summer Months.
  • The Need For Air Conditioning
  • The Production Of Carbon Dioxide Produced.
  • The Albedo (urban heat island) affect.

In Winter months the green roof provides thermal protection, although this is affected by the water content of the substrate (the higher the water content the higher the thermal conductivity). However UK cold condition studies carried out by Trent University indicated that on a day with a mean temperature of 0°C the temperature under the roof membrane of an unplanted system fell to –0.2°C whilst the temperature under the membrane beneath a 100mm extensive type green roof had a mean value of +4.7°C.

Factors believed to contribute to this thermal benefit include:

  • Reductions in wind strength across the roof surface due to plant layer.
  • Boundary/ still air layer within the planting/above the substrate surface.
  • Root respiration: a biological process within the substrate that keeps the soil temperature above that of the air.
  • Thermal insulation value afforded by materials in the green roof system.

Acoustic Performance

The addition of a green roof achieves benefits in both sound reduction and sound absorption.

Sound Reduction

Green roof substrate and planting (particularly sedum) provide mass, this is of critical importance where sound reduction is required. The green roof construction provides a natural sound barrier that can be tuned with the underlying construction.

Sound Absorption

Green roofs provide a naturally soft outer surface through their planting, providing a “built-in” solution to an age old problem of drumming noise created by rainfall on the external roof surface (such as sport halls).

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