A Green Roof Over Your Head

Roofs are valuable yet underutilised assets, I can’t help feeling that sometimes they are a bit ‘out of sight, out of mind‘. Encouragingly, I’ve been noticing more and more mentions of green roofs recently. I was also inspired and slightly surprised to spot a veritable jungle growing on the top of a high rise apartment block in Buenos Aires. This prompted me to find out a little bit more about green roofs and I thought I’d share with you my four favourite findings.

Green Roof collage A Green Roof Over Your HeadFirstly a little bit of background to put things in context:
Just under 3% of the Earth’s surface is covered with buildings. Conventional dark rooftops absorb and re-radiate the sun’s energy as heat, making them a major contributor to the phenomenon known as the Urban Heat Island Effect (UHIE) which is the difference in temperature between urban and rural areas. Urban areas have higher average temperatures than rural areas, caused largely by the replacement of green space with hard surfaces which have a high thermal mass. Other factors also contribute, such as cars, people, AC units but the buildings themselves are the major contributor.

Why does this matter? Well, roofs that absorb heat will themselves heat up and probably require cooling or additional refrigeration which consumes energy. In order to help meet current energy efficiency targets this is an area that can easily be addressed. Environmental issues aside… there’s money to be saved too. An alternative and effective way to reduce the UHIE is to paint roofs white which reflects more of the sun’s energy. But I’m more interested in green roof solutions as their benefits seem to far outweigh buying a can or two of white paint.

But what is a green roof? There are two main types of green roofs:
1. Intensive – using deep soil (normally from 150mm up to a metre in depth) capable of growing shrubs and even trees,
2. Extensive – using shallower soil (60-200mm depth) with a thin layer of vegetation such as sedum, moss or grasses. They are generally lower cost.

Green roof systems vary but generally comprise; a barrier layer to prevent plant roots penetrating the surface of the roof, an insulating layer, drainage matting (which can feature recesses to hold excess water from heavy rainstorms to be used by the plants in drier conditions, conserving water use), a thin layer of felt to prevent the drainage mats from filling up with soil and of course the growing substrate, such as lightweight soil developed for rooftops.

What are the advantages?
Lower energy consumption and a longer lasting roof – The plants in a green roof regulate their temperature via evapotranspiration and this evaporation of water has a cooling effect, keeping the roof at a near ambient temperature. This prevents the building itself from absorbing the sun’s energy and heating up, which in turn reduces the energy required to cool the building. This process also means that the roof will experience less extreme temperature cycles and therefore less thermal expansion and contraction stresses. This has resulted in predictions that a green roof could potentially double the life expectancy of a conventional black roof. A green roof therefore effectively acts as an insulator.

Green roofs can form part of an effective sustainable drainage solution – by reducing the amounts of storm water run-off and reducing peak flow rates. According to the Greater London Authority’s ‘Living Roofs and Walls’ technical report, in summer a green London roof can retain between 70-80% of rainfall run-off. A green roof city has the knock on potential to reduce the risk of localised flooding downstream and on a larger scale the need for expensive underground drainage infrastructure.

Other benefits include; the absorption of CO₂ and production of O₂, improved air quality, sound insulation, creation of social and/or productive areas, increased local organic and organic production of grown produce and of course the most obvious benefit of green roofs… increased biodiversity.

All this information starts to make sense when you look at some examples of green roofs, of which there are a huge variety varying in size and purpose. Here are four green roofs that caught my eye:

The London Climate Change Partnership state that “Summers by 2050 will be 1.5 – 3.5°C hotter… in central London the urban heat island currently adds 5 – 6°C to summer night time temperatures and will intensify in the future.” The Greater London Authority’s ‘Living Roofs and Walls’ technical report provides information for a building in Canary Wharf, suggests that “an 850m² retrofitted green roof has achieved an estimated reduction of 25,920kWh (11.46 CO₂ emission tonnes) a year through a reduction in heating and cooling of the spaces below the roof. The green roof was estimated to be saving up to £4,000-£5,000 per year in electricity“.

4 Lillie Rd Fulham London A Green Roof Over Your Head

Photo: Mathew Frith

Beaufort Court, Lillie Road in Fulham is a Peabody Trust social housing development. Completed in 2003, its sedum roofs reduce water run-off and are an award winning example of how green roofs can provide a simple, functional alternative to conventional roofing. They also demonstrate that green roofs don’t necessarily have to be green. Sedum is a succulent and can therefore store water in its leaves making it a low maintenance and hardy option for roofing.

But green roofs don’t have to be huge projects, I love this High Barnett garden shed roof. Click here to see a series of photos on Flickr showing the process.

New York
Conventional dark roofs in New York can reach 80°C at 1pm, even outside of high summer. The New York Heat Island Initiative worked out that an average 0.1-0.8°C reduction in surface temperatures could be achieved by providing 50% green roof cover within the metropolitan area. An 0.8°C reduction equates roughly to 396million KWh of energy being saved. Based on these findings New York now offers a tax rebate for building owners (within certain criteria) who install green roofs within the metropolitan district.

3 Brooklyn Grange NY A Green Roof Over Your Head

Photo: Cyrus Dowlatshahi

Brooklyn Grange is a commercial, organic, vegetable farm located on New York rooftops. Their goal being to “improve access to very good food, to connect city people more closely to farms and food production, and to make urban farming a viable enterprise and livelihood“. Brooklyn Grange has a 10 year lease from Acumen Capital Partners for their one-acre (40,000ft²) farm in Queens. The farm has been created with roughly 545 tonnes of lightweight soil and over 20,000 linear feet of green roofing material. No mean feat.

Toronto was a pioneering city in the use of green roofs and is the first city in North America to have a bylaw, brought in by Toronto City Council in May 2009, to ‘require and govern the construction of green roofs on new development’. The Bylaw applies to residential, commercial and institutional developments and requires new buildings to install green roofs covering a percentage of their roof area. For example, a building with a 2,000-5,000m² footprint would be required to install a green roof covering at least 20% of its roof area, a building with a 20,000m² footprint would need to install a green roof covering at least 60% of it’s available roof space.

1 MEC Toronto A Green Roof Over Your Head

Photo: Susanne Jespersen

The Mountain Equipment Co-op constructed its retail store in downtown Toronto incorporating a substantial green roof in 1998 (ten years before the city’s bylaw came into effect). All plants covering the 6,500ft2 ‘extensive’ green roof are native to Canada and suited to prairie conditions. This means that once established the eco-system can sustain itself as it would in the wild. MEC are true pioneers of green roofs, many of their retail outlets sport green rooftop installations, and award-winning ones at that!

Apparently, the average annual temperature in Tokyo has risen 3°C in the last century. The Organisation for Landscape and Urban Greenery Technology Development, based in Tokyo, estimates that ‘if half of the roofs in the city were planted with gardens, daytime temperatures in summer would fall by 0.84°C, saving 110 million Yen on air conditioning costs‘. Only 14% of central Tokyo has any planted or green areas, which is less than New York City or London (Brooke, New York Times, 2002). So it’s no surprise that the megacity has introduced policies requiring the installation of green roofs covering 20% of all new flat surfaces (over 1,000m²) on government buildings and covering 10% of all flat roofs on private dwellings.

1 ACROS Fukuoka A Green Roof Over Your Head

Photo: KnOizKi

ACROS Fukuoka, designed by Argentinian Architect Emilio Ambasz and Associates in 1995, is a 15 tiered office and retail building in the centre of Fukuoka, Japan. Using 30,000 plants of over 70 local species, including short trunked species to withstand typhoons, Ambasz created a sustainable (although it is supplemented with black water recycling at times of drought) space for the building’s tenants to use. Check it out on Google Maps’ satellite view. Impressive.

So, what are the considerations for installing a green roof?
Green roofs generally require planning permission, which can unfortunately be a slow process. For existing buildings a structural survey is also required to ensure the roof’s structure can bear the excess weight of a green roof. Once installed a green roof will require a certain amount of maintenance, although the extent of this depends upon the type of green roof installed. An ‘extensive’ sedum roof requires very little maintenance, typically less maintenance than a conventional flat roof, however an ‘intensive’ roof garden will require a much higher degree of maintenance.

If you are in the UK and want to find out more about green roofs the check out LivingRoofs.org which has a wealth of information on green and living roofs. If you’re considering creating your own green roof then their sister site, the Directory of Green Roof Resources, provides details of roofing contractors, green roof manufacturers and suppliers, architects and landscape designers who are active in the green roof industry.