Installing a green roof

Last updated Sunday 5 January 2003

Grow through the roof

Green roofs may have many benefits in warmer, drier countries, but are they any good in the cold, rain-soaked UK?

Talk to any green roof merchant and you'll wonder why you don't already have one. Not only are they 'superb' insulators, but they also apparently save endangered species, reduce flooding and extend your roof's lifespan. Green roofs even stop you getting headaches. Or so they say.

So why don't we all buy one? Because when it rains (which it does in Britain, 154 days a year), the science shows that green roofs don't save energy. In fact, research suggests that green roofs actually cool a house in the winter, requiring more heating and higher CO2 emissions.

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Photo: Installing a green roof

Doing this action could save 30 kg of CO2 a year

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In this article:

What is it?

A green roof is a roof consisting of various waterproofing and drainage layers topped with soil and plants. The plants can either be planted by hand on top of the roof, or you can buy readymade 'blankets' of vegetation that you simply roll out.

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How does it make a difference?

Greening the roof of an average home in Britain saves five times less CO2 than switching all of the lights in your house to energy-saving bulbs.

In wet climates, green roofs lose their ability to insulate. The air pockets in soil that normally trap heat are expelled when the soil is water-logged. What's more, green roofs actively lose heat when they're wet because plants evaporate moisture and cool their surroundings. That's why a green roof can actually emit 40% more heat in the winter than a regular insulated roof, according to research published in the journal 'Energy and Buildings' 2005. So unless you live in one of Britain's 'blue holes', to the east of a mountain range, a green roof is unlikely to cut carbon in the UK.

That's not to say that green roofs are a waste of time everywhere. Outside Britain, in hot, dry climates, the insulation provided by a green roof can significantly reduce how much air-conditioning is needed. In fact, a dry green roof can keep a home 4C cooler when it's over 25C outside, reducing how much energy your air conditioner uses by two-thirds, according to one Canadian study. Eco-roofs reflect sunlight more effectively than regular roofing materials, and they also use heat to evaporate moisture rather than trapping it in your home.

One way a green roof can reduce emissions is if you use it to grow your own fruit and veg. The Fairmont Hotel in Canada purportedly saves approximately 18,000 every year by growing its own greens on a grass roof.

Climate aside, green roofs offer a range of other perks:

  • They provide shelter for wildlife. One Swiss study found that 18% of the spiders sheltering in the green roofs examined were endangered. Bumble bees are attracted to wildflower roofs, according to research at Sheffield University
  • Green roofs soak up rain and delay runoff. In practice, this means less flooding, fewer overflowing sewers and less soil erosion. According to English Nature, green roofs soak up 75% of falling rainwater in the short-term, falling to 20% over the subsequent two months
  • Plants - whether they're on your roof or in your garden - improve air quality by removing pollutants like nitrogen oxides, airborne ammonia, and sulfur dioxide.
  • Green roofs can extend the longevity of a roof membrane by two to three times, according to the industry association, Green Roofs for Healthy Cities. A green roof shields the underlying bituminous roofing membrane from UV radiation and the daily stresses of expansion and contraction caused by thermal fluctuation
  • Having a flat green roof provides excellent protection from noisy aircraft fly-overs, according to 2009 research by Ghent University.
  • Green roofs could potentially make you happier. Research suggests that having a view of nature can reduce stress, blood pressure and muscle tension

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What's stopping me?

Your roof should be pitched at no more than 30 degrees, which rules out a large percentage of British housing that is semi-detached (one-in-three houses are semis).

The cost may also be a problem. While a basic roof can be rolled out for about 80 per square metre, an 'intensive' roof that can support trees and human activity can cost from 120 to 400 per square metre. On top of that you'll need to spend time and money weeding, mowing, trimming and watering. On the roof. Read more about costs in the Greater London Authority’s report.

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How do I do it?

Pub Fact

  • A 40-60 cm layer of grass grown on 20 cm of substrate can provide as much insulation as 15cm of synthetic mineral wool according to the green roof expert, Steven Peck
  • Greening three million square metres of Central London’s roofs could cut emissions of CO2 from energy use by over 8,000 tonnes - roughly as much as skipping 3,700 return flights to Thailand - according to a Greater London Authority estimate
  • English Nature estimates that 200 million square metres of existing roofs in Britain could be 'greened' with very little structural change
  • Green rooves soak up 75% of falling rainwater in the short-term, falling to 20% over the subsequent two months, according to English Nature
  • Green roofs were used by the military during World War Two to camouflage aircraft hangars
  • UK homes account for about a third of the total amount of energy used in the UK
  • One species of Sedum, a popular plant found on green roofs, was found to survive over two years without water in a greenhouse
  • Growing 30 cm of grass on about as much soil can insulate your roof as effectively as installing 15 cm of mineral wool insulation - but only when it's dry
  • Two thousand square metres of unmowed grass on a roof can trap four tonnes of particulate matter (airborne dust)
  • Check that your roof is pitched at less than 30 degrees
  • Find out how much weight your roof can support. This will determine how thick a layer of soil you can have in your green roof, what size of plant you can use, and whether you can walk on it. Most roofs can carry loads of about 75 kg per square metre, and the lightest green roofs weigh about 55kg per square metre. A green roof typically needs more than 15cm of soil to support large plants and trees
  • The more soil your green roof supports, the more insulation it will provide, but only if it stays dry
  • The lowest impact way to 'green' your roof is to allow it to be naturally colonised by native plants. It's worth bearing in mind, however, that your native plants may not be able to withstand the stress of living on a roof
  • Choosing your vegetation carefully can make a difference. Uniform evergreen vegetation provides more insulation than mixed vegetation like wildflower meadow, which tends to collapse to form a mat of vegetation with few air pockets. Vinca minor is one of the most effective plants at insulating, as are succulent species like sedum, sempervivum and delosperma. If your aim is to attract a particular species, make sure the vegetation reflects the species' needs
  • Plant in the spring or at the start of autumn to allow the plants to establish themselves before the frost arrives
  • Avoid fertilising your green roof. It encourages the 'wrong' type of plants and increases their vulnerability to drought stress, according to research published in HortTechnology, 2006

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Comments

Theresa, Suffolk 2009-05-30

We don't need to heat insulate our flat roofed garages. Maybe it should be part of building regs to extensively green your garage roof.

Baz, Shresbury 2009-02-12

It appears that the only advantage of a green roof in the UK is if you want to collect rainwater from it. The soil or growing medium will pre filter the water before it leaves the roof. It does seem a very costly way of doing this.

The other reason for having a green roof is to conceal the building, perhaps it may have been a necessary consideration for planning permission to be granted in a sensitive area.

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Glossary terms used on this page
Nitrogen oxides
Nitrogen oxides are compounds of nitrogen and oxygen, two elements that do not normally react with each other but will do so during high temperature combustion – such as in a car engine. Examples include nitric oxide (NO) and nitrogen dioxide (NO2), which contribute to air pollution, and nitrous oxide (N2O), which is a major greenhouse gas. Although its warming effect is far less than CO2, it persists in the atmosphere for far longer, so measured over 100 years its impact is 298 times greater.

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