Rodney and Diana Lord are not classic eco-warriors. They live in the middle of a sea of Victorian semis in southeast London. They wear loafers and smartly pressed shirts. And while they know they really should use energy-saving light bulbs, they just “haven’t got around to it”.

Yet in 2002 the couple became unwitting ecological trendsetters when, on a plot of land in Camberwell, they built a thoroughly modern house with a thoroughly modern roof.

The flat roof, covered in hardy sedum plants, acts as a natural insulator, keeping the house warm in winter and cool in the summer. When it rains, the plants absorb more than 50 per cent of the water, helping to prevent the kind of deluges that have wreaked havoc over the past few years. The rest of the water drains off and enters a rainwater-harvesting system where it is stored in an underground tank and pumped up for household chores.

“We didn’t design it with the environment in mind,” Mr Lord, 62, admitted as he stood on his roof. “It was about comfort, convenience and aesthetics. But now we find the environmental advantages really satisfying.”

Six years on, and Britain is rushing to catch up with the Lords. As the dual consequences of climate change — flooding and drought — manifest themselves with alarming regularity, and with water bills set to increase by about 6 per cent this year, homeowners, businesses and local and national authorities are looking to their roofs to manage water and cut costs.

As a result, Britain is experiencing one of the biggest booms in Europe in the “green roof” and rainwaterharvesting industries. Sales of rainwater-harvesting systems have more than doubled every year for the past four years. An industry worth about £500,000 in 2004 has now grown to more than £10 million.

The number of green roof companies has increased fivefold in as many years. Where fewer than 10,000 square metres of sedum blanket were laid annually, that figure is now approaching 100,000.

“We’re rapidly running out of water in Britain,” said Roger Budgeons, director of Rainharvesting Systems. “Water supplies are under severe stress, and people are more aware of the issue.”

In April last year, the Government’s code for sustainable homes made it mandatory for all publicly financed new buildings to cut water usage from 150 litres per person per day to 105 — a target most easily achieved through rainwater harvesting.

This month this became a requirement for privately financed social housing as well, and insiders predict that water legislation for private homes and companies could be introduced within a few years.

Many organisations have ploughed ahead in anticipation. Hundreds of thousands of employees and customers at Marks & Spencers, B&Q, Tesco, Asda and Sainsbury’s, as well as prison inmates, are already flushing waste, washing floors and watering plants with rainwater.

Buried in the ground beneath these buildings are storage tanks the size of tennis courts. When it rains, water pours into the systems at the rate of one bathtubful per second.

Lisa Farnesworth, a director at Stormsaver, which has installed many of these rainwater harvesting systems, said that the companies would be reducing their mains water usage by up to 80 per cent and saving more then £10,000 a year on their water bills. In private homes, where smaller harvesting systems cost about £2,000 to install, homeowners can cut their water costs by 40 per cent.

Green roofs might not reduce your water bills but, according to Dusty Gedge, the founder of Livingroofs.org and president of the European Federation of Green Roof Associations, they can cut your air-conditioning bills.

As well as absorbing up to 60 per cent of rainwater, green roofs can lower the surrounding air temperature by up to 11C. “2005 and 2006 were exceptionally hot summers, particularly in London and Manchester,” Mr Gedge said. “Comet was selling one air-conditioning unit a second. But if we all had green roofs, we wouldn’t need air-conditioning.”

Just last month, Britain’s first policy on green roofs was produced by the Mayor of London. Sheffield, Bristol, Manchester and Birmingham are all soon to follow. “There are thousands of square metres of roof going up all over the place,” Mr Gedge said. “It’s happening. It’s really happening.”

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