A building with a turf roof and straw walls has been dubbed one of the most environmentally-friendly in the country.

The Ecology Building Society headquarters in Silsden has won a top award being named the most sustainable building in the country.

It took the accolade in the small projects section of the competition run by Building magazine and presented at a glitzy ceremony in front of 700 people at the London Hilton.

The two-year-old building, in Belton Road, is constructed out of re-claimed and re-cycled materials, features renewable energy systems and has a living roof - lined with moss.

So effective is the structure at saving precious resources that Yorkshire Water was fooled into believing the water meter failed to work properly.

Jenny Barton, the society's marketing manager, revealed: "Yorkshire Water has come down twice because the meter reading is so low - they thought there was a fault."

"But they failed to take account of the water-recycling system, which involves filtering water from the turf roof and using it to flush the toilets."

The natural roof was constructed with the aim of replacing the habitat lost to wildlife when the building was constructed.

It is also more resistant to the changes in the seasons and climate.

The roof design is just one of a number of initiatives which help maintain its ecology-friendly pledge.

The latest is the new extension - a circular meeting room built of straw and, to keep it cosy, insulated with sheep's wool.

"The roof is made of cedar and the walls are straw, plastered on the outside with lime and clay inside - completely natural materials," said Miss Barton.

"When finished it will have no heating system because the straw and the roof are such good insulators. The place will be air tight. Just the light bulbs will be enough to heat it.

"We invited people round to help build the straw walls and with the plastering . Some people even camped on the site to help.

"The rest of the building doesn't need a lot of heating. The equipment, lighting and people generate enough.

"And of course we don't have air conditioning. It's great - it means we don't share colds as in other air conditioned buildings where, when one person is sick, the rest of the staff get it."

The main building was completed two years ago and enabled the staff - now 25 - to move from the old headquarters in nearby Cross Hills.

The walls are made from local stone and much of the timber structures use a special compressed substance manufactured from bark.

There are re-claimed bricks and clever lighting with sail-like shades which reflect the light, avoiding eye strain.

The floor is covered with linoleum made from natural linseed oil and hessian.

Chief executive, Paul Ellis, said: "We are delighted with the award and it is in recognition of the part we are playing in supporting cutting-edge "green" building and design".