In a National Trust experiment, the public are to be allowed into Tyntesfield as it undergoes a major restoration. David Clensy visits the mansion mid-makeover

A nyone who has ever struggled to put up a tent has to sympathise with Tim Cambourne, the senior building surveyor at Tyntesfield.

By the end of the summer, the rambling Neo-Gothic mansion between Bristol and Clevedon will look very different.

Tim and his team are to erect a 52-square-metre temporary roof over the entire building – a sort of giant umbrella, which will be the size of 10 tennis courts. It will enable them to begin the arduous task of re-roofing the 19th century pile.

"Visitors will notice the temporary roof as soon as they walk down the path," Tim says. "It's not something that you could miss.

"We'll be building £16,000-worth of scaffolding around the house which will act like an external skeleton upon which we can mount the temporary roof.

"Much of the existing roof is in a terrible condition and it regularly leaks. But we can't take the risk of lifting sections of the roof without having a major substitute to keep the rain out of the historic building and away from the treasures inside.

"It's a major undertaking," Tim adds, looking up nervously at the building's mishmash of turrets and pinnacles.

"Even the scaffolding will be revolutionary because we don't want to damage the Gothic stonework on the building, so it will be free-standing – it won't actually touch the building at any point." The new roof is just the tip of the iceberg when it comes to the work that needs to be done at Tyntesfield. Tim's team will tackle everything from replacing and restoring the damp decor to rewiring the property's entire electricity network.

The former home of Lord Wraxall, the late Richard Gibbs, Tyntesfield was bought by the National Trust in 2002 to be conserved for the nation.

The reclusive aristocratic bachelor, who died in 2001, had left the property in need of a serious makeover.

Cash from both the National Trust's own coffers and the National Lottery's Heritage Fund will cover the bulk of the £16.5 million renovation, with fundraising events and activities being planned to make up the £1.6 million funding gap.

If you've visited Tyntesfield in previous years, you probably realise the project is very much a work in progress. But 2009 is the year the trust hopes to make its big push on the property – and with the house opening for the new season on Saturday, that means welcoming the public into what could easily be mistaken for a building site.

"We've never attempted to open a property to the public that's been undergoing a restoration on this sort of scale," admits programme director Helen Bonser-Wilton.

"I'm fully expecting a sort of Marmite effect," she adds. "People will either love it or hate it. I think many will be fascinated to see the work being carried out. But there may be some visitors who wonder why they've paid to see a house that's covered in scaffolding and full of people wearing hard hats." In today's world of water-tight health and safety regulations, it may be surprising to hear the trust encourages visitors into a house where carpets are rolled up, floorboards are lifted and power tools litter every room.

"It's something we'll be very aware of," says visitor services manager Rebecca Aubrey as she leads the way through the mansion's labyrinthine corridors.

"We have a wonderful team of more than 500 volunteers on the estate, and with 100,000 visitors expected to come through the doors this year, we know our room supervisors will have an added element to their work.

"As well as interpreting the rooms and introducing visitors to some of Tyntesfield's treasures, they will also need to keep a very careful eye on how close people are getting to the work.

"They'll play a vital role, which will allow our contractors to carry on with their jobs without having to constantly watch out for children picking up their tools.

"Of course, we will also tape-off sections of a room immediately around where the craftsmen and women are working.

"But one of the real benefits from coming to see Tyntesfield at this time is that you will get to see this historic work being done at first hand. You will be able to talk to the crafts people about the project. Much of this is work that won't be carried out again in our lifetimes."

The Victorian mansion was bought by William Gibbs in 1843 after the prospering of the family's guano firm (selling seabird droppings to fertiliser companies) made them one of the richest clans in the country.

The family imported guano from South America to feed the blossoming trade in agricultural fertilisers.

The Gibbs family used their new- found wealth to transform the former Georgian mansion into an ornate neo-gothic 43-bedroom pleasure palace.

But the eccentric design of turrets and pinnacles makes the current restoration a real challenge.

"It's not until you get up on the roof that you realise just how crazy the whole building really is," says Tim Cambourne.

"We're not looking at one roof, in reality, we're looking at dozens of them, each individual in design and each offering its own challenges.

"Take this central roof with the dorma windows," he says, pointing to the building's iconic "frown".

"You can hardly see it at the moment, but the roofing tiles on this slope are actually designed in black and red tiles, which make a complex geometric design.

"That's the kind of thing of which we need to take careful note to ensure we are able to fully reconstruct it.

"In fact, the pattern will stand out much better with the newly restored tiles than it has done for generations."

The rewiring of the property will also be a major challenge. "The building was last rewired in 1953, so it's potentially dangerous and urgently needs replacing," Tim says.

"The sort of wiring we're taking out is a museum piece these days.

"When electric lighting was originally installed in 1890, Tyntesfield was very much ahead of its time. It was one of the first properties in the South West to have electricity and the estate even had its own generators.

"Because it was such a new idea in those days, the way the electrics are laid out is eccentric to say the least.

"That's something our electricians will have to get their heads around over the coming months."

As yet, the upstairs rooms will still be out of bounds to visitors, except for those on special tours as the trust continues to catalogue the vast range of items that fill the property.

"We have everything to catalogue, from the most beautiful pieces of china, all the way down to the Gibbs family's microwave. It may seem like a strange thing for the trust to keep, but to future generations it will be a real piece of social history.

"That's the sort of thing that Tyntesfield is able to offer which makes it truly unique among the National Trust's properties."

Tyntesfield opens to the public for the new season today (Saturday, April 4). It's open Saturday to Tuesday from 11pm to 5pm. Admission to non-members is £10.40 (including Gift Aid) and £5.25 for children. For more information, call 0844 800 4966 or visit www.nationaltrust.org.uk