WHEN it comes to unearthing the archaeological secrets of the past, digging underground is usually a good place to start. Or is it?

This year, the latest project at Chiltern Open Air Museum shows us that we needn't look to the ground to find out how our ancestors lived.

Rather, we can look upwards into the medieval-styled thatched roof that has just been reconstructed at the museum site in Chalfont St Giles.

For medieval thatched roofs are like an archaeological site, telling us many things about how our ancestors lived.

"They tell us a lot about the agriculture of the time," says John Letts.

John is an historic thatch consultant for the project, which is near completion and which began just before Easter when the museum acquired a 500-year-old medieval barn from Carters Hill Farm in Arborfield, Berkshire.

The barn is one of more than 30 rescued historic buildings housed at Chiltern Open Air Museum.

A thatch was put on the building in 1981 but it had deteriorated to such an extent that it needed a completely new coat of thatch.

But rather than employing today's method of using water reed to thatch the roof, the museum called in the help of experts John and his colleague Keith Quantrill, as well as local thatchers Roger Evans and Nick Surridge, to recreate the kind of thatching method that would have been used when the barn was first built.

"As far as we know, we are the first organisation to attempt to reconstruct medieval thatching for several hundred years," says Sue Shave, the museum's director.

Visitors to the museum in the next few weeks will be able to see the restoration work in progress and find out much more about the ways in which this type of thatching is carried out.

The barn shows how medieval buildings were thatched using two distinct layers: a base coat and an outer coat made of straw.

"The base coat was made out of whatever was available at the time," explains John.

This means anything from wood shavings that were a by-product of the barrel-making industry to local vegetation such as heather and bracken were used.

Broom, rushes, bracken, gorse, heather, bramble, and birch (from the museum site) can all be seen in the inside of the museum's barn, which resembles a patchwork quilt.

Just 250 examples of original base coats now survive in the UK, containing well-preserved examples of ancient cereals and crop weeds, many of which are now extinct.

John, however, has re-bred some 30 varieties of medieval wheat and rye especially for use on the outer coat of the museum barn. These were grown especially on a farm in Prestwood.

The result of this local crop is the straw that visitors can now see on the outside of the barn.

And thatchers Roger and Nick have been careful not to bind the roof too tightly.

"The problem with thatching is that if it's too tight it rots. It's a living roof and it needs to dry out quickly, so the water tension has to be right," explains John.

The thatchers have also used medieval methods to tie the first layer of straw onto the rafters of the barn, including Old Man's Beard, clematis, twisted hazel rods, dethorned brambles and willow.

"As long as it holds and ties it does the job. You can tie willow into rope and it lasts for 500 years," says John.

The team believe that we can learn a lot from these old methods, particularly from an environmental point of view.

Once a layer of thatching had worn down, our medieval ancestors would simply add on another layer meaning that some of the old roofs would eventually be up to two metres thick.

"It's a bigger roof every time you thatch it," says Keith.

"We have to look at the insulation value of roofs this thick," adds John.

It is no longer economically viable to grow crops just for thatching straw, which is why water reed is now often used to thatch roofs.

But as the Chiltern Open Air Museum show us, our ancestors were very innovative.

And so with the help of sponsors Lafarge Aggregates Ltd, the museum intends to make the barn a Lifelong Learning Centre for schools and adult education programmes.

"Chiltern Open Air Museum is very proud to be leading the way in the field of thatching restoration work," says Sue Shave.

For more details on the project or for the opening times of Chiltern Open Air Museum in Newland Park, Gorelands Lane, Chalfont St. Giles, telephone: 01494 871117