Two friends have realised a 30-year dream by having the roof of a medieval Oxford church dated - and found it was more than 700 years old.

Dr Martin Henig was a research student at Oxford University and Julian Munby a chorister at St Giles' Church, in St Giles, in the 1970s when they came up with the idea of getting the timber church roof dated using dendrochronology - or tree-ring dating.

But the science had not yet been sufficiently developed and as students, they had no means of paying for it.

Now, as part of a tiling project, the pair have worked with church warden Catherine Barrington-Ward and local tree-ring expert Dr Dan Miles to have the roof dated - to the year 1288.

This is the same century that saw Genghis Khan lead the Mongols to the conquest of most of Asia; and the Crusaders of Western Europe ransacking the Holy Lands - as well as the travels of Marco Polo to China.

Mr Munby said: "This very fine rafter-roof is the earliest visible in Oxford and is an excellent specimen of medieval oak carpentry, demonstrating typical techniques used in the late 13th century.

"Although the Chapter House at Christ Church has a roof with different jointing methods dating from 20 years earlier, it is hidden from public view by vaulting."

Mr Henig added: "This roof is one you can show to anyone who comes into the church. One of the timbers even had the edge of the tree which meant we could get an exact date for when it was felled."

The dating method cost £650 - £150 of which was provided by a grant from the Oxfordshire Architectural and Historical Society.

Society president George Lambrick said: "This is just what our scheme for tree-ring dating old buildings in Oxfordshire is meant to achieve. Knowing that the timbers were felled in the spring of 1288 is so much more vivid than just saying they are probably late thirteenth century."

The vicar, the Rev Andrew Bunch said: "We are delighted to know more about this ancient place of worship - it is a very telling testimony to the skill, dedication and spiritual inspiration of our medieval forebears."

Mr Munby believes the roof was provided by Godstow Nunnery, as rectors of the church, to complete a major rebuild 720 years ago.

He will be giving a free talk about medieval roofs in Oxford, in the church at 12.30pm tomorrow.

St Giles' Church ran a competition to guess the date of the roof beams as part of a fundraising effort.

The winner, who picked up two return tickets to New York from Virgin Atlantic, was John Pusey, who has been a bellringer at the church for 30 years.

Runner-up was Genefer Clark, of Walton Street. She won a crate of Champagne donated by Oxford Capital Partners.