TIMBER from trees felled in Ireland was used to build some of the magnificent roofs of Salisbury Cathedral, new research has revealed.

The wood was probably imported after a dispute halted local supplies, say scientists, whose new timber dating techniques prove the cathedral roofs are among the oldest in Britain.

Hidden above the stone vaults is one of the earliest crown-post roofs in the country and the earliest carpenters' marks in Arabic numerals ever discovered in the UK, they say.

The secrets of the cathedral were disclosed this week when a team of English Heritage experts reported the results of their tests.

They told how important advances in tree-ring dating (dendrochronology) techniques carried out for English Heritage have allowed scientists to confirm that much of the oak timber used in the two exceptionally fine roofs of the eastern chapels was felled in the spring of 1222 in the Dublin area.

In what was the 13th century equivalent of every builder's nightmare, the cathedral's original carpenters may have run out of home-grown timber because of a dispute between the chief carpenter Godardus and the warden of nearby Clarendon Forest.

And the cathedral had to import trees in order to keep pace with stonemasons building walls to support the vaults.

The new precise dating, say the experts, supports a 1224 record of the dispute and the subsequent importation from Ireland by a man called William of Dublin.

The wood also bears out documentary evidence that the chapels were completed in 1225 and shows that their roofs were designed as an integrated whole - the same trees, or at least from the same forest, appeared to have been used in both.

Peter Marshall, of English Heritage's scientific dating service, said: "The findings are among a raft of significant results to come from research commissioned for a programme of major repairs grant aided by English Heritage and the Salisbury Cathedral Trust.

"They will greatly increase our understanding of major historic buildings and are likely to have a profound effect on how they are repaired in future."

One of the most remarkable discoveries is that batten boards, used to support lead in the roof on the North Nave Treforium - described at the "most splendid medieval lean-to roof in any English cathedral" - were not post-Reformation replacements but part of the original 13th century fabric.

Incredibly, some have survived more than 750 years - even though exposed to the weather during the bleak Civil War years, when the lead would have been stripped away.

Dan Miles, of the Oxford dendrochronology laboratory and who has undertaken the dating work for English Heritage, suspected the boards' very early date.

He said: "In the past, boards like this would have been disregarded because they were generally thought to be replacements for the old ones thrown out when lead was stripped.

"Dendrochronology is beginning to show more and more detailed information about the importance of such material."

Analysis of the wood in the north porch, described by Dan Miles as probably the "jewel in the crown of all the Salisbury roofs", has revealed the earliest and one of the finest crown-post roofs in the country.

And the great oak west doors to the nave are made entirely of boards from the Irish wood.

Tim Tatton-Brown, consultant archaeologist to Salisbury Cathedral, who has conduced extensive research into its history and initiated the dating project, said: "This very important new series of dates from dendrochronology has given us for the first time an independent sequence of dates for the whole of the cathedral."