The oak is believed to come from a forest close to Canterbury
Archaeologists working Canterbury Cathedral have had parts of the structure dated to the time of William the Conqueror and the Domesday Book.
It had been widely believed the oak rafters in part of Anselm's Tower in the cathedral's south east transept were from medieval times.
The latest study confirmed the wood was from trees cut down in the late 11th Century.
Restoration work is currently being carried on Anselm Tower.
Nottingham Tree-ring Dating Laboratory was commissioned to date the oak supporting the structure's lead roof.
Their team of experts drilled small holes in the timbers before assessing their age using dendrochronology, which looks at the tree-rings contained within wood.
Rupert Austin, of Canterbury Archaeological Trust, said: "We have known for some time that this was one of the oldest parts of the cathedral, that we have this great medieval spire up there.
"But until the lead roof came off to be replaced we weren't able to have a proper look at it and do these tests."
He added: "The vast majority of the cathedral roof has been destroyed and rebuilt over the years through various mishaps, mainly fires, so there are very few sections of medieval roof that remain.
"Dendrochronology is generally accurate to the year. If it's a good sample it can tell you when the timber was felled."
The oak is believed to come from a forest close to Canterbury.
The 900-year-old cathedral is currently aiming to raise £50m for renovation works and recently received £460,000 for work on its library roof and windows.