Some of the lead will be recycled (picture Jason Dodd Photography)
Work to repair the roof of Canterbury Cathedral's south-east transept has been started.
The project to replace 20 tonnes of lead covering will cost £500,000 and is expected to take 20 weeks to complete.
Scaffolding currently encircles the transept, which was built between 1096 and 1130, and is one of the oldest parts of the cathedral.
A £50m global fundraising campaign was launched in 2006 to restore the 900-year-old landmark.
About £7.5m has been raised so far.
The transept's lead roof, which is last thought to have been replaced in the late 19th Century, is failing because of weathering, pollution and rotting roof timbers.
Once the stripped metal has been recast it will make up 60-70% of the 30 tonnes of lead needed to cover the roof.
Director of works at Canterbury Cathedral, William Roe, said: "It's a fascinating and important task to be working on that has taken months of planning.
"Like many things at the cathedral it is on such a vast scale and we have set ourselves a challenging target of 20 weeks.
He added: "If the weather decides to be kind to us it would make things much more straightforward.
"Once this roof is complete, attention will then turn to the stone masonry on the transept and we are already planning the investigatory work for the conservation of the famous Bell Harry Tower."
Fundraisers have said deteriorating stonework and a leaking roof mean parts of the Kent cathedral might have to be closed unless the cash is raised.
The cathedral, which was founded by St Augustine in 597 and is visited by about a million people each year, has been damaged by age, pollution and World War II bombs.