Castle Drogo started leaking only two years after it was completed
One of Devon's best known historical houses needs £10m to stop it leaking amid fears a ceiling could collapse.
Castle Drogo, which was finished in 1931, has suffered for years from damp caused by problems with the roof, windows and walls.
The leaks, made worse by the castle's exposed position near Dartmoor, have been traced to cracked asphalt and cement pointing on the roof.
All the castle's 13,000 window panes need replacing.
House manager Bryher Mason told BBC News: "I wouldn't be surprised to walk into a room one morning and find a section of the ceiling having fallen in because the metalwork in the ceiling has failed."
Castle Drogo was built over 20 years for wealthy grocer Julius Drewe and it was handed over to the National Trust in 1974.
Damp marks on the ceiling show the extent of the leaks
It attracts large numbers of visitors every year, both because it was the last castle to be built in England and because of its striking position high above the Teign Valley which means it has sweeping views of Dartmoor.
But leaks became apparent only two years after it was completed and the castle now requires substantial repairs.
The National Trust is raising funds for the castle by selling sections of the roof.
Spokeswoman Claire Bolitho said: "It is is quite ironic that castles were built to be impenetrable from invaders and yet Drogo is under attack from the rain.
"We are looking at lots of ways raise funds to help with the repairs and are planning to launch an appeal.
"The repairs are being carried out in phases. It's a long-term preservation project which we hope to have completed by 2016."