The owners of a listed mansion have lost a legal battle after taking a dispute over a leaky roof all the way to the High Court.
The owners of Wyndcliffe Court near Chepstow wanted to reroof the Grade II* listed house with man-made slates.
However Monmouthshire council and a government officer said they must use natural stone slate to "honour" the Arts and Crafts architectural style.
Judge Milwyn Jarman dismissed the owner's complaint.
The National Garden Scheme, the Historic Houses Association and the Royal Commission on the Ancient and Historical Monuments of Wales all supported owners Anthony and Juliet Clay.
However, they must now use natural slate from the Delabole Quarry, in Cornwall, or Westmorland Green slates.
Monmouthshire council said they are "more in keeping with the spirit of the architectural style" and are "a better solution" for the important building.
Built in 1922 for shipping magnate Charles Clay, Wyndcliffe Court in St Arvans overlooks the Severn estuary and is regarded as a prime example of the Arts and Crafts style.
The stone roof tiles need to be replaced, however, new tiles from the same Northamptonshire quarry that supplied the originals are now in limited supply.
Monmouthshire County Councils refused listed building consent for the couple to reroof the building using reproduction slates in March 2017.
It claimed manufactured tiles would have "a detrimental impact on the special character" of the listed building.
A government planning inspector agreed, saying the Arts and Crafts movement was about a return to craftsmanship and a move away from mass production and industrialisation.
Challenging the decision at the High Court, the couple claimed the inspector had failed to properly consider the alternatives and gave inadequate reasons for his decision.
However, Judge Jarman could detect no flaw in the planning inspector's reasoning.
There was, he added, no "positive obligation" on the inspector to weigh the advantages and disadvantages of man-made tiles against natural slate.
He concluded: "There are no grounds for interfering with the inspector's decision."