Lead thieves left historic house Insole Court at the mercy of torrential rain over the weekend.
The Victorian building was flooded by the downpours after the gothic-inspired mansion was targeted by crooks for the fourth time in a month.
The crooks did not get away with their loot because the gates were locked.
One neighbour said the 1850s mansion was now “like a swimming pool inside”.
A source said: “It was in the early hours of Sunday morning they stole the lead off the roof.
“They took the lead off and put it into nice little rolls and put it in the big wheelie bins.”
They rolled their booty – worth about £700 – to the main gates but they were closed.
A neighbour saw what was going on at the Grade II listed structure and called police.
“It was a Sunday and there was no-one to make the house watertight,” the insider said. “And what happened Sunday night? It bucketed down.
“They now have a swimming pool at Insole Court. There are about 50 buckets dotted around the place. It is the second time it has happened in a week.”
The council discovered the damage on Monday.
Before that a theft was reported to them on Monday, January 21. And on December 23 an attempt was made to break into the building but nothing was taken.
But the next day – Christmas Eve – it was targeted again. A television set and lead were taken.
Sir Norman Lloyd-Edwards is chairman of Insole Court Trust.
He said: “It’s worrying because every time something is stolen it is more cost to put it right.”
Cardiff’s Insole Court Trust is set to take over management of the property at the end of 2014.
It has raised £761,724 to repair the stables for community and educational functions and is hoping to hit a £5m fundraising target.
It already has planning approval to convert adjacent outbuildings into a visitors’ centre and tea rooms, as well as to help restore the gardens.
Cardiff council’s Huw Thomas, the cabinet member for sport, leisure and culture, said: “Insole Court is an extremely important and historical landmark in Cardiff and I am disgusted that these thefts have taken place.
“The community have really come together over the restoration plans for the facility and this comes as a real kick in the teeth for them.
“It is mindless vandalism from a small minority with no respect for their own society.”
Sir Norman thought CCTV should be installed at the site. South Wales Police are investigating.