The incident happened on the top floor of property in Church Road, St Sampson’s, at about 11pm on Monday
White watch acting commander Paul Bishop said a water tender, a water carrier and a rapid intervention vehicle attended.
When the crews arrived they found the fire had got under the slates and was spreading.
Two firefighters were sent in with breathing apparatus, followed by another pair in breathing kit, all carrying hose reels.
It took about half an hour to extinguish the blaze.
Yesterday morning Ian Skipton, who cares for the property, was checking the damage and contacting the insurers.
The top floor is under the eaves and is made up of two bedrooms either side of a staircase.
Mr Skipton said it appeared that a candle had been placed close to the sloping roof and this had started the blaze in the roof. It spread across the rafters and to the other side of the hallway, causing damage to both upstairs room.
There were holes in the roof in several places, and the walls were stained with soot and floor covered in debris and slates.
The joists were almost burnt through in one area.
‘The whole of the roof is going to have to have to replaced,’ Mr Skipton said.
While the top level was seriously damaged, the next floor down seems to have escaped the worst of the incident, with just water damage in the kitchen from the firefighters’ efforts.
Mr Skipton said the property was fitted with fire alarms and fortunately his father had boarded between the flat and the next flat, so the fire had not spread beyond the affected flat.
He added that the important thing was that everyone was safe and had avoided injury, but he did urge people to think carefully when lighting candles.