A fire at the Crumlin Road courthouse in Belfast is being treated as suspicious by police.
The blaze at the derelict property started at about 2300 BST on Friday night.
A number of fire crews dealt with the incident and the road was closed until 0430 BST on Saturday.
Police say the building is to be examined by engineers later due to concerns about the safety of the structure.
Michael Graham of the Northern Ireland Fire and Rescue Service said there was a "large amount of damage throughout the property".
He said the "vast majority" of the centre section of the roof was now "completely burnt away".
"Due to the skill of the firefighters we managed to prevent it spreading throughout the entire property," he added.
"We were actually tied up there for approximately seven hours throughout the night.
"We believe this fire was started deliberately due to the nature of the building itself and it being unoccupied.
"We are led to believe that it was a fire started deliberately by an individual or individuals."