They also believe it could have been a major incident and spread to other buildings.
Group Commander David Harbinson told BBC News NI that firefighters had worked to stop the fire spreading across the roof.
"We could have had a building engulfed in fire, which could have spread to adjoining properties," he said
"It was really down to the crews who stopped it becoming a major incident," he added.
The Northern Ireland Fire and Rescue Service (NIFRS) sent eight appliances to the scene.
No-one was hurt, with all staff and guests exiting the building safely.
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