Saint John firefighters called to Canterbury Street at 9:20 p.m. on Sunday night for a reported structure fire.
Black smoke could be seen coming from the roof at the building located between Queen and Duke streets.
One person was hurt as the building involved and those nearby were evacuated and he was sent to the hospital to be checked out.
Acting platoon chief Roy Byers says crews remained on scene for about an hour.
The cause was determined to be a malfunctioning oil-fired water heater.