The two-alarm fire broke out in a building that houses Powerhouse Gym on McKee Drive in Mahwah around 7 a.m.
Firefighters had it knocked down in under a half-hour.
Island Road was temporarily closed. No injuries were reported.
Mahwah firefighters brought their fan boat to ventilate the building.
Mutual aid responders requested either at the scene or in coverage included firefighters from Ramsey, Montvale, Ridgewood and Ho-Ho-Kus, as well as Hilburn, Hillcrest, Sloatsburg, Suffern and Tallman.
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