Debris escaped from a burn barrel and set a cardboard box containing trash on fire to start the blaze that did more than $300,000 worth of damage to Postlewait Trucking of Turtlepoint.
State police fire marshal Cpl. Greg Agosti says the fire then spread to a plastic tote containing a residue of used motor oil, and then to the outside of the building.
A passing motorist saw the fire at just after 3 p.m. and alerted employees inside.
The fire also destroyed several vehicles parked along the side of the building.
No one inside the building was hurt, but one firefighter from Coudersport sustained a minor injury while battling the blaze.
Agosti ruled the fire accidental.
The Star Hose Company of Port Allegany was called at just after 3 p.m. Companies from Smethport, Eldred Borough, Eldred Township, Otto Township, Coudersport and Roulette were called in as well.
Firefighters remained on the scene for about three hours.