Work had only just started for the day when the roof came down and no-one was hurt in the incident at the Littleburn Industrial Estate at Langley Moor on Monday.
Durham Fire and Rescue Service said the roof of the building was thrown on to nearby vehicles in winds gusting up to 60mph.
More than 25 firefighters were called to the scene amid fears workmen were trapped.
However, the spokesman said all had been accounted for.
He told the BBC: “We got a call just after 9.30am to say a roof had fallen on cars at the Littleburn Industrial Estate.
“The wind lifted the pitched roof from the building under construction, it landed on 15 cars, which we believe belong to staff of a nearby bakery.
“A controlled demolition of the two ends of the building will take place later on today as the structure was deemed unsafe in high winds.”
The north-east of England has been hit by strong winds from the tail-end of Hurricane Katia.
Firemen check no-one was trapped
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