UPDATE: Part of the roof on a Chattanooga warehouse collapsed Thursday morning and one of the employees is responsible.

Fire officials say around 20 people were inside the Riverbend Materials facility when a portion of the roof caved in just after 11:00 am.

They say the partial collapse was the result of a fork lift operator accidentally backing into a support beam in the building.

"A fork lift operator had driven into a support for the roof structure knocking that structure loose and causing a partial roof collapse," Assistant Fire Marshall Chuck Hartung told Channel 3.

Employees inside were evacuated and no one was injured, but they were sent home after a city inspector looked at the damage.

"City inspectors came out and it's been deemed unsafe for anyone to occupy both sides of the building," he explained.

From a street view, the building looks virtually undisturbed, but a bird's eye view and photos taken inside show a different perspective.

"Because the roof is not supported on that structure, it is still actually moving. It's a dynamic load. The water that was on the roof has actually caused the roof to be sagging, so it is still actually falling. They will actually have to go in there and shore it up with a structural engineer," Hartung said.

He told Channel 3 the building's sprinkler system was also damaged as a result of the collapse--adding another reason to keep people out until repairs are made.

"They will have to get a structural engineer out here to certify that both sides of the building is ok to be occupied," he added.

The portion of the roof that caved in was about 45 feet wide. The entire building is a little more than 70,000 square feet.

Channel 3 reached out to Riverbend Materials to see when the building's roof will be fixed and when they'll resume operations. We have not heard back at this time.

Stay with the WRCB app for updates to this story.

ORIGINAL STORY: A Chattanooga fire official says about 20 people were inside Riverbend Materials when the roof caved in just before 11 o'clock Thursday morning.

The official says the roof collapsed after forklift hit a beam which brought down a support beam causing the roof to cave in.

The area of the collapse is about 45 feet in diameter.

The fire department has deemed the area unsafe and employees are not allowed back in until repairs are made and a structural engineer has checked it out.

The sprinkler system was also damaged as a result.