A 100ft by 65ft section gave way at an Audi dealership in Milton Keynes, Buckinghamshire earlier this month.
Staff realised the danger and activated the fire alarm to evacuate the building about 20 minutes before part of the building collapsed.
Ian Wilson, from Buckinghamshire and Milton Keynes Fire and Rescue Service, said: “People working within a workshop heard a loud bang and promptly hit the fire alarm call point.”
Jardine Motors Group, which is the dealership’s parent company, said it was “investigating the cause of the collapse and the extent of the damage.”
In a statement on its website, the company said: “We have enacted our Business Continuity Plan, including the support of the Audi dealership team, to ensure that we can provide a suitable service level to all customers as soon as possible.
“All customers directly affected by this incident will be contacted in person.”
The group’s commercial director Jason Cranswick told The Guardian: “There was a bang, at which point one of the managers set off the fire alarm. It was after everybody was evacuated that the collapse occurred.”
Reports suggest firefighters propped up what was left of the building to stop it falling completely.