Apparently part of the roof has "dropped a metre", and all 3,500 workers have subsequently been evacuated from the site. Multiplex are calling it a "weld failure".
It seems no one was hurt but an investigation, as you might imagine, is underway.
Update: Apparently the incident happened around 9am this morning when "The guys on the roof heard a loud bang and the roof fell about 3 feet." That's according to 'Steve Pryle, a spokesman for the GMB union, which represents steelworkers on the site.
The GMB union said in Jnauary that Wembley's steel work would be completed by March 31. Pryle said today that "all bets are off'' about the completion deadline.