MULTIPLEX is looking at its method statement for completing the remaining work on the Wembley stadium roof after part of it fell on Monday.
A 30 m-long rafter, identified as R24, dropped as a welding team was about to begin its morning shift to fix the beam permanently into place.
The rafter was being held in a temporary position by steel connections when the incident happened.
No one was injured when the 50tonne girder fell 0.5 m in the north-west part of the roof but Multiplex evacuated all 3,500 workers from the north London site as a safety precaution.
Multiplex's UK managing director Martin Tidd said it would examine the process behind the welding along with its steelwork contractor Hollandia and engineer Mott Stadium Consortium.
One site source said: 'It's almost certainly to do with Hollandia's method statement. The welder would have been following orders.' And one steel expert added: 'It seems Hollandia hasn't thought it through properly. This is a problem with the erection process, not the design.
'The rafter will have had locked-in stresses and these have caused the weld to break. A weld is only so strong. It doesn't sound like bad welding, it's an over-stressed weld. It will cause concern because Multiplex will have to do a fair amount of checking but, if it had been a real balls-up, the roof would have been on the ground.' Checks on the affected section of the roof are expected to last until the end of next week.
Mr Tidd said this part of the roof would be closed off until a forensic analysis of the temporary works had been completed.
He added: 'A structural engineering investigation has revealed that this was an isolated incident and the situation has not occurred anywhere else on site.'
The R24 beam equivalent in the north-east part of the roof, known as R9, has not been affected.
Mr Tidd said the closure would cause some disruption to workers, who returned back to the site on Tuesday, in the affected area. These men have now been relocated to other parts of the job.
Multiplex, Mr Tidd added, will only be able to work out much time it has lost as a result of the incident once the permanent weld was in place.
Hollandia replaced original steelwork contractor Cleveland Bridge in August 2004. It will have been paid close to £100 million by the time it finishes work.