Firefighters were called to reports of a partial building collapse on West Heath Close in Hampstead.
A 6m rolled steel joist weighing around 4 tonnes had collapsed onto builders. No one was trapped.
Four people were treated at the scene by London Ambulance Service crews.
Firefighters made the scene safe and a hazard zone was set up as a precaution.
The Brigade was called at 1114 and the incident was over for fighters by 1209. Three fire engines from West Hampstead, Kentish Town and Hendon fire stations were at the scene.