The crane was being used to build a four-storey block of flats in Northwood, north London when the incident happened at 4pm on Friday.
Nobody was injured although the tip of jib struck a house forcing police to evacuate a family from the building.
It is being speculated that the luffing jib hoist brake or cable gave way, allowing the jib to drop.
The crane is the same type that was involved in an almost identical incident in Wapping last year when the jib dropped its load onto a busy public footpath
The Health and Safety Executive visited the site run by Essex company Clarke Design and Build.