The operation to rescue a builder trapped under rubble in Little Falls in Roodepoort would continue into the night, Gauteng police said on Thursday.

Inspector Karen Jacobs said the builder was still trapped under the rubble by 5pm on Thursday.

Around 8am on Thursday half of an office block under construction collapsed, injuring 14 workers and killing one.

One builder had not been accounted for and police were searching for him under the rubble.

Emergency rescue workers removed rubble by hand in an attempt to find the builder.

Dogs had reacted to a number of areas of the rubble but only a bag belonging to one of the workers was found.

Emergency workers would excavate all the areas of rubble that the dogs had reacted to but focused on a specific area of the rubble in the centre where bystanders last heard the groaning of an injured man while helping injured workers out of the rubble.

Rescue efforts were further being hampered by rubble continuously shifting and settling into the hole where the missing man was presumed to be located.

Heavy equipment was removing rubble in areas where the dogs had not reacted, to decrease the chance of the equipment accidentally injuring the missing worker.

The Labour Department's building inspectors were waiting for the engineer plans to ascertain whether there may have been a fault with the design of the building.

Sapa