Emergency crews have rescued a man who was trapped after an addition to a house that was under construction collapsed on Lucknow Street in south-end Halifax on Monday afternoon.

Halifax Regional Fire and Emergency officials said he was the only one who was trapped. The man was conscious and alert, said Phil McNulty, a spokesperson with Halifax fire.

McNulty said the structure was a three-storey wood frame addition to an existing house. Crews could see some movement from the injured man, he said.

"He's up in the air a bit as we need either the aerial platform or to reach him via ladder," he said earlier on Monday.

Building collapse

A neighbour says work began on the addition to this house in the winter. (Anjuli Patil/CBC)

Crews lifted the man over power lines as they rescued him from the roof.

The man was taken to hospital with a cut to his forehead and complaining of an ankle injury, said Lloyd Currie, Halifax Regional Fire division commander.

Crews found the man next to a wall that had come down and landed on top of the concrete polisher he was using, he said.

"It's all likelihood that the wall might have hit him on its way down," said Currie.

The man was running a machine to polish a concrete floor that had just recently been poured when the collapse occurred, he said.

The Department of Labour has issued a stop work order.

The Canadian Red Cross says six people have been displaced from the home. Disaster volunteers helped four men who needed emergency food and a place to stay. Two other men were able to make their own arrangements.

Neighbour Nina Zeh lives next to the site of the collapse and says construction work on that section of building started in April.

She said she heard the sound of the structure coming down and came out of her house.

"We were running around with some neighbours and some construction people and were looking around there, behind the house and seeing if someone had gone underneath or not," she said.

Police and fire crews were quick to respond and she watched the rescue, said Zeh.

"We saw that he was reacting and talking and moving so that was good," she said.

Building collapse

Halifax fire spokesman Phil McNulty said the injured man was conscious and alert. (Rebecca Kendrick/Twitter)