The gantry at a factory in Dongguan City in Guangdong Province fell on the two-storey building made of containers during the thunderstorm, which brought with it winds of up to 100 kmph, said Luo Bin, deputy secretary-general of Dongguan municipal government.
In all, 139 workers were in the building when the mishap happened, said Luo, also the rescue department spokesperson.
Eighty-eight of them managed to escape. Of the remaining trapped, 18 died and 18 were being treated at a hospital.
The rescue operation has concluded, state-run Xinhua news agency reported.
The factory is owned by the Fourth Harbor Engineering Co Ltd, a subsidiary of China Communications Construction Co Ltd.
The fallen gantry is estimated to weigh 80 tonnes.
The company has allocated 20 million yuan (USD3.1 million) to handle the situation in the mishap's aftermath, Luo said.
Dongguan municipal government has set up a team to investigate the accident.
