The collapse in Qiqihar, Heilongjiang Province was caused by construction workers illegally placing perlite — a form of volcanic glass — on the building’s roof, state news agency Xinhua said.
Heavy rain then led the perlite to expand and increase in weight, causing the roof to collapse, it added.
Footage aired by state broadcaster CCTV showed the roof — made of concrete slabs, according to state media — gave way in its entirety.
Rescue workers broke through large chunks of concrete and twisted metal in a bid to reach victims, the footage showed. One worker used a pneumatic jackhammer.
“A total of 11 people died in the accident,” state broadcaster CCTV said.
An in-depth investigation of the accident was in progress, state media has said, with those in charge of the construction company having been placed in police custody.
The gym at the No. 34 Middle School collapsed just before 3 pm (0700 GMT) on Sunday, Xinhua said.
Nineteen people were at the gym when the accident took place, local newspaper Heilongjiang Daily said. Four escaped and four others survived after being pulled from the rubble.
The search and rescue operation, which had involved nearly 160 firefighters and 39 fire trucks, has now wrapped up, CCTV said.
One unnamed source told the state-owned Chongqing News that concerns had been raised prior to the collapse over the materials being piled up on the roof.
“The gymnasium was built more than 20 years ago, and had long fallen into disrepair in many places,” the source was quoted as saying.
“Everyone was worried that something could happen with all the stuff piled up on the roof.”
Industrial accidents are common in China due to lax safety standards and poor enforcement.
Last month, an explosion at a barbecue restaurant in northwestern China left 31 dead and prompted official pledges of a nationwide campaign to promote workplace safety.
In April, a hospital fire in Beijing killed 29 people and forced desperate survivors to jump out of windows to escape.
And one of the worst such accidents took place in 2015 in Tianjin, where a gigantic explosion at a chemical warehouse killed at least 165 people.
