While the exact cause of the inferno is still unclear, footage of the fire offers some clues to how it became so deadly. Experts point to the lethal mix of combustible materials, like substandard scaffolding netting and polystyrene foam boards installed on windows.
The fire tore through the Wang Fuk Court housing estate with astonishing speed, leaping from building to building, each of which was 32 stories high.
Flames not only raced up the sides of the high-rises but sped through the interiors, consuming multiple floors. That was particularly deadly because it trapped residents inside and made it harder for firefighters to slow the blaze and reach survivors.
The fire burned for more than 40 hours, ultimately killing more than 150 people. It burned with such ferocity that some bodies were charred beyond recognition or even reduced to ashes. The disaster has led to public anger about unsafe construction practices and why the government failed to prevent this.
Here is how the design of the buildings and the construction materials used may have contributed to the fire’s spread:
The Upward Race
Wang Fuk Court, a dense housing estate in northern Hong Kong, was home to more than 4,000 people, most of them older and of modest means, according to census data. The Nov. 26 blaze started at Block F.
Officials say netting on a lower floor caught fire first, a moment that a passerby apparently captured on video.
The video shows the fire at an alcove between two wings of the cross-shaped building and in front of a staircase.
That semi-enclosed space may have created a “stack” or “chimney effect” of vertical air flow that pushed the fire quickly upward, experts say. The gap between the scaffolding and the exterior wall could also have caused that effect.
The fire raced rapidly upward within minutes.
“That exterior gap chimney effect is real and is the key to what happened,” says Jonathan Barnett, managing director of Basic Expert, a fire engineering consulting firm.
“Something that would normally not burn very quickly will burn very quickly because of the radiant heat feedback inside this chimney,” he said, referring to how the heat from the fire would have bounced back and forth between the walls of the alcove.
Every tower in Wang Fuk Court featured these recessed alcoves. During the fire they became vertical express lanes for heat and smoke, fire experts say.
“The recessed corner acts like a vertical channel that rapidly draws hot smoke upward,” said Lung-ken Tsai, the chairman of the Taipei Civil Engineering and Architectural Society.
Similar columns of fire were visible in the other buildings, too.
Flammable Construction Material
The eight apartment towers at Wang Fuk Court, under renovation since last summer, were sheathed in bamboo scaffolding and plastic netting to prevent construction material from falling to the ground.
Investigators said that after a summer typhoon, contractors had replaced some of the netting with cheaper material that did not meet fire-safety standards.
So many windows were covered with the flammable foam panels that in the one unaffected building, the police found them on windows in the elevator lobby on every floor.
As the panels caught fire, they helped spread the fire vertically. And as the boards melted into liquid, the material likely became like gasoline, spreading fire downward as well, according to Mr. Barnett.
They also caused the windows to overheat, breaking the glass and allowing the fire to enter the buildings, officials said.
The foam boards meant that many residents could not see what was going on outside even after the fire had started. Photos taken from inside apartment units before the fire show how the foam boards completely blocked the view.
Others have blamed the bamboo scaffolding, which is not highly flammable but can still ignite at high temperatures. Temperatures within the building reached as high as 930 degrees Fahrenheit.
As pieces of bamboo caught fire and broke off, they blocked fire exits and made it dangerous for firefighters to enter. The scaffolding also made it hard for firefighters to position rescue ladders.
In a city where most buildings are made of reinforced concrete, making such exterior fires rare, the firefighters struggled.
“We never imagined there would be this kind of situation,” said Chan Man, a civil engineer in Hong Kong who has worked on government projects. Mr. Chan said most of the city’s safety regulations are geared toward interior fires, such as those caused by faulty appliances.
The windows of the elevator lobby and stairwell of each building posed another problem: As they shattered in the heat, smoke and fire entered the corridors, cutting off residents’ escape routes.
The fires on multiple floors prevented firefighters from moving to the higher levels, and narrow corridors limited the number of firefighters that could be on a given floor, officials said. It was so hot that apartment units kept reigniting, slowing the firefighters even more.
A fire climbing the side of a high-rise and igniting multiple floors at once can undermine some of the most basic strategies used to slow the fire and rescue survivors, said Charles Blaich, a former deputy chief of the New York City Fire Department.
One standard practice is to connect hoses to standpipes, or water supplies in the building, two floors below the fire, Mr. Blaich said. Then firefighters climb the stairs, pulling the hoses with them.
“Usually it’s a burning apartment,” Mr. Blaich said. “You go down the hall, you enter the apartment, and you extinguish the fire.”
But none of that works if the fire is burning on floors above and below the firefighters.
A Cascade of Other Failures
Experts say the fire jumped from one high-rise to another in a “domino effect” as falling embers and burning debris reached the scaffolding netting or polystyrene foam of other buildings — similar to how wildfires spread.
The intense heat from the fire may also have “preheated” neighboring buildings, between 100 and 30 feet away, making it easier for those burning embers to start new fires.
“The mesh, the polystyrene, all the outside of the building are heated up because they are facing the flames in the adjacent building,” said Albert Simeoni, a professor and department head in fire protection engineering at Worcester Polytechnic Institute, outside Boston. “That makes the threshold of ignition lower.”
There were other failures. Investigators found that the fire alarm systems in the eight buildings were not working properly. Residents described being notified of the fire by family members outside the building who saw news of the blaze.
Within three hours, seven of the eight buildings were ablaze.
In the aftermath, residents are demanding to know how a fire of such magnitude could have ripped through a housing estate for some of the city’s most vulnerable, and whether warnings had been ignored. More than a year before the fire, residents had raised questions about the safety of the netting and foam that were being used for renovations.
The disaster also poses a challenge to the Hong Kong authorities, who, under tighter control by Beijing, must prove they can prevent it from turning into a political crisis. The police have arrested at least two people who demanded more government accountability as officials pledged to pursue anyone who “maliciously smears” the government.
Some experts warn that the shrinking space for dissent could itself be a safety risk. With open demands for government accountability now far more constrained, the problems that enabled this disaster may be harder to spot in the future.
“There’s no dissent in Hong Kong,” said Mr. Chan, the engineer, referring to the silencing of critical voices. “I think it will affect the safety of buildings.”