It took just 90 minutes for fire to race up 20 storeys of the Grenfell Tower in June 2017, killing 72 people.
Last month, a cigarette sparked a blaze that quickly jumped five floors of Melbourne’s Neo200 building.
Both buildings — and plenty of others like them — were covered in aluminium composite cladding.
It’s not the only type of flammable cladding out there, but what is it about aluminium composite that encourages fire to spread particularly quickly?
Most aluminium composite cladding consists of a layer of plastic sandwiched between two aluminium sheets.
And it’s the plastic layer, usually made of polyethene (also called polyethylene), which encourages fire to spread so quickly.
Polyethene isn’t only found in cladding — not by a long shot. It’s everywhere, making up roughly a third of all manufactured plastics.
What makes it so versatile is its molecular structure: long chains of “monomer” units comprising carbon with hydrogen atoms hanging off them.
If the chains branch off into loads of smaller chains, you get low-density polyethene. That’s the flexible plastic that you find wrapped around food, for instance.
The stuff used in cladding is high-density polyethene, or HDPE.
HDPE’s chains don’t have many branches, so they pack beside each other tightly. This gives the plastic rigidity — a quality you want in construction.
But it also has plenty of less desirable qualities.
Fire spreads in a few different ways
Materials engineers use a figure called the “combustibility ratio” to give an idea of how flammable a fuel is, said Kate Nguyen, who leads a fire and facade engineering group at RMIT University.
Put very simply, the ratio is calculated by dividing the amount of heat released from burning material by the amount of heat needed to ignite it.
The higher the number, the more flammable the material. Red oak, for instance, is pretty low: it has a combustibility ratio of 3.
“The combustibility ratio of HDPE is 25,” Dr Nguyen said.
And this is a big reason HDPE fires spread so quickly. A burning chunk of HDPE provides more than enough heat to ignite any HDPE sitting close enough to it, which can ignite the patch next to it, and so on.
Another danger of using polyethene in construction is the way it falls apart when exposed to heat, called its “pyrolysis” mechanism.
Usually, the long chains of carbon and hydrogen in HDPE are tightly packed together. That’s what makes the material so stiff.
But when HDPE is on fire, those chains break apart at random points and the solid plastic turns to liquid, Dr Nguyen said.
Cladding fires aren’t only spread by HDPE.
Aluminium can help things along because it’s an excellent conductor of heat. If it gets hot enough, aluminium can convey that heat into the HDPE underneath.
Then when the aluminium melts, it exposes the pre-heated HDPE to oxygen, accelerating its combustion.
“Top hats”, which secure cladding to the building, create cavities behind cladding. These cavities funnel heat and smoke up the outside of a building “like a chimney”, Dr Nguyen said.
“And you might have two or three cavities, like between combustible insulation and cladding, which add more types of fuel into the system.”
So a fire starting in aluminium composite cladding on the fifth floor of a building, for instance, can spread all directions: up, thanks to flames and smoke in cavities; down, because hot, liquid HDPE ignites more HDPE as it drips down; and even sideways, as aluminium spreads heat.
So what can be done?
An obvious place to start is to lower HDPE’s combustibility ratio by introducing a flame-retarding material.
Some flame retardants, like aluminium trihydroxide, work a couple of ways.
“Having aluminium trihydroxide in the HDPE will reduce the amount of polyethene in the system, so there’s less fuel,” Dr Nguyen said.
But if HDPE contains a flame retardant, it’s still combustible — just less so.
At the moment, buildings deemed highest risk by cladding audits are having all their flammable cladding taken off and replaced.
It’s expensive, costing $1 million or more for apartment blocks.
Dr Nguyen says there’s a cheaper way that’s just as safe: introduce vertical firebreaks.
These work in the same way as firebreaks around houses in grasslands and bushland — create a barrier that stops fire spreading.
“If you replace half the cladding with firebreaks, then it is possible to stop the fire spreading,” Dr Nguyen said.
“We think that if you really understand the facade and the cladding, you can make buildings and the people living inside safe.”