Why do some houses burn down and not others?

You’ve seen the pictures.

The gleaming white mansion on a Palisades hillside, completely surrounded by the rubble of burned homes.

The Malibu miracle house.

The red-roofed home in Maui.

Why some homes burn and not others can seem like a mystery, a miracle or pure luck. But experts who study wildfire damage say multiple factors influence whether houses burn — and they might not be what you’d expect.

First, embers

Both the Palisades and Eaton fires ignited on undeveloped land and spread quickly into dense neighborhoods nearby.

According to an early analysis by the Insurance Institute for Business & Home Safety, which sent researchers into burned neighborhoods to assess damage, the first few houses that burned down in both fires were ignited by wind-blown embers.

Embers, the tiny fiery particles that melt your puffy jacket when you’re sitting around a campfire, are the leading cause of home ignition, according to fire scientists interviewed by LAist.

In a wind-driven fire, embers fly miles ahead of where the wildfire is burning. They can sneak into open windows, through cracks in a garage door, or wedge themselves beneath an eave on a porch.

“A lot of houses actually catch fire from inside the structure first without even catching on fire from the outside, because the ember gets in,” explained Alexandra Syphard, a research ecologist with the Conservation Biology Institute and an adjunct professor at San Diego State University.

In rural areas, where houses are spaced widely apart, ember-ignition accounts for nearly all of the houses that burn down, Syphard’s research has shown.

But once a house in a dense suburban neighborhood is lit on fire by an ember, the mechanism by which subsequent houses burn down changes.

Then, radiant heat

After a house is ignited by an ember, it begins to put out a lot of radiant heat.

Our homes “contain a lot of flammable, combustible products,” explained Kimiko Barrett, a senior wildfire researcher and policy analyst at Headwaters Economics. “So they're going to burn really, really hot and they're going to burn for a really, really long time. And so when they do start to burn, they therefore threaten any nearby flammable objects, including other homes themselves.”

Sometimes, the heat from a burning house will ignite a privacy hedge, wooden fence, play structure, pile of firewood, or a car parked in a driveway.

Finally, direct flames

With that wood fence aflame, what experts call a "connective fuel," it acts as a bridge — helping flames leap from one house to another.

At this point, what began as a brush fire has turned into a full-blown urban conflagration.

This is exactly what happened once the Palisades and Eaton fires hit more urban neighborhoods.

“ The wildfire is often just the catalyst for what turns into something that really looks a lot like our urban fires of centuries ago,” said Ian Giammanco, the managing director of standards and data analytics at the Insurance Institute for Business and Home Safety.

So how to explain the 'miracle houses?'

Houses that survive may have a number of things going for them.

They may have been more air-tight than their neighbors, so embers weren’t able to penetrate the home.

There may have been more space between the house and nearby flammable objects, such as cars, fences or bushes.

They also may have been built with more fire-resistant materials.

In an analysis of the damage from the 2018 Woolsey Fire, which burned in the Santa Monica Mountains, Syphard found that building materials, especially double-paned windows, enclosed eaves and vent screens, played a large role in determining which homes survived.

How to ‘fireproof’ a house

Experts say it’s simply not possible to completely fireproof a house, especially if it’s built in a very high fire risk area (although that hasn’t stopped some people from trying). However, there are evidence-based steps you can take to reduce the likelihood your home will burn down.

As my colleague Jacob Margolis documented in this article and in his wildfire podcast, The Big Burn, and as you can watch in the video below, trying to prevent embers from getting into your house is the most important step.

• Install a Class-A, non-wooden roof.
• Create a 5-foot-wide, ember-resistant barrier around your house by removing mulch, wood chips, plants and other flammable items. 
• Put fine mesh screens over vents to catch embers.
• Install a 6-inch tall strip of non-flammable material around decks and walls.
• Enclose eaves, which easily trap embers and allow them to ignite roofs.

Here’s a list of other ways to harden your home against wildfire.

Notably, these are not things you can do at the last minute before evacuating — they involve advance planning.

What you can do in a hurry and why hosing down your roof doesn’t help

If you’re running out the door and have time to do a couple of things, though, Barrett recommends removing anything flammable that may be resting against your house, such as deck chairs.

But don’t waste time hosing down your roof.

“The heat coming off of a wildfire front or from a neighboring house burning is going to overwhelm any sort of sprinkling or exterior watering system,” said Barrett.

Don't put yourself in danger, she said, in order to hose down your roof. "If it's between that and evacuating, evacuate," she said. "Save yourself; save your life."

What about defensible space?

Creating “defensible space” involves removing flammable vegetation up to 100 feet around a home. It’s required for houses located in California’s very high fire hazard severity zones, a technical term that refers to the highest areas of wildfire risk.

A 2020 state law requires all homeowners living in high fire risk areas to create a five foot ember-resistant barrier around their houses,; however, it has yet to take effect.

But experts say that not all defensive space is created equal, and that creating an ember-resistant barrier immediately around your house is more important than clearing vegetation farther away.

 The other function defensible space serves is to provide a safe place for firefighters to move around, which is yet another explanation for the existence of “miracle houses.”

Perhaps a firefighter was there, hosing down embers before they could turn into a full-blown fire.