Editor’s note: This article has been updated to reflect that Erica Fischer and Amy Metz of Oregon State University led this study.

When a wildfire ravages a community, the loss of homes can be one of the most devastating impacts. The destruction displaces residents and can drive businesses and services away from those who still live there.

But fires don’t affect all homes and neighborhoods equally. Some burn to the ground, while others are left untouched. 

In a recent study, engineers from Oregon State University and CU Boulder aimed to find out why.

“There's no such thing as fireproof,” said Abbie Liel, a co-author of the study and professor of civil, environmental and architectural engineering at CU Boulder. “But I really believe we can protect our communities better.”

In the study, which was published in Fire Technology, the research team, led by Erica Fischer and Amy Metz of Oregon State University, examined the patterns of destruction left by the December 2021 Marshall Fire, which killed two people and destroyed more than 1,000 homes in Boulder County. 

The researchers used machine learning models to predict whether 1,055 homes in the Marshall Fire burn area would have survived based on 23 different factors, such as the homes’ construction year and type, the presence of wooden fencing, nearby vegetation, how isolated the neighborhood was and what jurisdiction it was located in.

When the researchers entered the true data from the fire, the models accurately predicted 99% of the houses that would be destroyed and 54% of the homes left standing. When they removed certain factors, they discovered that some factors, such as where a house is located, mattered more than others.

The characteristics of the homes themselves made a difference — for example, the materials a home is made from or the landscaping nearby — especially in more densely populated wildland-urban interface, or WUI, areas like Louisville and Superior.

In these areas, homes are closer together, and fire spreads more easily from one home to the next. Because the houses themselves act as fuel for a fire, and the most common way fires spread in WUI areas is through embers that can be carried miles away, it’s even more crucial for homes in these areas to be built in ways that reduce risk, the researchers said.

Fischer of Oregon State University said there are a few simple and low-cost ways homeowners can safeguard their homes: They can clean gutters, keep rooftops free of debris that could catch fire and remove easily combustible material, such as bark mulch, from within 5 feet of a home. They can also add screens over attic and crawlspace vents to block embers from getting inside the house and sparking a fire.

Residents can also learn about their risk level for fires. Colorado has a Wildfire Risk Viewer that can help residents learn whether they are in a WUI area.

Abbie Liel is pictured doing field work in areas affected by the Marshall Fire in January 2022. (Credit: Abbie Liel)

Other methods, like upgrading to fire-resistant siding and roofing or removing flammable landscaping, can be more expensive but can also help protect homes. Liel of CU Boulder calls wooden fences a “conduit” for fire spread and recommends changing them out for fire-resistant fencing if possible. The researcher’s study indicated that newer houses were somewhat more likely to survive, indicating the benefit of more modern construction.

Another major takeaway of the study, which was chosen as one of the best studies of 2024 in Fire Technology: Community-level actions and policies can play an even bigger role than individual homeowners’ actions. 

The researchers found that during the Marshall Fire, the layout of communities and the spatial arrangement of houses determined which homes survived, especially in Louisville and Superior, which are more densely populated WUI areas.

Houses’ proximity to open space mattered, but so did the distance between houses: Areas like Louisville and Superior with more dense housing and smaller distances between houses were more susceptible to wildfire spread. In unincorporated Boulder County, where lot sizes are larger and homes are farther apart, the fire did not spread in the same way.

But Liel said she doesn’t see reducing housing density as the best lever for mitigating fire risk. Denser housing can create livable, walkable communities and also be more affordable, especially for lower-income people and families.

Homes in denser areas can be built to be more fire-resistant, and these communities can be protected from fires through other means, including vegetation and open space management as well as keeping roads and water sources accessible for firefighters.

Colorado, a home-rule state, has no statewide building code, and local jurisdictions are allowed to set their own standards, although efforts to create a statewide wildfire code are underway. Those standards could include wildfire hardening policies implemented at a city or county level. Numerous cities, including Boulder, are currently weighing such policies.

Broader legislation can also influence which regulations homeowners’ associations can impose on residents. For instance, a new Colorado law passed last year blocks HOAs from prohibiting building with fire-resistant materials, so they can’t require higher-risk features like wooden fencing.

Additionally, Fischer said, jurisdictions can find ways to shore up their water capacity for fighting fires, and neighborhoods can be designed in a way that makes them easy for firefighters to access.

Even when there’s not a shortage of water in nearby reservoirs, a sudden surge in demand for water can overwhelm municipal water systems and lead to a drop in water pressure, causing fire hydrants to run dry. The Los Angeles wildfires in January offered a painful illustration of this lesson.

“Typical public works departments are built such that they're fighting one house fire at a time. They're not fighting an entire city block or an entire cul-de-sac at the same time,” Fischer said. “The water system is just not made for it.”

Regardless of the means, the researchers agreed that people and communities need to take action to make themselves safer from fires.

“People really do want a safe home, and they really do want their community to be safe,” Liel said. “We need to take individual and collective actions, starting with the things that are most achievable and moving up to things that are harder.”