How Mansions Can Intensify Wildfires

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(Bloomberg) — A neighborhood of $4 million homes that burned Wednesday in a Southern California wildfire highlights the vulnerability of large suburban dwellings to climate-driven blazes, according to fire experts.

The Coastal Fire destroyed at least 20 homes in a gated community in Laguna Niguel, a wealthy Orange County enclave near Laguna Beach. Houses in the Coronado Pointe development line a ridge overlooking the Pacific Ocean. Homes in the neighborhood are palatial, ranging in size from about 4,000 square feet to 10,000 square feet, and sit on large lots with room for swimming pools with coastal views. But the mansions are wedged together with relatively little space between buildings. 

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When a fire broke out near a wastewater treatment plant on Wednesday, it raced up a chaparral-covered hillside until it encountered an explosive source of fuel – Coronado Pointe.

“Homes made out of wood are like dead trees,” said Michael Wara, director of the climate and energy policy program at Stanford University who studies wildfire risk. “And there’s a lot more dead trees in a 10,000-square-foot house than a 2,000-square-foot house.”

The conflagration started on a cool day with high humidity — conditions not usually conducive to wildfire. But gusty ocean winds propelled what started as a small fire up the hillside below Coronado Pointe. California’s record-breaking drought had turned brush into tinder and the steep terrain intensified the fire and funneled it uphill, raining embers on homes. 

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“The biggest challenge with this fire and most wildfires is the confluence of wind, dry fuels, and topography,” Sean Doran, a fire captain with the Orange County Fire Authority, said in an email. “There are many factors that affect ignition including, home hardening, fuel loads in and around the homes, access, proximity to one another, size of the homes, size of the fire, and winds.”

Stephen Quarles, an authority on fire-resistant construction, said that the closeness of the houses to one another probably was a factor in supercharging the fire. 

Quarles, a former official at the University of California at Berkeley Center for Fire Research and Outreach, said when homes are close together a burning a house transfers heat through the air to adjacent structures and exposes them to flames. “With buildings burning and vegetation burning, embers can be transported further and have enough energy ignite other things,”  he said.

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Michele Barbato, a professor of structural engineering and structural mechanics at the University of California at Davis, said the size of the Coronado Pointe homes likely contributed to the extent of the destruction.

“Larger homes have generally more combustible material and burn for a longer period of time, increasing the amount of released [embers] and the time that other houses need to resist ignition,” Barbato, who works on developing fire-resistant building material, said in an email. “Large homes also tend to have more weak spots, like wooden decks, longer gutters, more irregularities in the roofs.”

Most of the houses in Coronado Point were built in the 1990s before California adopted stringent building codes for homes constructed in the wildland-urban interface, the overlap between housing development and fire-prone wilderness. That includes a 10,000-square-foot mansion built in 1999 that was for sale for $9.9 million when it burned Wednesday.

Wildfire risks have only grown in coastal Orange County in recent decades as housing development continues to push into canyons and ridge tops and real estate values soar.

Following firestorms in 1993 that destroyed more than 400 homes and forced the evacuation of Laguna Beach, a report on the catastrophe issued a prophetic warning: “Orange County’s wildfire history has repeated itself for thousands of years. Because of continued wildland-urban encroachment, this type of disastrous wildfires are likely to increase in frequency and severity.”

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