Researchers use models that combine how weather, topography and types of vegetation raise the risk that a fire will grow large and threaten neighborhoods.

Julie Johnson, Sriharsha Devulapalli San Francisco Chronicle

Feb. 25—With streaks of yellow, orange and red, Cal Fire’s new fire risk maps make plain the extent of fire danger in the Bay Area.

The origin of the maps date back to one of the first major wildfires to burn into neighborhoods in the state: the 1980 Panorama Fire in San Bernardino County. That fire triggered extensive research into which types of homes burned and which survived. Shake roofs made from wood shingles, for example, made homes especially vulnerable to flying embers.

Cal Fire’s maps, which are still in draft form, establish what the state calls fire hazard severity zones, which measure the likelihood of a major wildfire hitting certain areas and neighborhoods. State and local governments require homeowners to follow stricter building codes and landscape rules in some zones — such as banning more wood shake roofs in the riskiest areas.

What do these maps tell communities about fire risk?

Cal Fire’s maps characterize the threat of a potential fire that ignites in the wildland and comes into communities, said Dave Sapsis, a research manager with Cal Fire who was part of the technical team that developed the model.

They use models that combine how weather, topography and types of vegetation raise the risk that a fire, once ignited, will grow large and threaten neighborhoods. Steep terrain and eucalyptus trees, for example, add to fire risk.

But the maps have limits. They do not yet incorporate data for what happens once a wildfire becomes a true urban conflagration spreading from home to home, according to Sapsis.

“They’re modeling a buffer from the wildland to urban areas to account for ember cast,” he said.

He said that scientific models for urban conflagrations are still being developed.

The maps also aren’t meant to show that there is no risk from wildfires in areas not designated in a fire zone, and they focus on the heightened risk for places close to forests, grasslands, parks and other open spaces.

“The closer you are to that wildland edge, the higher the hazard is,” Sapsis said. “The more likely that wildfire that impinges will make it to you.”

Do Cal Fire’s maps require residents to make immediate changes to their properties?

No. Homeowners generally do not have to act until they make big changes, like when properties are built, renovated, purchased or sold. That’s when stricter building codes kick in requiring better roofs, fine mesh vents, multi-pane windows and fire-resistant siding to be installed.

Cal Fire provides a basic outline of the various codes.

The primary audience for the maps are city and county governments, and especially permitting departments, which inform residents about rules for the projects they undertake. For governments, the maps guide planning and development, and inform infrastructure requirements, such as those for roads.

But these zones could eventually impact a vast number of California yards.

The California Board of Forestry and Fire Protection is drafting new rules for the area within five feet of homes. The current plan is to ban most plants (apart from mature trees) and flammable materials like bark mulch and wood fencing within the five feet of residential structures. Studies have shown that embers that land in this area are less likely to set the home on fire if there isn’t material there to burn.

Called an ember-resistant zone, or “zone zero,” the rules would apply only in “very high” fire hazard severity zones in areas generally protected by local fire departments and in all areas listed on the maps as state responsibility protected by Cal Fire. Residents would not have to comply right away. Once adopted, zone zero requirements would first apply to newly built homes and then to existing properties three years after they are enacted.

Where did fire risk grow?

Some cities in the Bay Area gained more acres in the highest tiers of fire risk, including San Jose, Orinda and Half Moon Bay as well as counties including Sonoma and Napa, where multiple major wildfires have broken out in recent years.

In Sausalito, fire risk now extends nearly to the shoreline, according to Cal Fire’s map. The city has about 702 acres in one of the three fire hazard severity zones, whereas it had none listed in the earlier maps released between 2007 and 2011.

Marshall Nau, deputy fire marshal with the Southern Marin Fire Protection District, said the city’s steep terrain, dense neighborhoods and thick vegetation make it particularly fire-prone given the community’s proximity to wild open spaces.

The fire district considers fire risk to extend even further into the flatlands than Cal Fire’s maps show.

Nau said that he’s tried to reassure the many callers to the district who are more worried since the fires in Los Angeles. Residents can harden their homes in meaningful ways with fire-resistant materials and by clearing flammable brush away from structures, he said.

“We’re trying to bring the anxiety down so people can think clearly and take action,” Nau said.

What areas in the region show less fire risk?

Cities including Oakland, Berkeley, El Cerrito and Hillsborough have fewer acres designated in fire hazard severity zones.

In Berkeley, Cal Fire’s new modeling decreased the number of acres listed as having “very high” fire risk from 1,269 to 454. Some of those acres were spread into lower tiered categories, high and moderate.

Berkeley Fire Chief David Sprague said he was surprised to see homes along Panoramic Hill mostly designated as “moderate,” though the city considers this area to be at a major disadvantage should a wildfire ignite. The neighborhoods are densely packed on steep terrain with thick vegetation. Many of the homes are over 100 years old and weren’t made with modern fire-safe standards. Evacuation routes are very limited.

“Cleary, that’s not accurate,” Sprague said.

Cal Fire’s map is just a draft. Local jurisdictions will submit feedback and Cal Fire is expected to finalize the maps at some point later this year.

When will Cal Fire release the next batch of fire risk maps?

The maps released Monday covered cities and towns protected by local fire departments in 17 counties in the San Francisco Bay Area and along Northern California’s coast.

Cal Fire will release another batch of maps March 10 covering central California including Sacramento, and then a final group of maps covering Southern California will be published March 24.

Cal Fire published fire hazard severity zone maps for the areas of the state it protects , which includes most but not all of unincorporated California.