Firefighters protect a structure off Tubbs lane in Calistoga, California. Picture: Kent Porter/The Press Democrat via AP
Santa Rosa, California - The fire had already come down one side of the hill and been beaten back. Now, it was backtracking across the gully, low tongues of flame threatening a house with gray shutters at the end of the cul-de-sac.

Firefighters watched the smoke and assessed wind patterns, raking dead leaves and branches away from the blaze in hopes of stanching its charge once again.

The men, members of a fire company from the nearby town of Windsor, estimated that they had been awake for more than 70 hours and hadn't eaten for the first 16.

Like the other 21 wildfires ravaging northern California, the Tubbs fire has burned largely out of control for days, stretching fire crews and challenging traditional efforts to tame the flames. At least 31 people have been killed in what is now the deadliest wildfire incident in California since 1933, with the fires collectively consuming an area larger than Chicago. More than 20,000 people have been evacuated across the area.

On the ground, though, many firefighters said they hadn't seen the news or heard the statistics. Most had been on the clock since the fires started on Sunday night, sneaking away for swigs of Gatorade and 15-minute naps while steeling themselves for a long haul fighting fires of enormous size and scope complicated by drought and development.

For them, the Tubbs fire is a particularly personal one. Windsor firefighter Mike Stornetta's parents lost their house of 30 years, the home where he grew up, as a firestorm swept through the Santa Rosa neighbourhood of Fountaingrove on Sunday night.

"Our first assignment was two blocks away," he said during a pause in patrol. "While we were evacuating an elderly care facility home, we could see down into the glow of the neighborhood where I knew my parents lived."

They weren't home, but his grandmother was housesitting and just barely escaped. His parents lost everything except the clothes they were wearing.

Stornetta and his crew were fighting back the flames at Woodley Place, a line of modest houses surrounded on three sides by wooded hills. The prevalence of this kind of development - a low-density combination of homes and wild vegetation - has increased in California in recent years, said Jonathan Cox, battalion chief and spokesman for Cal Fire. Called "Wildland-Urban Interface," or "intermix" in firefighter parlance, these environments are among the factors that have made the Tubbs fire in Sonoma and Atlas fire in Napa so difficult to contain - along with five years of brutal drought, powerful winds and resources stretched thin from simultaneous fires around the state.

Although hard-hit Santa Rosa neighborhoods like Coffey Park are more traditionally urban, intermix areas are part of an upward trend throughout California.

"Areas that would 20 years ago have nothing now are interface environments," Cox said. "Take the sheer number of square acres that are involved with intermix and wildland-urban spaces, combine that with the frequency and intensity of fires increasing - it's a recipe for disaster."

Firefighter Josh Perucchi thought the house had probably been one story tall, perched on a bluff surrounded by forest, with vaulted ceilings and a deck out back overlooking the green hills of Annadel State Park in the Oakmont neighbourhood of Santa Rosa. It was hard to tell now. Just a couple of brick columns remained, plus a rubble of concrete, tile, metal and the hulk of what was once a washer-dryer. The deck was gone; the hills across the way were on fire. Just beyond the house, scorched eucalyptus trees blended into a tangle of burned vegetation, a reminder that the fire had recently been through here.

Perucchi and two other men from the Petaluma Fire Department were at the house on "mopping" duty, where firefighters revisit scenes of previous fires to target still-smoldering hotspots that might throw embers and light new fires in windy weather. The evening's forecast called for high winds, meaning the crew found itself back at a house where they had already fought a fire, turning their hose on the bushes that covered the still-warm ground and on the house's ruined foundation.

As they went, they sifted through the broken tile and brick for anything they might be able to salvage for the house's owners. Perucchi reached into detritus with a puzzled look, holding up a partially intact appliance.

"Pasta maker," firefighter Dale Keithley said.

"Is that what that is?" Perucchi replied.

Keithley, the acting captain, suggested that Perucchi put the pasta maker to one side; perhaps its owners would want to refurbish it. He had similarly spearheaded the mission to save the burned but intact silver convertible that now sat in the driveway,pushing it out of the garage and away from the flames during the fire that had taken the house down.

Burned by the Tubbs fire, only a pool remains among the ashes of an Old Redwood Highway complex in Santa Rosa, California. Picture: Karl Mondon /San Jose Mercury News/AP

Fires move quickly through wildland, and in the case of intermix, their continuous source of fuel is broken up only by houses, making those structures especially difficult to defend and those fires especially difficult to stop.

And even though fire codes require houses in intermix areas to have fire-resistant roofs, noncombustible siding, and 100 feet of vegetation clearance around their structures, that doesn't change the major challenge: Firefighting tactics for vegetation and structure fires are fundamentally different, and combining them makes their execution more difficult.

"It's not just put a line on the ground and the fire is contained," Cox said referring to the tactic of cutting down a line of vegetation to limit the fire's fuel. "You have essentially a jigsaw puzzle of fire and homes and infrastructure, all mixed together, and then you add in topographical features like slope and hills and trees."

In early evening, Keithley's men from Petaluma joined a group of other firetrucks to refuel, refill water tanks, and await next assignments in a staging area on Highway 12, a pitted field of brown, dry grass featuring a line of porta-potties and a few withered oak trees. Trevor Hayes, of the Petaluma company, napped between the hoses at the back of his truck, his hat over his eyes. Nearby, Engine Capt. Greg McCollum of Santa Rosa Fire changed his boots and charged his phone in anticipation of a full night's work.

Even after 24 years, the sheer size and power of the Tubbs fire has humbled him.

"This is a once-in-a-career fire," McCollum said. "One of the other guys said it's a once-in-two-careers fire."

He pulled back, circumspect. "Well, I'm no historian, but I know a damn big fire when I see one."

Like Cox, he also saw a connection between the growth of urban-rural interface development and the fire's scale.

"There was a fire that came over the hill [from Calistoga, Calif.] similar to the Tubbs fire in 1967," he said. "Now, the urban interface is growing - people moved out here to live in the country. There's a lot more exposure for structures, and Mother Nature doesn't care."

The other Santa Rosa firefighters sat on a nearby stonewall under a murky pink sunset, checking their phones and chewing tobacco. A passing firetruck honked its horn in greeting, and the men waved back and settled in to wait for the next call. They knew it was coming soon.

WATCH: Footage from Northern California shows entire neighborhoods have been destroyed by the ongoing wildfires.