In Denver, two construction workers died in March when a partially built wood apartment building erupted in flames. Several other workers were injured as they jumped from the second and third floors to escape. Dozens of cars melted from the heat.

Last year, three fires in the Boston area destroyed three unfinished housing projects and forced evacuations of hundreds of people in nearby buildings.

And last week, a fire destroyed half-built Oakland townhouses, in the ninth destructive inferno at an East Bay construction site since 2012.

Alameda County had more suspicious fires than any other region, said Robert Solomon, director of building fire protection at the National Fire Protection Association. But it’s far from alone: Large fires have destroyed 45 major housing projects nationwide since 2009, according to M-Fire Suppression Inc., which sells a fire-suppressing spray. More than half of those fires occurred over the past three years, and seven significant construction fires have caused over $100 million in damage in 2018 alone. Some fires have been accidents, while others were found to have been intentionally set.

Fires are doing more financial damage as developers build denser projects to accommodate housing demand in cities. The rising number of fires has not only destroyed thousands of unfinished homes around the country, but it’s had a broader effect of increasing insurance premiums and security costs by hundreds of thousands of dollars on some projects. Those cost increases, along with a national construction labor shortage, could lead to higher rents and sales prices passed on to tenants in already expensive areas, according to developers.

Longtime Oakland builder Madison Park Financial Corp. experienced a relatively small fire on Tuesday at its Hollis Oak project that is 10 blocks from the townhomes that burned down. The fire damaged only one room. But the company’s insurance premiums, which cover the cost of repairs for fires, have risen about 10 to 15 percent in the last year because of the other fires in the area, according to Simon Chen, the chief financial officer.

Chen said it now costs about $300,000 to insure a 100-unit housing project for 20 months. The company also spent $200,000 for taller fences and $60,000 a month on security including guards.

Those rising costs, along with other expenses like impact fees and the labor shortage, mean that Madison Park has no plans to propose more housing projects in Oakland. It’s instead focusing on cities such as Portland, Ore. “We would likely not go out and buy a piece of land at full land price” in Oakland, Chen said.

For developer Trammell Crow Residential, a project in a “hot spot” city where fires have happened could cost $400,000 to $450,000 to insure for a year, up from $150,000 to $200,000 in 2015, according to Scott Woodward, Trammell Crow’s risk management and legal director in Dallas. Some insurance carriers are refusing to insure any housing projects in the wake of fires, Woodward said. Other insurance companies require that a sophisticated camera system be installed. He declined to identify the companies.

“In all of my years, I’ve never seen that before,” said Woodward, who has worked in real estate since 1982.

Country’s toll 22,500: U.S. fires intentionally set in 2017, excluding fires with unknown causes, up 13 percent from the previous year 280: Number of U.S. civilian deaths in 2017 from fires, down 10 percent from the previous year $582 million: Property losses from fires in 2017, up 23 percent from the previous year Source: National Fire Protection Association

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Trammel Crow developments have not had any fires, but its Alexan Webster project in Oakland is across the street from developer Wood Partners’ Alta Waverly project, which burned down in July 2017. Trammel Crow’s site wasn’t damaged, but it has increased its security. A side benefit is that other incidents, like theft of building materials like copper, have stopped, Woodward said.

Woodward declined to speculate on motives for arson, but said that more fires are also a byproduct of more construction activities and denser apartment projects, especially in cities with high job growth.

“The escalation coincides with the boom in multifamily construction coming out of the recession,” Woodward said.

All of the projects in Oakland and most of those around the country that have burned down have been made mostly of wood. It’s the most popular building material for mid-rise projects — seven stories and under — because it’s cost-effective and a renewable resource. Taller buildings, those with more than eight stories, typically use more expensive steel and concrete frames, which are needed to support additional floors and don’t burn as easily. No partially built steel or concrete towers in Oakland have had fires.

Woodward said it’s very unlikely that many developers would abandon wood in mid-rise projects, because using steel and concrete is far more expensive and requires costlier worker wages.

“Fire is a danger for all buildings and construction sites, regardless of building material,” said Heather Stegner, vice president of communications at the American Wood Council, which promotes the building material.

The council and 60 other organizations have teamed up to develop safety standards, she said.

Completed wood-frame buildings that have drywall and sprinklers installed are generally safe from fires, said Solomon from the National Fire Protection Association.

But if a fire does spark in a wood-frame building that doesn’t have additional materials built, firefighters are relatively helpless and focus on saving adjacent buildings.

“There’s no stopping it,” Solomon said.

Roland Li is a Chronicle staff writer. Email: roland.li@sfchronicle.com Twitter: @rolandlisf