The Associated Press

SAN DIEGO—Building walls and roofs with fire-resistant material and clearing brush are vital defenses against wildfire damage, but alone they offer insufficient protection against wind-driven embers that accompany massive blazes like the ones that ravaged Southern California in the fall, fire officials said.

Authorities believe that embers driven by raging winds through small openings or against exposed wood were responsible for igniting a majority of the 1,125 homes leveled by October's Witch fire, the most destructive in California this year. In many cases the embers smoldered for hours before causing homes to burn.

A home "has to have a weakness for it to burn," said Ernylee Chamlee, California's chief of wildland fire prevention engineering. "It's less random, or a case of luck, than you might think."

More than half of the 497 structures that burned in unincorporated areas of San Diego County during the Witch fire had fire-resistant walls and roofs, according to the analysis of government data by the Los Angeles Times.

Information on construction materials has not been compiled for neighborhoods inside the cities of San Diego and Poway, but senior fire officials estimate that well over 75 percent of the destroyed homes had fire-resistant exteriors.

The analysis of the Witch fire's pattern of destruction points to deficiencies in long-held beliefs that brush clearing and fire-resistant materials are enough to fend off wildfires. Even the smallest openings in a home's exterior must be sealed to keep burning embers from entering, fire officials said.

Understanding the danger posed by burning embers is gaining in importance as local governments continue to let developers build in fire-prone areas.

The California Department of Forestry and Fire Protection estimated in 2003 that more than 3 million homes in California—the largest chunk in Southern California—were at "significant risk" from wildfires.

And the number is growing, with 40 percent of all home construction in the West pushing into wild lands, according to a 2002 federal report.