SAN FRANCISCO (CBS SF) – Less than 20 percent of California homes are affordable to teachers earning an average salary, and none of those homes are in Silicon Valley, according to a new analysis.

Real estate company Redfin found in the state’s most populous counties, only 17 percent of homes for sale are affordable on the average teacher salary of $73,536.

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“A typical home in California costs more than half a million dollars—$200,000 more than the average teacher can afford,” Redfin chief economist Nela Richardson said in a statement. “Due to the yawning gap between incomes and home prices in communities across the country, our public servants can’t afford to live in the communities they serve. It’s a problem of national importance.”

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Researchers found the percentage of affordable homes has dropped in the last four years, despite teachers earning more money. Redfin found it takes four full teacher salaries to buy a typical Bay Area home, a whole salary more compared to 2012.

The real estate company defines “affordable” as a monthly mortgage payment that is no more than 30 percent gross income, with a 3.5 percent interest rate and a 10 percent down payment.

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Based on MLS listings on September 6th, Redfin found zero homes in San Mateo and Santa Clara counties affordable for teachers, echoing an analysis released earlier this month that found several Silicon Valley communities are among the most expensive places in the U.S. to buy a home.

In San Francisco, only one of 571 of homes for sale fit the “affordable” definition, and only 4 out of 787 homes in Sonoma County.

Solano County is the only one in the Bay Area where more than 10 percent of homes for sale were deemed reasonable on a teacher’s salary (88 out of 755 homes.)

Central Valley counties remain relative bargains for teachers. Redfin found about six out of 10 homes for sale in Fresno and Merced counties were affordable on an average teacher’s salary. In Sacramento County, nearly a third of available homes were.

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Tim Fang is a digital producer for CBS San Francisco and a native of the Bay Area. Follow him on Twitter @fangtj.