With rain flooding the Washington area, many homeowners may end up looking to their insurance to cover water damage to their homes or belongings.

But standard homeowners insurance doesn’t cover flooding, warns the federal government Web site FloodSmart.gov, and that can be a costly omission.

“Just a few inches of water from a flood can cause tens of thousands of dollars in damage,” the government site says. “From 2007 to 2011, the average residential flood claim amounted to almost $30,000.” And nearly a quarter of flood losses take place in areas considered to have a low to moderate risk of flooding, according to Allstate.

Costs can run big for insurance companies because many people do think ahead and cover themselves.

Last week, the shares of major insurance companies dropped as Hurricane Sandy looked set to batter the East Coast.

Allstate, the second-largest U.S. home and auto insurer, fell 0.9 percent on Friday, closing at $40.15 a share. Its shares fell further Monday on European markets.

Travelers shares fell 0.8 percent Friday, and Chubb, which insures many expensive homes, sank 1.8 percent.

The storm, which is affecting more than 50 million people, could inflict $10 billion to $20 billion in total economic damages and result in $5 billion to $10 billion in insured losses, according to an estimate by ­Eqecat, a firm that models ca­tastrophe risks for the insurance industry.

For homeowners, the time to have gotten flood coverage was a month ago.

Typically there’s a 30-day waiting period before newly purchased flood insurance goes into effect, so homeowners who lack such coverage now are out of luck when it comes to Hurricane Sandy.

Both the government and private insurance companies warn homeowners not to depend on federal disaster assistance to cover losses.

“Federal disaster assistance is only available if the president declares a disaster,” Allstate says. “Less than 50 percent of all flooding incidents are awarded a federal disaster assistance declaration.”

Moveover, even if the president declares a federal disaster area, most disaster assistance is provided in the form of a loan that must be fully repaid — plus interest, Allstate says.