Granger and Sheila Martin of Olney returned home last weekend from a trip to North Carolina to the shock of discovering that two feet of raw sewage had flooded their finished basement, ruining most of its contents and causing an estimated $15,000 in damage.

But that wasn't the only bad news: Normally, the Washington Suburban Sanitary Commission would be liable to pay the cost of damage in situations such as this, in which a sewer blockage occurred on city property but backed up into a home.

In this case, however, the family will be eligible for only $1,500 from WSSC because the Martins have the unfortunate distinction of being the first customers to suffer a sewer backup in their immediate vicinity, according to WSSC spokeswoman Marjorie Johnson. Had such a backup happened along the same sewer line previously, WSSC would have been in a position to accept liability and negotiate a settlement, Johnson said. The Martins have lived in the four-year-old Olney Oaks development for 1 1/2 years.

State law allows the bicounty water and sewer agency to pay up to $1,500 to help with cleaning costs, and to deny claims beyond that for first-time occurrences. The law is intended to shield WSSC from full responsibility in cases where the commission is not aware of previous problems in a sewer line.

"We have successfully defended our position before in cases like this when they've gone to court," Johnson said.

The Martins were told by their State Farm Insurance agent that city waterline breaks and backups are not covered on the homeowner's policy, Granger Martin said.

Granger Martin is upset because he has to pay 90 percent of the estimated $15,000 bill. "It is ridiculous," he said. "We're caught between the devil and the deep blue sea. Or should I say the deep sewage sea."

"If a dog kills somebody -- without ever biting before -- that dog should be killed," Granger Martin said, arguing that in extreme cases WSSC should be fully liable.

"All government agencies are protected from bankruptcy," said WSSC's Johnson. "In a first instance we can't possibly know there is a problem . . . . Liability means you knew these people were going to come home and find two feet of sewage in their basement." Now that sewer line will be watched and some preventive maintenance may be done on it, Johnson said.

The backup this past weekend was caused by a buildup of grease blocking the line, she said.

A WSSC crew unclogged the line and drained the basement, while the Martins and their two young children stayed elsewhere Sunday night because of the unsanitary conditions, Granger Martin said.

Workers from WSSC later cleaned and disinfected the basement of the house on Shallow Brook Lane, hauling away two six-foot piles of items destroyed by the sewage, he said. "They cleaned it up with some Lysol so that it changed the fragrance."

Among the estimated $15,000 worth of damage are belongings including stereo speakers, a sofa bed, the water heater, a cordless telephone, a movie camera and projector, books, albums, Christmas decorations and more. The Martins, who both work for IBM, are also part-time Amway distributors and about $2,000 worth of products were ruined, Granger Martin said.

He said he has learned three lessons. If there is a shower and toilet in a basement, a back-water valve should be installed to prevent a backup, grease should not be poured down the drain, and belongings should be stacked at least a foot above floor level in the basement, he said.