The owners of 17 houses in Vienna and a house in Great Falls have sued one of the nation's largest builders, alleging that they were promised homes built with genuine stucco but instead were given a synthetic version that caused water damage and rot.

The lawsuits, filed in the past month in U.S. District Court in Alexandria, contend that Toll Brothers Inc. and its subsidiary, Hunter Mill Limited Partnership, tried to pass off the synthetic stucco, known as exterior insulation finish system, as the real thing.

"Some of my clients have water leaking in and mold," said Peter C. Grenier, who is representing a Great Falls couple and homeowners from the Hunter Mills Estate subdivision in Vienna in 18 lawsuits against Toll Brothers. "One client has water penetrating into the basement and water damage."

Edwin A. Sheridan, an attorney for the defendants, declined to comment on the lawsuits. "It's brand-new litigation, and I'm not at liberty to make any comments at all," he said.

Damage caused by synthetic stucco has been the subject of hundreds of lawsuits, including one statewide class action suit in North Carolina. Manufacturers of synthetic stucco have denied that their product is defective and have blamed problems on the contractors who apply it.

Synthetic stucco is cheaper to install than real stucco, which is generally made of cement, sand, hydrated lime and water.

Grenier said his clients never worried about the well-publicized problems caused by synthetic stucco because they believed their homes were made with genuine stucco. The lawsuits allege that Toll Brothers literature and contracts described each of the $500,000 to $600,000 homes in the Vienna subdivision as having "stucco detailing." The houses are two to five years old.

Each of the Vienna plaintiffs is asking for $600,000 in damages. The couple that owns the Great Falls house is seeking $850,000.

Grenier said his clients in Vienna learned that their houses were not constructed with genuine stucco in November, when Toll Brothers sent a letter to the homeowners asking them to inspect their properties for potential "moisture penetration" caused by the use of synthetic stucco.

In the letter, Toll Brothers offered to pay for repairs if the builder determined that improper installation of the synthetic stucco had caused moisture to penetrate the houses.

In March, company officials sent a second letter saying they had identified a series of repairs they planned to make, such as caulking, and they offered to reinspect homes in a year to be sure that "the system is performing as designed." After that, the letter said, it would be the homeowners' responsibility to inspect and maintain their houses to prevent moisture from seeping in.