Neighbors pounded on Dara Padwo's door at 3 a.m. last Saturday, screaming that their town house was being flooded and that the water was coming from her home.

Before the water could be turned off, the neighbor's den ceiling, situated below her kitchen, had collapsed with a thundering crash.

"I thought, 'No, not again, this can't be happening again,' " said Padwo.

It was the third time a plastic water line had separated in the brand-new Silver Spring town house that she and her husband, Eric Pripstein, moved into six months ago with their year-old son. In each case, a copper band joining sections of the plastic pipe -- approved only two years ago by Montgomery and Prince Georges county authorities -- had come undone, sending a steady flow of water into their home.

County officials say they know of at least 12 such incidents in the county since the plastic pipe was approved in 1982, but estimate that the number of unreported cases is much higher.

"We've had no problems with the plastic pipe itself, but the copper crimps, which are difficult to install, have come undone," said Michael P. Boswell of the Washington Suburban Sanitary Commission, which regulates plumbing in the two counties. "The problem is that the crimps have been badly installed."

Hard copper crimps on plastic pipes were banned six months ago by the commission, which now requires plumbers to use a soft copper crimp that is easier to install correctly. But Boswell estimates that plastic water pipes were installed with the troublesome hard copper crimps in thousands of single-family houses and town houses in the two counties before the ban was put into effect.

"I wouldn't say it's a time bomb out there," said Boswell. "But it is possible those hard copper crimps were installed incorrectly, and they may seperate either tomorrow or over a period of years."

Plastic piping, usually made of polybutylene, has been a boon to the construction industry nationwide since it was introduced 10 years ago.

"It's light, installs quickly and costs 20 to 25 percent less than traditional copper pipes," said John W. Fraser, general sales manager of Vanguard Plastics Inc. of Kansas, a leading manufacturer of the pipe. "It won't rust and can expand when its cold, so there is less of a chance of a rupture when it freezes," he said.

Building officials in Maryland and in other local suburbs agree that the hard plastic pipes are an excellent substitute for copper. They say that the problem is that in many cases plumbers have not been installing the pipes correctly when using the hard copper crimp to join pipe sections.

"If the hard copper is installed correctly it is safe and sound," Fraser said. However, he said the industry has voluntarily discarded the hard copper crimps in favor of a soft copper band that is easier for plumbers to install. "We also developed a new tool for them, so it's a snap to do right," Fraser said.

The voluntary change by the industry was not quick enough, say authorities in the two Maryland counties, who changed the law first. "We were the last jurisdiction in the world to approve plastic pipes," said Boswell of the WSSC. "We wanted to be sure it was safe. We're disappointed now that there was still a problem the industry had not told us about."

Boswell said the faulty hard copper crimps have caused untold water damage in the two counties. He said a pipe in one Montgomery home separated last year and was discovered two days later when the owner returned from vacation.

"We aren't talking about a leak, like in a copper pipe," he said. "We're talking about a steady outpouring of water."

Maryland plumbing inspectors, like those in Virginia and the District, require builders to test plumbing systems by applying 150 pounds of pressure to the pipes in any new home. The water pressure in a private home averages 50 pounds and cannot exceed 60 pounds by law in Maryland, said Boswell.

In addition, Boswell said inspectors will spot-check about 50 percent of all copper crimps installed in the county.

"Usually that's enough to catch the problems," he said. "But steady pressure day in and day out can result in breaks thet don't show up during inspections."

A spokesman for James A. Federline Inc., the plumbing company that has installed much of the plastic piping in Maryland and the District area, including the pipes in Padwo's town house, would not comment.

"It's really a manufacturing concern," said Vice President Ronald Bryant. "They, not the plumbers, should talk about it."

Officials at Artery Organization Inc., the large building firm that constructed Padwo's $70,000 home in the Townes of Glouster subdivision off Route 29, were unavailable for comment.

Other area builders, however, said they've experienced few problems with the plastic pipes, which they praise as allowing them to cut the price of new homes.

"We are always looking for ways to make plumbing -- or any other aspect of a home -- less expensive and trouble-free," said Tony Ahuja of the Northern Virginia Builders Association. "After all, we don't want to have to go back and fix pipes any more than the consumer wants it."

The plastic pipes have been permitted in Virginia since the state building code was amended in 1981 and in the District since the pipes were introduced 10 years ago. Officials in both jurisdictions said they have had few complaints about the plastic pipes.

"Like any new technology, there were some problems to be worked out by the industry," said Claude Cooper, chief plumbing inspector of Fairfax County, who said the state does not specify what type of crimp must be used. "Most of the plumbers had some problems at first, but not now."

The District's chief plumbing inspector, James C. Sutton, said only 20 to 30 percent of newly installed pipes in D.C. are made of plastic. "Most plumbers seem to prefer copper because they're familiar with it," he said. "But we've had no problems with the plastic."

But for Padwo and Pripstein, the plastic that has revolutionized the building industry has caused nothing but heartache in their first home. The first break caused their living room ceiling to collapse, the second was caught before it caused any major damage and the third has ruined their kitchen.

Artery has paid for fixing the pipes, which are under warranty, and for repairing the water damage.

But the couple has moved temporarily to a hotel, for which they may or may not be reimbursed.

"We don't know when the next one is going to go," Padwo said.