Home builders recognize that even the best-built houses are not perfect. And many builders will tell you that wet basements are among the most common imperfections.

One engineer with a professional interest in housing believes that half of all single-family houses may have wet basements at one time or another.

After more than three inches of rain fell on an already snow-laden Washington area last weekend, thousands of area basements had taken on water, as they say in the Navy.

Some had to be pumped out by firemen. Some had dampness or puddles. Others required mopping or vacuuming. Sump pumps were running in many hundreds of houses here.

"We had an incredible number of calls," said one home inspection specialist.

Eleanor Ulrich, who owns a year-old house in the Burke Station area of Northern Virignia, said that 2 1/2 inches of water accumulated on her basement floor, with all four walls contributing to the peoblem. She said she and her husband now plan to sell the house -- whose basement has been "overly wet" five times in 12 months. "We've had it," she said.

Howard Simon, president of Guardian Construction Co. which built the Ulrich's home, said that the firm had offered to repair the basement walls last fall but that Ulrich refused to permit workmen to disturb her tulip bed. "We'll fix that basement but we have to dig around the foundation," Simon said.

Not all leaky basement problems are beyond fairly easy solutions. Marketing specialist John Gornall said that his Treasure Oak town house in the section of Potomac near Seven Locks Road escaped the weekend with only one damp spot on a wall. Previously the basement of the six-year-old dwelling had taken on water during heavy rains.

"I got some professional advice and followed it," said Gornall. "It was worth the $85."

Last summer Gornall got an inspection by Home-Tech Systmes Inc., a firm that specializes in home inspections but does not do any work itself. Gornall and his wife got a succinct 300-word report on their problem from Home-Tech and followed the advice.

Gornall extended downspouts, kept gutter clean, did some new landscaping and had a trench dug in the yard to divert water from the exterior walls. He said he spent less than $250 to improve runoff and drainage.

Jack Reilly, a member of the Home-Tech firm, had some advice for anyone with a wet basement problem. For starters, he siad, "a house is not a boat." it can't float on water and is susceptible to exterior water build-up and hydrostatic pressure.

Reilly and other professional inspectors urge homeowners to keep their gutters and downspouts clean to provide maximum runoff. But William Papian of the Claxton Walker Associates inspection firm, added: "Not even clean guttering is capable of handling all the roof runoff during a severe rain."

Railly suggested that homeowners should grade around their houses again because house locations are back-filled after the foundations are dug and built. Even if the back-filling is done in the recommended stages and tamped strongly, the house is likely to settle during the first five years -- "maybe as much as 5 to 7 inches," Reilly said.

If debris and porous material are in the original backfill dirt, the problem may become exacerbated."Anything that holds or sheds water in the ground contributes to the build-up and deters natural drainage," he said. Of course, an unusually high water table or underground springs present special problems.

Reilly suggested that it is incumbent on owners to put in new fill dirt ("good old clay dirt that we have around here") and keep the grade line sliping down and away from the house. He said that sump pumps and French drains can be installed to take care of extreme problems.

Reilly is skeptical of the value of interior basement wall waterproofing because the main objective is to move the water away from the exterior walls rather than to have the foundatin walls soak up water.

Papian, an engineer with the Walker home inspection firm, said that those concrete splash blocks under downspouts tend to "walk away." He also said that they are often too short to do a good job of moving water from downspouts away from the house.

A member of the Home Owners Warranty staff of the National Association of Home Builders said that 10 percent of the claims against that insurance program to protect new home buyes have concerned wet basement problems.

Many owners of homes with base ments experience water seepage problems. Some spend sizable sums of waterprooding and sump pump instal lations. Jack Reilly said his experience has shown him that many wet basement problems can be solved or substantially alleviated, except in times of hurricane-like rains, by improving guttering and downspout efficiency and by landscaping and regrading to drain water away from the foundation of a house.