Bob Sparks is used to getting ripped off.

Usually they clean out his trailer. Or help themselves to his lumber or tools. But when they wrested the furnace right off a wall in one of his unfinished town houses, the Northern Virginia builder said he knew how serious the problem had become.

Fairfax County police say Sparks' experience is not an exception. With development and construction at record levels in the county, thieves are taking off with everything that is not nailed down at some construction sites. And a lot of things that are.

"That's really where the crime problem is right now," said police spokesman Warren R. Carmichael. "And I don't see that there's going to be any curtailment of this problem any time soon . . . . It appears we're going to have a large number of construction sites with us for years to come."

Despite decreases in many categories of crimes in the county during the first six months of the year, Fairfax police say the overall level of serious crime rose 12 percent over the previous year. They attribute much of that increase to larcenies reported from construction sites.

Last year there were 554 reported thefts at construction sites in Fairfax, compared with 334 the previous year, according to police; this year police already have logged more than 405.

Police caution that these numbers are probably low, because some larcenies were lumped under a general theft category and therefore were not included in these construction figures. Also, many thefts at construction sites go unreported, police say.

A June raid of a fencing operation in the District gave a glimpse of the problem. About 5,000 items, taken from Northern Virginia construction sites and valued in the millions of dollars, were confiscated. Among them were refrigerators, stoves, microwave ovens, mosaic tiles, windows and doors.

D.C. police inspector Edward J. Sperlock said that a large portion of the confiscated items came from Montgomery County. "It's easy everywhere," Sperlock said of construction site theft. "Easy. When you're talking new construction, you're talking easy."

Construction theft, he said, is so widespread it is "a pain" for police, "especially if you're talking whole developments."

Lt. Bill Edmonston, formerly supervisor of the criminal investigation section of the West Springfield Police District in Fairfax, knows what Sperlock means. That district is one of the most rapidly developing areas in the county.

Edmonston said security at construction sites in Fairfax is "terrible." The average construction site has no security, he said.

Although many builders purposely delay until the last minute the delivery of such costly appliances as microwave ovens and dishwashers, Edmonston said that is not enough.

He said he encourages job foremen, who frequently ask police for advice, to use more locks and fences, keep better records, get better lighting, mark their materials and, whenever possible, hire guards.

"I'm afraid to put appliances in 'cause they'll come back and get them," said Sparks, superintendent of the TysonsTowne construction project, sitting next to a shattered window in his job site trailer. "When you hit the finishing stages, that's when the money gets in there."

Charles T. Meyers, vice president of construction with Signature Communities Inc., said the theft problem seems to be worse these days than he's seen in the past five years. He said his company is experimenting with several new security systems.

But another problem, he said, is knowing when something has been taken. "With me," he said, "it's a question of knowing if something has really been stolen."

Builders say it's difficult to keep track of -- and mark -- such things as two-by-fours, Black and Decker drills, circular saws and Craftsman hammers, which are regular items on the police blotter. Officers say the lack of ownership identification on such materials makes them hard to trace.

"One serious problem with a construction job is you just don't know what you have," Sparks said. "Some items get broken, some get stolen. You don't know what was stolen. You can only guess."

While Sparks reported his stolen furnace, valued at about $1,200, including labor, some builders say it is not worth the trouble to report many thefts because the claims are likely to drive up insurance premiums.

"Normally, if somebody steals 15 sheets of plywood, at $15 a sheet, my firm would not collect," said Fulton Gordon III, executive vice president of Gordon Builders. "If we made a claim every time we turned around, they'd cancel us ."

Insurance agent Karl Nelson said that while no single builder seems to have been hit dramatically, his company, Smith-Field Insurance Agency, has noticed an overall increase in theft. Some Monday mornings there is a flurry of calls from builders who have had losses during the weekend, Nelson said.

"Insurance companies are losing money as a result of theft," he said, "and they are really going to increase prices."

Sparks said that when insurance rates go up, he will have to make an adjustment: "You have to pass it on to somebody."

"Believe me, they builders aren't taking the loss," he said. "The consumer's the one who suffers."

Al Hearren, superintendent of the Sequoia Building project across the street from Fairfax City Hall, agreed that, ultimately, the consumer pays. "If you're in an area where you think there is the likelihood of theft, as with anything else, you take that into account," he said.

Hearren said that if a builder experiences more theft than anticipated, the loss comes out of the profits. "Next time around, you're going to look harder," he said.

Meanwhile, Sparks, who is building 104 rental town houses near Tysons Corner, said police have taken fingerprints, tire prints and blood samples from the area where his furnace was ripped from the wall.

He said that he has propped the windows closed with sticks and is using locks, and he will have someone remain overnight at the site when the appliances for his town houses are delivered.

"It makes you lose faith in mankind," Sparks said of the theft problem. "You're scared to death."