Claude Shaffer, president of Alexandria Asphalt, juggled his crews like mad last week, but he found himself without enough men to do the work scheduled.

On Tuesday alone, the Northern Virginia parking lot paving company had jobs worth $500,000 to do, including a project at the Tysons Corner Holiday Inn that was to start that day. But, no matter how Shaffer figured it, he didn't have enough workers -- and he couldn't find enough to do the job.

In booming Fairfax County, where what was once 400 square miles of rolling farmland is teeming with new shopping centers, subdivisions and high-rises, employers in the blue-collar and service industries say they are suffering from the worst labor shortage in recent memory.

The March unemployment rate in the county was 2.4 percent, a rate that trails state and regional unemployment figures and compares to a nationwide average of 7.5 percent. In the District, the March figure was 8.1 percent.

With so few people looking for work in the mostly white-collar county, some employers bring workers from as far away as North Carolina or the depressed steel towns of Pennsylvania.

Between 3,000 and 5,000 Northern Virginia jobs are currently listed with the Virginia Employment Commission, and observers say there are many more out there. In Fairfax County, the lack of workers has slowed home and swimming pool construction and has left department stores and restaurants seriously understaffed.

"It's a big problem when you've got work to get out and nobody to do it," said R.B. Anderson, of Anderson Excavating and Paving in Falls Church, who needs a crew of 12 but has only seven.

"I'm just flabbergasted," said Jay Goyert, maintenance foreman at The Fishel Co., a contracting firm on Old Lee Highway that installs underground utility and water lines. Goyert's average daily work force is 65, but it should be 120. "I've never seen anything like this."

The problem is not unique to Fairfax County, but observers say it is felt most keenly there because of demographics.

Fairfax's population is wealthy, homogeneous and well educated. An estimated 77 percent of Fairfax workers are white-collar, according to the 1980 census. The estimated median family income for 1985 is $54,100, according to the county office of research and statistics. Four out of five Fairfax high school students go on to college.

Fairfax employers complain that, with that kind of population, the sources of workers for entry-level labor are slim.

Most Fairfax housewives who seek work through the Re-Entry Women's Employment Center, a county agency that offers career counseling to about 5,000 women each year, already have bachelor's degrees and strong professional or functional skills, said Leia Francisco, director of the center.

High school students can fill gaps during the summer, but many shun entry-level jobs in fast food restaurants in favor of work that pays more.

"I think the income level in this area is such that families help support the kids. Many of them don't really need jobs," said William Sheppard, personnel manager for JC Penney Co. at Fair Oaks Mall.

The problem is not new, but it is growing. The Tayloe Murphy Institute reports a 35 percent increase in Northern Virginia "help wanted" classifieds since last spring -- the biggest one-year increase since 1980.

To fill those jobs, many Fairfax employers have traditionally journeyed west and south, to places like Manassas and Culpeper, and even to North Carolina, to find refuse, construction and other workers.

At Baird Concrete, for example, a 15-passenger van makes the round trip each day between Fairfax and Colonial Beach, Va., about 25 miles east of Fredericksburg. Another Baird van travels weekly across the North Carolina border to Enfield. During the week, those workers are provided with company housing, say Baird officials. At The Fishel Co., roughly 40 percent of the work force is from out of state, primarily from West Virginia's coal country or from Pennsylvania.

"Laborers really can't afford to live in Fairfax County," said Baird's Wayne Mosser. Last year, the estimated median value of a home in the county was $108,100. "It's just too expensive to live in," Mosser said. "And, these are guys who make pretty good money: $12, $13 and $14 an hour."

While Fairfax employers are willing to go outside the area to find laborers, they say there are many reasons they do not siphon more workers from the District, where about 26,000 persons were unemployed in March.

"That's a very sensitive and difficult area to address," said Fairfax developer and lawyer John T. (Til) Hazel. "The District labor pool . . . much of it is unskilled, and some of it is unlearned."

"Many of the people who are unemployed, at least some of the younger people, don't have much of a work history," said Thomas Muller, an economist at The Urban Institute. "Some of the young people don't have a work ethic that employers look for. They haven't held stable jobs."

According to a 1983 D.C. government study, 34 percent of unemployed black men in the District have been without work for between one and two years. More than 40 percent of those, considered to be "discouraged" workers, have either some college or a college degree.

Fairfax employers also say the Potomac River provides a "philosophical and psychological" barrier to District workers. Also, public transportation to the outer reaches of Fairfax and its neighboring counties can take hours for low-paid workers who often cannot afford cars. "How am I going to get anybody here?" asked Merwin Decker, owner of the Anchorage Motel on Lee Highway.

Montgomery County, with much the same demographic profile as Fairfax, has an easier time attracting District workers, with more Metro stops and without the obstacle of the Potomac to consider.

The time taken by long commutes from the District to Fairfax are of special concern because 61 percent of unemployed black District women and 54 percent of "discouraged" black women have dependent children under 18, according to the D.C. government study, which was conducted by the Washington Urban League.

The Greater Washington Board of Trade is studying job accessibility as part of a larger examination of the region's unemployment, said spokeswoman Susan Pepper. "It would be premature to say that we could come up with a whole new scheme to get people out of the city and to the jobs," she said.

Another problem is the absence of a regional job bank and insufficient coordination among area job service agencies, said Matthew Shannon, director of the D.C. Department of Employment Services. He said the best Fairfax jobs are often snapped up by Virginia workers. "We tend to get the crumbs that fall off the table," he added.

Virginia's "right-to-work" laws also present a barrier, said Ron Richardson, secretary-treasurer of the Hotel and Restaurant Workers, Local 25, which represents 10,000 persons, mostly in the District.

Richardson said a union hotel maid, working in the District, can make $6.55 minimum, plus enjoy job protection and an assortment of benefits. In Virginia, the same maid usually makes $3.35 to $3.50, but with no job protection and minimal benefits.

"So, when you look at these differences and consider that your job in Virginia is tenuous at best, people really hesitate to get off public assistance, if that's what they're on, or food stamps," he added.

Thomas McNutt, president of the United Food and Commercial Workers, Local 44, which represents 30,000 area persons, says D.C. residents are more aware of their rights. "They have been involved with unions, and there's a substantial resistance in Virginia to import that kind of person to your work force because it might become organized," he said.

Joslyn Williams, president of the Metropolitan Washington AFL-CIO, which represents 200,000 persons, said fear of hiring poor, desperate, inner-city blacks is a factor. "Given the choice, they might be more willing to hire a suburbanite who went to suburban schools and who might be white," he said.

Meanwhile, Fairfax employers have resorted to novel ways of finding help. At Goodfriend Temporary Services in McLean, manager Ken Harhai said he hopes to lure summer students by offering the chance to win $500 college scholarships.

At Bradlees, which has five Fairfax County stores, personnel supervisors are soliciting at senior citizen homes for part-time workers, said Leonard Calabree, the regional personnel manager.

"We're continually trying all kinds of new ideas to get people," said Alexandria Asphalt's Shaffer. "If a guy will bring another guy in, we'll pay him a $25 to $50 commission."