Before sunrise each morning in rural towns of Virginia's Piedmont and the Shenandoah Valley, vans subsidized by major building contractors from the Washington metropolitan area load up with people willing to commute the 150 miles or so a day to work on construction sites in Northern Virginia.

Other laborers from as far afield as Pittsburgh, Philadelphia, North Carolina and the Eastern Shore, attracted here by advertisements in their local newspapers placed by Northern Virginia contractors, are renting inexpensive motel rooms by the week along Alexandria's Route 1 while they work here during the construction boom.

A fraction of the distance away in the District of Columbia are 27,800 jobless people, 8.6 percent of the District's population, but contractors say that work force is unreliable. They prefer to recruit further afield.

"It's a crying shame," said Robert J. Murphy, the largest carpentry contractor in Northern Virginia. "I've looked toward D.C. numerous times, but it's never worked out because these people are not accustomed to showing up on time and being dependable." Murphy has run a van through Luray and Stanley, Va., for the past year to collect skilled and unskilled carpenters.

United Masonry, a pioneer in busing labor from rural areas, ships in about 10 to 12 percent of its 1,000 workers daily in 12 vans from Frederick and Hagerstown, Md., and from Front Royal and Winchester, Va. Although the president of the company, Douglas A. Hottle, does send some vans to the District, he agrees with Murphy that the quality of that labor force is unpredictable.

"We have trouble with a lot of inner-city people working as hard. It's basically manual labor, carrying material . . . the core of unemployed there is not willing to do the kind of work we need," Hottle said.

Northern Virginia building contractors blame D.C. laborers' alleged unreliability on a work ethic that shuns heavy labor such as hauling materials under a hot sun, while blue-collar workers from agricultural areas or heavily industrialized cities are accustomed to those conditions. Contractors also point to inadequate public transportation to the sites for unskilled workers who cannot afford their own cars.

On the other hand, the executive secretary for the Washington Building Construction Trades Council, Robert S. Parker, blames Northern Virginia builders for paying menial wages to unskilled workers, offering no benefits and failing to invest in training.

"We don't have those problems with District labor. These people are just picking up people off the streets and expecting them to be qualified. If they're paying substandard wages, they'll get substandard work," Parker said. His council represents 22,000 construction trades workers, about 25 percent of whom are D.C. residents.

Under contracts with the Washington Building and Construction Trades Council that cover 75 percent of the District's construction workers, companies pay a beginning apprentice $5 to $6 an hour, plus health benefits and a four-year training program with night classes and yearly pay increases, Parker said. In Virginia, a right-to-work state without unions in the homebuilding industry, there is no equivalent program. An unskilled worker starts at $4.50 to $5 an hour and receives no fringe benefits or formalized training and has limited prospects unless a foreman takes a personal interest in an individual, according to Ken Weeden, a specialist hired by the Northern Virginia Builders Association to establish training programs.

Next week, the first group of unskilled workers begin an eight-week crash program to learn basic construction techniques while building a home in Lake Ridge, Prince William County. By summer's end, NVBA expects to put 50 to 60 people through the course. Weedon also is organizing courses for skilled tradespeople on blueprint reading and trim carpentry.

The program is costing NVBA $1,000 to establish. Although the program is meager compared with the comprehensive training that unions in the District offer to unskilled workers, the NVBA's executive officer, Samuel A. Finz, said it shows the new commitment that Northern Virginia builders are making to training.

In the long term, this will ease the Northern Virginia building contractors' recruiting problems. In the meantime, they say they have to cope with turnover rates among workers from the District and rural areas that can run as high as 80 percent. "They come for a day or two, then leave," Hottle said.

The director of the District's Office of Employment Services, Matthew F. Shannon, agrees that Washington youths adjust badly to strenuous work conditions on construction sites, but he says it reflects less on the young people's willingness to work than on their expectations. With the federal government as the District's largest employer, its residents grow up expecting clean, air-conditioned, white-collar, service-oriented jobs, Shannon said.

Racism on the construction site also discourages D.C. residents from returning to work in Northern Virginia. "Many of the foremen are rednecks from West Virginia who tell jokes" about blacks, Weedon said. "There's also a macho thing here. It's 'I went through hell and you should too. It'll make you a man.' But kids today aren't prepared to put up with that."

On a construction site, racial slurs are doubly threatening, Shannon said. If they called you a name "the day before and you're 20 stories up on scaffolding, you're going to fear for your life. That can be a major disincentive to returning to work the next day," Shannon said.

All contractors interviewed agreed that there are racial tensions on the job site, but said they doubt that it's any worse than other segments of society.

Said Murphy: "The industry was prejudiced and harassed when I started, but you have to eat it, you have to take it . . . the jobs are there if they want them and are prepared to work for it."