Every Tuesday and Thursday night, James Starkey attends training classes at the Ironworkers Union hall in Northeast Washington and then drives the 96 miles back home to State Line, Pa. After 15 years as a construction worker, Starkey, 37, has become a union man.

"I tried to get in the union for years, but I couldn't because they didn't have the work and weren't letting people in," Starkey said during a break in his welding and blueprint-reading class. " . . . Now that I'm in, I love it. The training is better, the jobs are safer and I love the benefits."

This newly certified journeyman ironworker is part of a surprisingly strong recovery under way for Washington area construction unions. Long in decline, the unions have rebounded in the past two years from roughly 20 percent unemployment for their members -- and up to 40 percent in some trades -- to what they describe as full employment.

Thanks to an estimated $3 billion-a-year commercial building boom in the region and to innovative cost-cutting agreements offered to contractors, the 20 building trades unions, which have 25,000 members, have been able to recruit several thousand new members. Among them are many from outlying jurisdictions such as Starkey, who is currently welding steel railings at the new Hyatt Regency in Bethesda and is considering moving to the Washington area.

"We have everyone working. For the past 18 months, we have been virtually fully employed," said John P. O'Connor, secretary-treasurer of the Washington Building and Construction Trades Council.

"We were losing people who ceased being members and went nonunion. But they have returned for the simple reason they can support their families again" and usually receive better pay, pension and health benefits than nonunion workers, he said.

The reason for the union recovery, O'Connor said, is that "the times are better. But it's also because we have gotten competitive" by cutting the cost of union labor by as much as 20 percent, sometimes without cutting members' wages.

Changes in work rules, increased use of lower-paid helpers and apprentices, and in some cases wage freezes or 10 percent pay cuts have succeeded in dramatically reducing the cost advantage of lower-wage nonunion builders, according to company and union officials.

Local unions in recent years had lost at least 10 percent of their membership, but they have recovered those losses and expanded because unionized construction firms that employ them are booming, company and union officials say. They add that cost-cutting union contracts, which include "no-strike" guarantees, enable the construction firms to win some jobs that previously went to nonunion companies.

The cost-cutting agreements, which originated in 1984 and are incorporated into union contracts with hundreds of unionized contractors and subcontractors, are part of a long-range "market recovery" program promoted by the AFL-CIO in 27 metropolitan areas to help rejuvenate the slumping construction unions.

Results of the program have not been compiled, but an AFL-CIO spokesman said there have been "encouraging signs" in many areas, including Washington.

"We are getting jobs now that we would not have gotten" before the union concessions, said Jack Greenup, vice president of Turner Construction Co., a unionized firm whose work here has doubled in recent years to $100 million. "I was very pleased and glad to see that the management of the unions understood the economics and understood what was required if they wanted to preserve their work."

Unions dominated the construction scene in Washington into the 1970s but then went into sharp decline as developers sought to curb skyrocketing construction costs and turned to the "open shop" -- nonunion -- builders, particularly in the growing suburbs.

The growth of nonunion companies, including "double-breasted" unionized firms that established nonunion affiliates to win more jobs, has reduced the union share of the construction market to about 25 to 30 percent, according to the Associated Builders and Contractors of Metropolitan Washington, a predominantly nonunion industry group.

Donald Manger, the group's executive director, said that the pace of the building boom in the Washington area has been so frenetic that there is more than enough work to go around for union firms and nonunion firms. He said he doubted that the recovery of union contractors has taken much work away from nonunion ones, although no data is available.

"It is a record year. A huge year," said Robert Herr, president of Donohoe Construction Co., a leading nonunion firm. Herr said he believes that union concessions are not deep enough to persuade more developers to build all-union projects. Only about 10 percent of projects in the region are all-union, according to the Associated Builders and Contractors.

But Herr said that the union concessions have apparently enabled union subcontractors in each craft, such as electrical, plumbing, and carpentry work, to win a larger share of the contracts.

Some union members opposed the wage concessions and new rules, O'Connor said, "but we feel we would have been frozen out of work. We were going downhill fast . . . . Obviously, there's a building boom, but we would not have been a big part of it without this program."

The program allowed companies such as Garlinski Steel of Alexandria, one of several hundred firms under union contract, to cut costs sharply after the rodmen's union Local 201 agreed to a pay freeze. It allowed the firm to hire more lower-paid trainees to assist the $14.61-an-hour journeymen rodmen, who install reinforcing iron rods.

The rodmen's union, seeking to protect jobs, previously allowed companies to use only one trainee for every five veterans. The union agreed to cut the ratio to 1 to 3, which creates substantial savings because trainees earn only about half the pay of journeymen.

Other costly and long-established union rules have been scrapped by some unions, such as the requirement that if weekday work were canceled by bad weather, the Saturday makeup day would be paid at time-and-a-half. Now, weekend work is paid at regular rates.

"If we didn't have this agreement, we would only have about 33 percent of the work we have now," said Kenneth Wood, vice president of Garlinski. Wood said his firm's prices are still often 5 to 10 percent higher than nonunion competitors, but he said Garlinski is winning jobs because "we have highly skilled labor. We can use less men. get it done quicker, bring the job in on time. And everyone is happy."

The recovery has allowed the rodmen's union, 40 percent of whose members had been unemployed, to provide jobs for all 350 members and for 125 new arrivals, many from Ohio, West Virginia and Pennsylvania, said George Hindle, the union business manager.

"It was hard to sell" the wage freeze and new rules to many members, he said. "Nobody likes to work for less than you know you are worth." But opposition has lessened, he said, because the recovery enabled the union to get a 5 percent wage increase in its new contract this year.

Ironworkers Local 5 has the most work for its members since it was created in 1901, according to business manager Billy Joe Walker. He said records show that his 900 members have logged nearly 1 million work hours in 1985, an increase of 40 percent over two years ago. Training classes have expanded from 40 participants to 320, membership has grown by 100, and more than a dozen new firms have signed union contracts, he said.

"My costs are now compatible with an open-shop firm, and I am building it faster and that enables me to cut costs," Greenup said. "It may cost me $120,000 more to use union labor, but I can finish the job two months earlier . . . . "

Wood, who has supervised nonunion firms and union firms, said Garlinski is profiting more from union work: "With the nonunion, we never made a dime. The work force was inexperienced . . . . They didn't show up. You would pay them Wednesday and not see them Thursday and Friday, and I could not find enough qualified people for some jobs."

However, Manger, of the Associated Builders and Contractors, said the growth of nonunion firms has allowed them to improve training and the caliber of workers, and said their dominance of the market is evidence that the quality of work is comparable to that produced by union workers.

The key attraction of construction unions is that they usually can guarantee a builder a sufficient labor supply through the union hiring halls, contractors said. But the higher wages, the depletion of union membership and the traditional threat of work stoppages had dampened enthusiasm for union labor, they said.

The no-strike clauses are particularly important because of developers' fears of costly delays, officials said.

"No work stoppages. No walkoffs, no slowdowns," O'Connor said. "Disputes are handled by an ongoing labor-management committee . . . and they have given the developer the assurance that his job won't be stopped."

Starkey said his year-long grind of five hours a week of night classes and commuting nearly 200 miles every day has been worth the effort.

"I've worked all over the U.S.," he said " . . . On the nonunion side I got no training. You go out on the job and learn by mistakes . . . and accidents. This is more professional. You don't go out with untrained and unskilled people, and I found out what I'd been missing for 15 years."