Union Station's leaky roof has made parts of that magnificent old structure "unsafe for human occupancy" and some offices will have to be evacuated, National Park Service engineers have determined.

However, the water problems have not reached the public areas of the National Visitors Center in the station's old main hall or in the gallery behind it. The Metro subway station and Amtrak passenger operations also are unaffected, according to station officials.

However, one domed section of the portico - the covered walkway that runs the length of the building in front - has been barricaded for "safety reasons," signs warn.

Water damage can clearly be seen in the dome of the barricaded section and Park Service officials are keeping watch on a widening crack in that dome.

James C. Gross, general manager of the Park Service's National Visitors Center, recently sent what amounts to an eviction notice to some out-of-sight offices in the east and west wings of the building. A copy of his letter was obtained yesterday.

Because of "water intrusion," Gross wrote the Washington Terminal Company, evacuation of "all occupied offices in the west wing of the center and the public and private areas of the Railroad YMCA should begin immediately."

The Washington Terminal Company is a joint venture of the Chessie System and Amtrak. It owns the new Amtrak station behind Union Station and the rail yard behind that.

Union Station, or the National Visitors Center, is now owned by the Department of Interior, which wants to give it to the Department of Transportation, which wants to fix it up for $52 million. Congress is presently mulling that proposal.

Meanwhile, the roof continues to leak.

The Washington Terminal Company's accounting office - about 20 people - is located in the west wing, along with the Railway Employees Federal Credit Union - about six people. Both organizations are looking for new places to go, their officials said yesterday.

The Railroad YMCA, in the east wing, is where Chessie and Amtrak crews lay over between shifts. A couple of desultory card games were taking place amid its peeling paint and crumbling plaster yesterday afternoon.

"We need a major structural study," said Gross as he led a tour of back corridors and locked, abandoned offices. "You can only patch up the old lady so long before she needs some real attention."

Part of DOT's $52 million proposal would include a major structural study, repair of the roof and replacement of the electrical system.

It is the combination of leaking water and ancient electrical fixtures that particularly disturbs Gross. "There is jury-rigged wiring all over this place," Gross said. "We have a couple of fixtures that were literally dripping with water."

There has been at least one fire. It was in an old sauna and exercise room on the fourth floor of the east wing. The damage is still there to see.

C. W. Shaw Jr., manager of the Washington Terminal Company, said his greatest concern is to find room for his accounting department.

"We're trying to buy some time," he said. "There's a computer to move and we can't have these operations too far from the terminal itself."