At the Norwegian Embassy in Washington, Per Aasen, the press officer, said that he was aware of the custom here, but added, ''I didn't know we had exported it.'' In Norway, he said, the custom is prevalent not only for major buildings, but also for private homes.

''When my parents built our home in the 1930's, this tradition was there,'' he said. The building owner's obligation was to provide a festive meal for the construction workers at such a time, he said, or, if they did not plan to host a party, to mount a scarecrow on the roof instead.

According to Christopher Gray, an architectural historian and expert on New York from 1875 to 1935, the ceremony may mean more now than it did in the last century. ''I've often wondered, in reading 19th- and early 20th-century real-estate and construction periodicals, why I don't see any mention of topping out,'' he said.

Mr. Gray believes that the custom then was widespread but informal. In those days, he said, the fuss was made over laying the cornerstone. Of topping out, he said, ''maybe it was a workman's ritual that was only adopted later as an opportunity by the powers that be.'' A topping- out now, he added, is ''a media event,'' a milestone that builders use for publicity.

In addition, he noted, until the advent of the skyscraper in the second decade of the century, builings were low and flat. ''Topping out meant a lot less when buildings weren't so tall to begin with,'' he said.

The flag was the focal point by the time the skyscraper came on the scene, and a major preoccupation was its height. The New York Times for June 30, 1912, in reporting the topping out of the Woolworth Building, then the world's highest, carried a report headlined: ''Flag to Fly 830 Feet Up.'' The story detailed to the inch the height of the building, the ornamental tower, the flagpole and two other ''tall'' downtown buildings.

The Empire State Building had three topping-out ceremonies in the fall of 1930. In the first, at sidewalk level, Alfred E. Smith, then a popular former Governor, cemented in place a stone containing a time capsule as 5,000 people watched. Nine days later, steelworkers raised a 48-star flag 1,048 feet above Fifth Avenue to mark the completion of the steel frame. In November, another flag was raised when the last piece of steel - intended as a mast for mooring dirigibles - was placed in the tower.