Frank Kind, shown in 1995, washing a window on the 46th floor of a building in Midtown Manhattan. Credit Andrea Mohin/The New York Times
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As a pair of window washers clung to a scaffold dangling outside the 68th floor of 1 World Trade Center on Wednesday, the captivating drama left some below wondering: Why were they up there at all?

In an age when a few clicks on a cellphone can solve myriad problems, it seems fair to ask why people are still descending from the roofs of skyscrapers to rub soapy water on glass and wipe it off with a squeegee. Can’t robots take on this simple, repetitive task and relieve humans of the risk of injury, or death, from a plunge to the sidewalk?

The simple answer, several experts said, is that washing windows is something that machines still cannot do as well as people can. The more complicated answer is that high-rise buildings are more complicated than they used to be.

“Building are starting to look like huge sculptures in the sky,” said Craig S. Caulkins, who consults with building owners on how to maintain their exteriors. “A robot can’t maneuver to get around those curves to get into the facets of the building,” he said

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Man on an Edge

Man on an Edge

Brent Weingard, of Expert Window Cleaners, has battled dirt and grime high above New York City for over 35 years.

Video by David Frank on Publish Date August 9, 2013.

Mr. Caulkins, the president of C. S. Caulkins Co. in Irvine, Calif., said “the robots have problems.” Most notably, he said, robotic cleaning systems tend to leave dirt in the corners of the glass walls that are designed to provide panoramic views from high floors.

“If you are a fastidious owner wanting clean, clean windows so you can take advantage of that very expensive view that you bought, the last thing you want to see is that gray area around the rim of the window,” Mr. Caulkins said.

Indeed, prime examples of that particular shortcoming are the towers of the original World Trade Center; the Port Authority of New York and New Jersey equipped those buildings with a mechanical apparatus for cleaning their windows, but it worked so poorly that human window washers had to follow behind to catch the spots the machine missed, said Steven Plate, director of the World Trade Center construction department at the Port Authority.

“It was never effective,” Mr. Plate said in an interview on Thursday. “It basically didn’t clean the building.”

One of the men who kept the glass of the twin towers clean was Roko Camaj, whose hazardous duty was the subject of a children’s book “Window Washer: At Work Above the Clouds.” The book, published in 1995, quoted Mr. Camaj saying that “Ten years from now, all window washing will probably be done by a machine.”

On that point, Mr. Camaj, who was killed when the towers were destroyed on Sept. 11, 2001, was not a prophet. Mr. Caulkins said “there may be several dozen” skyscrapers in the entire country that employ robots to clean their windows.

Another reason for the sparse use of robots is that buildings require a lot more maintenance than just window cleaning, Mr. Caulkins said. Equipment is needed to lower people to repair facades and broken windows, like the one that rescue workers had to cut through to rescue the window washers on Wednesday, he said.

“At some point you need workers up on the side of that building,” Mr. Caulkins said. If a building requires a mechanism on the roof to lower a platform for that sort of work, the owner is unlikely to invest in a second, mechanized system for cleaning the glass, he said.

In all, there are about 700 scaffolds for window washing on buildings in New York City, said Gerard McEneaney, a field representative for about 500 washers who are members of S.E.I.U. Local 32BJ, a union representing building service employees. Mr. McEneaney said he understood why the owners of 1 World Trade Center would employ humans to clean the tapered glass facade.

“They want that building sparkling, sparkling clean,” he said.

His members are willing to do the work because it pays well: as much $26.89 an hour plus benefits. Many of the window cleaners are immigrants from South America. “They’re fearless guys, fearless workers,” Mr. McEneaney said.

Having spent nine years on scaffolds in New York City, he said he knew firsthand how scary an ordeal like the one on Wednesday could be. The first time he descended on a scaffold at 1 Penn Plaza, he said, his end of the platform did not stop when it should have, tilting the scaffold.

His partner reacted quickly and lowered his end of the platform to keep it level and avert the need for a rescue, he said. “In a job where you know your life hangs in the balance, you just kind of accept it and rely on your partner and pray that your training has prepared you.”